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Effects of heat treatments on the microstructures and mechanical properties of Mg–3Nd–0.2Zn–0.4Zr (wt.%) alloy
Abstract
Microstructure and mechanical properties of as-cast and different heat treated Mg–3Nd–0.2Zn–0.4Zr (wt.%) (NZ30K) alloys were investigated. The as-cast alloy was comprised of α magnesium matrix and Mg12Nd eutectic compounds. After solution treatment at 540 °C for 6 h, the eutectic compounds dissolved into the matrix and small Zr-containing particles precipitated at grain interiors. Further aging at low temperatures led to plate-shaped metastable precipitates, which strengthened the alloy. Peak-aged at 200 °C for 10–16 h, fine β″ particles with DO19 structure was the dominant strengthening phase. The alloy had ultimate tensile strength (UTS) and elongation of 300–305 MPa and 11%, respectively. Aged at 250 °C for 10 h, coarse β′ particles with fcc structure was the dominant strengthening phase. The alloy showed UTS and elongation of 265 MPa and 20%, respectively. Yield strengths (YS) of these two aged conditions were in the same level, about 140 MPa. Precipitation strengthening was the largest contributor (about 60%) to the strength in these two aged conditions. The hardness of aged NZ30K alloy seemed to correspond to UTS not YS.
... 200°C 9 14 h) and the T7-treated (over-aged treatment, 540°C 9 10 h ? 250°C 9 10 h) alloys are finer b 00 precipitates (Mg 3 Nd, hcp, D0 19 , a = 0.64 nm, c = 0.52 nm, hexagonal prism) [11] and relatively coarser b 1 precipitates (Mg 3 Nd, fcc, a = 0.74 nm, {10-10} a plate) [12], respectively. The stress amplitude of the T6-treated alloy is increased in the beginning of cyclic loading. ...
... Similar to NZ30K (Mg-3Nd-0.2Zn-1Zr) alloy [11], the microstructure of the ascast Mg-3.68 wt%Nd-0.58 wt%Zr alloy mainly consists of a-Mg matrix and hard be-Mg 41 Nd 5 eutectic particles (tetragonal, I41m, a = 1.47 nm, c = 1.04 nm) (on {1-100} a and {11-20} a planes) [12] (as shown in Fig. 1a), while the T4-treated alloy merely consists of a supersaturated solid solution and a-Mg matrix [11]. ...
... alloy [11], the microstructure of the ascast Mg-3.68 wt%Nd-0.58 wt%Zr alloy mainly consists of a-Mg matrix and hard be-Mg 41 Nd 5 eutectic particles (tetragonal, I41m, a = 1.47 nm, c = 1.04 nm) (on {1-100} a and {11-20} a planes) [12] (as shown in Fig. 1a), while the T4-treated alloy merely consists of a supersaturated solid solution and a-Mg matrix [11]. For the T6-treated alloy (as shown in Fig. 1b), the fine b 00 precipitates (on {1-100} a and {11-20} a planes) have a completely coherent relationship with the matrix, resulting in a lattice distortion and effectively blocking dislocation movement to achieve the purpose of strengthening [11,12]. ...
Low cycle fatigue behavior of the Mg–3.68 wt%Nd–0.58 wt%Zr alloy was investigated in both as-cast and various heat-treated conditions and the T6-treated (peak-aged) Mg–3.69 wt%Nd alloy in terms of cyclic stress responses with microstructure evolution. Dislocation-slip mechanism was found to be responsible for the fatigue deformation in the grain-refined material in both the as-cast and the T4-treated (solution) conditions and in the non-grain-refined material in the T6-treated condition. Dislocation interaction with precipitates and twins dominating cyclic deformation was only observed in the grain-refined material with aging hardening conditions. The constant stress amplitudes observed in the as-cast and the T7-treated (over-aged) conditions in the grain-refined material are attributed to the competing mechanism of work hardening and stress release caused by the cracks and slip bands, respectively. The transformation of precipitates from β′′ to β1 and β and the coarsening of β1 precipitate leads to the cyclic softening in the T6-treated alloy. The shearing mechanism of dislocations is the main reason for the change of precipitate in the alloy.
Graphical Abstract
... The as-cast EV31A Mg alloy, trapped α-Mg, and Eutectic phases such as Mg12Nd and Mg41Nd5 are segregated along grain boundaries; increasing the stress concentration in that vicinity is responsible for uneven deformation and premature failure [23,39]. Only secondary phase and grain boundary strengthening mechanisms contribute to stir-cast EV31A alloy [40]. Because Nd is less soluble in the alloy system, solution heat treatment has a more substantial effect [35,36]. ...
... The as-cast EV31A Mg alloy, trapped α-Mg, and Eutectic phases such as Mg 12 Nd and Mg 41 Nd 5 are segregated along grain boundaries; increasing the stress concentration in that vicinity is responsible for uneven deformation and premature failure [23,39]. Only secondary phase and grain boundary strengthening mechanisms contribute to stir-cast EV31A alloy [40]. Because Nd is less soluble in the alloy system, solution heat treatment has a more substantial effect [35,36]. ...
... The more significant the difference in radius between Mg and REE solute atoms, as well as the higher concentration of REE solute, the better the mechanical properties of the EV31A alloy [41]. The grain boundary and solid solution strengthening mechanisms correspond to the T4-heated EV31A alloy strengthening [40]. During the aging process, the supersaturated solid solution's controlled decomposition to precipitation occurs in the Mg alloy system, resulting in a relatively larger nominal grain size. ...
This study aims to prepare a stir-cast EV31A magnesium alloy and investigate the effects of the T4 condition (solid solution strengthening) and T6 condition (solid solution strengthening cum age hardening) on the phases, microstructure, mechanical properties, and fractography. The solid solution at 520 °C for 8 h allows the Rare-Earth Elements (REE) to dissolve in the Mg matrix, but the solubility is limited by the presence of Zn. This phenomenon is responsible for the T4 heat-treated alloy’s strengthening, which raises the UTS to 212 MPa. The formation of new grains within the grains causes an increase in grain boundaries and dislocations during the T6 heat treatment process, increasing the strength (UTS) of the EV31A alloy to 230 MPa. In all three test conditions, the fractography of tensile samples revealed a cleavage-ductile/mixed mode fracture. As expected, the fine-grained T6 sample exhibited superior strengthening at the expense of ductility.
... The mechanical and structural properties of Mg alloys and their response to stresses are currently a hot topic [5][6][7]. Many studies have shown that rare earth (RE) elements could significantly strengthen Mg alloys [8][9][10][11][12]. Although adding a large number of RE elements can effectively improve the mechanical properties of an alloy, the cost increases greatly, which limits the wide application of Mg alloys with high RE content [13,14]. ...
... Therefore, a comprehensive understanding of the thermal deformation behavior of Mg-Nd-Zn-Zr alloys was necessary for the development and engineering application of low RE content Mg alloys. Up to now, the research on Mg-Nd-Zn-Zr alloys mainly focused on the effect of casting and heat treatment on the microstructure and properties of the alloy [11,14,20]. Cast alloys generally have many defects (e.g., coarse structure, low strength and poor plasticity, etc.) [11,14,18,20,21]. ...
... Up to now, the research on Mg-Nd-Zn-Zr alloys mainly focused on the effect of casting and heat treatment on the microstructure and properties of the alloy [11,14,20]. Cast alloys generally have many defects (e.g., coarse structure, low strength and poor plasticity, etc.) [11,14,18,20,21]. Hot extrusion was shown to be an effective method to refine the microstructure, eliminate casting defects and improve the properties of alloys, which can significantly improve its hot formability [21]. ...
Isothermal hot compression experiments were conducted on Mg-2.5Nd-0.5Zn-0.5Zr alloy to investigate hot deformation behavior at the temperature range of 573–773K and the strain rate range of 0.001s−1–10s−1 using a Gleeble-3500D thermomechanical simulator. The results showed that the rheological curve showed a typical work hardening stage, and there were three different stages: work hardening, transition and steady state. A strain compensation constitutive model was established to predict the flow stress of the Mg-2.5Nd-0.5Zn-0.5Zr alloy, and the results proved that it had high predictability. The main deformation mechanism of the Mg-2.5Nd-0.5Zn-0.5Zr alloy was dislocation climbing. The processing maps were established to distinguish the unstable region from the working region. The maps showed that the instability generally occurred at high strain rates and low temperatures, and the common forms of instability were cracking and flow localization. The optimum machining range of the alloy was determined to be 592–773K and 0.001–0.217 s−1. With the increase in deformation temperature, the grain size of the alloy grew slowly at the 573–673K temperature range and rapidly at the 673–773K temperature range.
... For the SSAT process, the aging temperature was 100-400 • C with temperature intervals of 100 • C that were obtained by air cooling. For the TSAT process, the pre-aging temperature was 200-300 • C with temperature intervals of 25 • C that were obtained air cooling, the pre-aging times were 0.05 h, 0. 10 As-cast T5 - ...
... As the composition of the Zn element in the ZM6 (Mg2.6Nd0.4Zn0.4Zr) alloy was 4%, which is under the equilibrium solidification condition, all of the Zn was dissolved in the Mg matrix, and the solidification rate of Zn was much higher than that of equilibrium solidification, with a small Mg-Zn mesophase being present along the grain boundary [10]. As seen in Figure 2b, the formed precipitated-phase Mg12Nd contained a large number of Mg atoms with a high melting point and good thermal stability [10,13]. ...
... alloy was 4%, which is under the equilibrium solidification condition, all of the Zn was dissolved in the Mg matrix, and the solidification rate of Zn was much higher than that of equilibrium solidification, with a small Mg-Zn mesophase being present along the grain boundary [10]. As seen in Figure 2b, the formed precipitated-phase Mg12Nd contained a large number of Mg atoms with a high melting point and good thermal stability [10,13]. Generally, the melting points of rare earth elements are relatively high, reaching 798-1663 °C, and their diffusion in the Mg From Figure 2a, according to the Mg-Zn binary phase diagram, the maximum solubility of Zn in Mg is 6.18 wt% under equilibrium conditions. ...
In the present study, different solid solution and aging processes of as-cast and as-compressed ZM6 (Mg2.6Nd0.4Zn0.4Zr) alloy were designed, and the microstructure and precipitation strengthening mechanisms were discussed. After the pre-aging treatment, a large amount of G.P. zones formed in the α-Mg matrix over the course of the subsequent secondary G.P. prescription, where the fine and dispersed Mg12(Nd,Zn) phases were precipitated at the grain boundaries. The pre-aging and secondary aging processes resulted in the Mg12(Nd, Zn) phase becoming globular, preventing grain boundary sliding and decreasing grain boundary diffusion. Meanwhile, precipitation phase â″(Mg3Nd) demonstrated a coherent relationship with the α-Mg matrix after the pre-aging process, and after the secondary aging phase, Mg12Nd increases and became semi-coherent in the matrix. Compared to an as-cast ZM6 alloy, the yield strength of the as-compressed ZM6 alloy increased sharply due to an increase in the yield strength that was proportional to the particle spacing, where the dislocation bypassed the second phase particle. Compared to the single-stage aging process, the two-stage aging process greatly improved the mechanical properties of both the as-cast and as-compressed ZM6 alloys. The difference between the as-cast and as-compressed states is that an as-compressed ZM6 alloy with more dislocations and twins has more dispersed precipitates in the G.P. zones after secondary aging, meaning that it is greatly strengthened after the two-stage aging treatment process.
... Owing to a relatively high combination of strength, ductility and creep resistance, recently developed NZ30K (Mg-3Nd-0.2Zn-Zr) alloy with lower RE content is in the spotlight of research in weight-critical structural applications [13], A considerable amount of literature has been published on cast NZ30K based alloys since casting is accepted as a highly cost-effective approach to prepare structural components with complex (cavity) shape [10,14]. However, its insufficient strength and thermal stability should be further improved in order to meet the increasing demand for applications to various structural components. ...
... To explore the morphology and composition of these "cloud-shaped phases", SEM observation and EDS analysis are conducted, which are illustrated in Fig. 4. It is noteworthy that the "cloud-shaped phases" are actually rod-shaped phases with no fixed orientation relationships ( Fig. 4a), which are highly enriched with Zr and Zn according to the map-scanning results illustrated in Fig. 4b-f and point analysis shown in Fig. 4g. In accordance with previous work of Mg-Nd-Zn-Zr alloy [13], these rod-shaped phases should be Zn 2 Zr 3 phases. Besides, there is a bright-white particle near the Zn-Zr phases shown in Fig. 4a, which can be proved to be undissolved Zr particles by the EDS analysis ( Fig. 4b and h). ...
... Fig. 7c and d displays the SAED pattern and BF image recorded from the [2110] α direction. Owing to the specific orientation relationship of (0001) β ′ ′ //(0001) α between β ′′ and α-Mg matrix, finely dispersed β ′′ distribution is revealed to parallel to {0001} α matrix planes, which is in accordance with previously published works [13,28]. The average area number density of predominant β ′′ phase is estimated to be approximately 5.8 × 10 15 #/m 2 . ...
This work was primarily aimed at evaluating the effect of heat treatment on the microstructure, high cycle fatigue behavior and mechanical properties of a novel sand-cast Mg-3Nd-2.6Gd-0.2Zn-0.5Zr alloy. Results indicated that the as-cast studied alloy consisted of non-dendritic α-Mg grains and considerable eutectic phases distributed along grain boundaries, most of which could been successfully dissolved into matrix accompanied by the appearance of rod-shaped Zn-Zr phases after subjected to the optimized solution treatment (535 °C × 10 h). Examinations of microstructure revealed a finely dense dispersion of predominant β′′ and minor β′ phases in the peak-aged studied alloy (isothermally aging at 200 °C for 14 h). Tensile properties were significantly improved, and the relatively best combination of strength and ductility was obtained in peak-aged condition (YS = 220 MPa, UTS = 303 MPa and EL = 4.1 %). Heat treatment conditions were found to exert a pronounced influence on the high cycle fatigue properties, and the highest fatigue strength σf (approximately 121.5 MPa), was achieved in peak-aged studied alloy, which is about 24% higher than that of commercial NZ30K-T6 alloy. Owing to the specifically significant response to heat treatments, the fatigue strength of the studied alloy was confirmed to increase in a near linear relationship with the increasing yield strength and ultimate tensile strength. This is also well consistent with the characteristic fatigue life predicted by Weibull statistics in different heat treatment conditions.
... Alloys with small additions of RE (1-2 wt%), however, have shown improved mechanical strengths and ductility at elevated temperatures compared to conventional magnesium alloys. 2,7,8 For example, the addition of RE element Nd resulted in improved mechanical properties with additions as small as 1 wt% making it possible to produce an alloy suitable for structural applications at a reduced cost. 8 Despite the increased interest of using magnesium alloys for automotive applications, the economic hurdles for magnesium alloys remain high and go beyond the cost of the alloying constituents. ...
... 2,7,8 For example, the addition of RE element Nd resulted in improved mechanical properties with additions as small as 1 wt% making it possible to produce an alloy suitable for structural applications at a reduced cost. 8 Despite the increased interest of using magnesium alloys for automotive applications, the economic hurdles for magnesium alloys remain high and go beyond the cost of the alloying constituents. Solution heat treatment with artificial aging is often required to obtain high tensile strengths with maximum ductility along with improved corrosion resistance for structural magnesium alloys. ...
The heat treating response of a magnesium-rare earth alloy that underwent rapid heating to the holding temperatures using a fluidized bed heat treating process was investigated. Cast tensile samples underwent various solution treating and aging times within the fluidized bed process, and resulting mechanical behavior was measured by uniaxial tensile testing. The measured heating rates were faster than conventional processes with the cast tensile samples heated to both solution and aging holding temperatures within three minutes. From the measured mechanical behavior, the optimum solution treating time within the fluidized bed was two hours, and this holding time is considerably less than that recommended for standard conventional heat treating. Microstructural analysis showed that the solute phase dissolved rapidly within the Mg matrix during solution treating that allowed for a significant reduction in holding times using the fluidized bed process.
... This combination increases the peak strength and improves the castability and ductility of the material [4][5][6][7]. Adding Nd as a low solid solubility (3.6 wt % [8]) RE is a cost-effective solution, because a low amount of alloying addition is sufficient to induce secondary-phase formation in order to improve elevated temperature strength [9,10]. The presence of Zn further decreases the solubility of Nd [11]. ...
... However, under realistic casting conditions, the Mg 12 Nd metastable phase is formed [12]. Given that the presence of Zn stabilizes the metastable Mg 3 (Nd, Zn) phase [13], extensive research has been recently devoted to the Mg-Nd-Zn system, aiming to characterize the mechanical behavior of the alloys at ambient and elevated temperatures under different load conditions and fatigue [9]. Because Nd is also biocompatible, these alloys are considered to be prospective implant materials for future medical applications [14]. ...
In situ synchrotron radiation diffraction was performed during the compression of as-cast Mg–3Nd–Zn alloys with different amounts (0, 0.5, 1, and 2 wt %) of Zn addition at room temperature. During the tests, the acoustic emission signals of the samples were recorded. The results show that the addition of Zn decreased the strength of the alloys but, at the same time, increased their ductility. In the earlier stages of deformation, twin formation and basal slip were the dominant deformation mechanisms. The twins tended to grow during the entire compression stage; however, the formation of new twins dominated only at the beginning of the plastic deformation. In order to accommodate the strain levels, the alloys containing Zn underwent nonbasal slip in the later stages of deformation. This can be attributed to the presence of precipitates containing Zn in the microstructure, inhibiting twin growth.
... After that, the size of β′ phase in Fig. 10(c) significantly increases; however the volume fraction does not change but the number density of β′ phase slightly decreases. Thus, the process after 10,000 s could be considered as coarsening process for precipitation of β′ phase and age hardening is supposed to be observed at about 10,000 s, which is validated by the experimental [77]. Fig. 11 shows the calculated results where the peak-age at 200°C is observed around 10,000 s. ...
... Calculated age-hardening response of Mg-3Nd-0.2Zn alloy at 160°C, 200°C, and 250°C, compared with experimental data[77]. sequence for ATS0.25: Mg-7Al-2Sn-0.25Si ...
This paper presents an overview on the application of CALPHAD (CALculation of Phase Diagrams) methodology in the design and development of advanced lightweight metallic materials including magnesium, aluminum, titanium, aluminum-based metal matrix composites, and high entropy alloys. In this work, CALPHAD methodology has been established and summarized from the construction of databases describing thermodynamics, atomic mobility, kinetics, thermo-physical properties (such as viscosity) to the application of computational design of lightweight materials. The examples in this paper have demonstrated the effectiveness and capability of CALPHAD methodology in accelerating the design of advanced lightweight materials by optimizing the compositions and various heat-treatment conditions, modifying the evolution of microstructures during processing , and finally predicting the mechanical properties (e.g., yield strength and hardness) of the lightweight components. Although the examples are given in lightweight alloys for structural applications, the fundamental methodology and modeling principles are applicable to all materials and engineering applications. Thus, the future of the advanced material design will be strongly based on development of CALPHAD methodology such as the construction of reliable databases coupled with CALPHAD-based models for various applications.
... Alloys containing Nd and Y are most widespread in industry [5]. Neodymium is one of most efficient strengthening element for magnesium among REMs of the cerium group [3,6]. These materials are wellstrengthened during heat treatment [7]. ...
... However, even a small amount of Zn in Mg-Zr-REM alloys increases the creep resistance [11,21]. In addition, the presence of zinc also somewhat increases the alloy strength [11] and hardness after aging [6]; therefore, its use in the alloy composition makes it possible to decrease the content of high-cost yttrium. ...
Samples of ML19 magnesium alloy with composition, wt %, (0.1–0.6)Zn–(0.4–1.0)Zr–(1.6–2.3)Nd–(1.4–2.2)Y have been investigated. The influence of Nd, Y, Zn, and Zr on equilibrium phase-transition temperatures and phase composition using Thermo-Calc software is established. The Scheil–Gulliver solidification model is also used. We show the significant liquidus temperature increase if the zirconium content in alloy is higher than (0.8–0.9) wt %. Thus, a higher melting temperature is required (more than 800°C). This is undesirable when melting in a steel crucible. The change in equilibrium fractions of phases at different temperatures in ML19 magnesium alloy with a minimum and maximum amount of alloying elements are calculated. Microstructures of alloys with different amounts of alloying elements in as-cast and heat-treated condition has been studied using scanning electron microscopy (SEM). We investigate the concentration profile of Nd, Y, Zn, and Zr in the dendritic cell of an as-cast alloy. The amount of neodymium and zinc on dendritic cell boundaries increased. A high concentration of yttrium is observed both in the center and on the boundaries of the dendritic cell. A high zirconium concentration is mainly observed in the center of the dendritic cells. A small amount of yttrium is also present in zirconium particles. These particles act as nucleation sites for the magnesium solid solution (Mg) during solidification. The effect of aging temperature (200 and 250°C) on the hardness of the samples after quenching was studied. Aging at 200°C provides a higher hardness. The change in the hardness of quenched samples during aging at 200°C is investigated. Maximum hardness is observed in samples aged for 16–20 h. The two-stage solution heat treatment for 2 h at 400°C and 8 h at 500°C with water quenching and aging at 200°C for 16 h is performed. This heat treatment enables us to get tensile strength 306 ± 8 MPa and yield strength 161 ± 1 MPa with elongation 8.7 ± 1.6%.
... 2.1.1 Development of high-performance magnesium rare-earth (RE) alloys Shanghai Jiao tong University developed the JDM1-JDM4 series alloys [7][8][9][10][11] to address the lack of strength, plasticity, heat tolerance, and corrosion resistance of magnesium alloys, to satisfy the needs of the automobile industry for lightweight structural components. The typical tensile mechanical properties of these magnesium alloys are shown in Table 2. JDM1 [7,8] is an Mg-Nd-Zn-Zr based alloy. ...
... Development of high-performance magnesium rare-earth (RE) alloys Shanghai Jiao tong University developed the JDM1-JDM4 series alloys [7][8][9][10][11] to address the lack of strength, plasticity, heat tolerance, and corrosion resistance of magnesium alloys, to satisfy the needs of the automobile industry for lightweight structural components. The typical tensile mechanical properties of these magnesium alloys are shown in Table 2. JDM1 [7,8] is an Mg-Nd-Zn-Zr based alloy. It is synergistically strengthened by dispersed Zr-containing particles and metastable prismatic β′′ (Mg 3 Nd)-phase precipitates; trace amounts of Zn and Zr addition promote the activity of the non-basal dislocations at room temperature, which improve the ductility of the alloy. ...
... %) exhibits excellent plasticity and formability, and thus is widely used in aircraft and automobile manufacturing [6][7][8][9][10]. In order to further improve the high-temperature strength and creep resistance, Mg alloys containing rare earth (RE) have been developed [11][12][13][14][15], including Mg-Gd, Mg-Y and Mg-Nd systems [16][17][18][19][20]. However, age hardening in Mg-RE alloys usually deteriorates their plasticity. ...
Heterostructured materials are an emerging class of materials with superior performances that are unattainable by their conventional homogeneous counterparts, and have recently attracted extensive attentions from the materials community. However, due to the hcp crystal structure, the conventional processing methods of heterostructured materials are difficult to be applied in Mg alloys. To overcome the negative effects, namely poor formability and easy oxidation, in Mg alloys, diffusion bonding under an ultra-vacuum condition was employed to prepare multi-layer heterostructured materials with different alloys, i.e. AZ31 (Mg–3Al–1Zn wt. %) and GW103 K (Mg-10Gd-3Y-0.4Zr wt. %) alloys. Moreover, the reinforcement of interface was further improved by interaction of alloying elements during post-annealing. The new formed interfacial phase was found to be the main reason for the reinforcement of interface. Without a decrease of strength, the ductility of post-annealed samples was increased to more than twice of the diffusion bonded ones. Based on high resolution TEM observation, the crystal lattice of the interfacial phase was determined as a fcc structure with a lattice parameter of a = 7.9 Å.
... The contribution of β′ phase on inhibiting the dislocation motion was stronger than that of β 1 phase. The similar phenomenon had been observed in the Mg-Nd-Zn-Zr and Mg-Gd-Zn alloys [27,28]. For the peak-aged Mg-Nd and Mg-Gd series alloy, the major strengthening phase was the β′ phase. ...
As an indispensable pre-treatment for aging, homogenization treatment has a significant effect on precipitation behavior of the Mg-RE alloys. Herein, the influence of homogenization temperature on the microstructure evolution and mechanical performance of a novel Mg-2.0Nd-2.0Sm-0.4Zn-0.4Zr (wt.%) alloy has been studied systematically. The results indicated that the as-cast alloy was mainly composed of α-Mg matrix, β-Mg12(Nd,Sm,Zn) phase and Zr-containing particles. Upon increasing the homogenization temperature from 500 oC to 525 oC for 8 h, the average grain size of as-homogenized alloy increased from 76 μm to 156 μm, and the content of β phase decreased gradually. It was worth noting that the homogenization temperature exceeded 515 oC, the β phase at the grain boundaries was completely dissolved. After aging at 200 oC for 18 h, numerous of plate-like β' phases were observed in α-Mg matrix. The rise in homogenization temperature was conducive to nucleation and growth of the β' phase. However, excessive homogenization temperature significantly coarsened grain size. The aged alloy under homogenization treatment at 515 oC for 8 h achieved optimal mechanical properties. The values of ultimate tensile strength, yield strength and elongation were 261 MPa, 154 MPa and 5.8 %, respectively. The fracture mode of the aged alloy mainly exhibited a typical transgranular cleavage fracture.
... Generally, the two categories of precipitates grow preferentially along the prismatic planes of α-Mg matrix, and can efficiently block the basal glides of dislocations. Besides, some ternary Mg-Nd based alloys containing the transition or divalent metals, such as Ag [13,14], Zn [15][16][17], Ni [18], Mn [19] or Ca [20], can exhibit the more optimized comprehensive mechanical properties since a series of ternary plate-like precipitates including γ ′ , γ ′′ and γ ′ ′ ′ precipitates, as well as long-period stacking ordered (LPSO) structure can be formed on the (0001) α basal plane [21][22][23]. With only a few nanometers thick along [0001] α , these basal plates can serve as some effective barriers to non-basal slips of dislocations. ...
In Mg-Nd-In alloy, the conventional β′ strengthening phase has been replaced by a basal plate with an introduction of In atom. In this work, the basal plate is elucidated to have a ternary composition and a close-packed structure using atomic scale scanning transmission electron microscopy. The stacking layer containing three parallel (0001)α atomic planes is stacked alternately and the two adjacent stacking layers have a 180° rotational symmetry along [0001]α, constituting a perfect hexagonal stacking structure with an AoBrAoBr stacking sequence. It is confirmed that the hexagonal structure has a space group P6322 and parameter lattices of aH = 1.134 nm and cH = 1.606 nm. However, in case that a 180° rotation does not occur between two stacking layers, a stacking fault or a rhombohedral configuration of an AoBrAoCoAr stacking sequence can appear. In addition, the basal plate generally has a thickness of less than 40 nm and an aspect ratio of ~20, as well as can maintain a specific orientation relationship to α-Mg matrix, which are also discussed particularly based on the interfacial mismatch between both phases.
... According to elemental distribution images, the Y and Sm atoms mainly concentrated on the grain boundaries, while part of the Y and Zn atoms were observed at the lamellar parts, which corresponded to the positions of LPSO. The irregular particlesconsisted of the Y, Sm and Zr atoms and were identified as the Zr-containing compound[31]. ...
An abnormal texture with c axis of the grains parallel to extrusion direction (ED) was found in extruded Mg–Y-Sm-Zn-Zr alloy. The mechanisms for the formation of this abnormal texture were investigated based on the dynamic recrystallization (DRX) mechanisms and deformation modes during extrusion using electron backscatter diffraction (EBSD) and a viscoplastic self-consistent (VPSC) model. The microstructure evolution during extrusion indicated that the abnormal <0001>//ED texture was dominated by DRX grains. With the strain increasing, the intensity of this texture enhanced. Based on the EBSD results analysis, discontinuous dynamic recrystallization (DDRX) played the dominated role in nucleation of the new grains at the initial stage and then continuous dynamic recrystallization (CDRX) was activated at high strain. The formation of abnormal <0001>//ED texture was attributed to the activation of <c+a> slips and it could promote the rotation of c-axis of grains to ED, which contributed to the formation of <0001>//ED texture. The simulated texture predicated that the CRSS of basal slip was higher than that of pyramidal <c+a> slip, which played an important role in contributing the formation of the <0001>//ED texture component.
... The microstructures of the WE43B and WE43B + 0.6wt%Zn alloys after T6 heat treatment (solution treatment at 525°C for 8 h with water quenching and aging at 250°C for 15 h) are shown in Fig. 7. Precipitates in the WE43B alloy exhibited a low backscattered electron (BSE) contrast and are indicated in Fig. 7(a) by white arrows. In the WE43B + 0.6wt%Zn al-loy, precipitates formed elongated rods with a high BSE contrast, as shown by the white arrows in Fig. 7(b); this microstructure is typical of other Mg-RE-Zr-Zn alloys [11,32]. In the WE43B alloy, precipitates were uniformly distributed in (Mg) (Fig. 7(c)); however, in the WE43B + 0.6wt%Zn alloy, precipitates were located around (Zr) phase particles ( Fig. 7(d)). ...
Zn is a commonly used alloying element for Mg alloys owing to its beneficial effects on mechanical properties. To improve the mechanical and corrosion properties of WE43B Mg alloys, the effects of 0–0.7wt% Zn addition on the microstructure and properties of sample alloys were investigated. Addition of Zn to as-cast WE43B alloy promoted the formation of the Mg12Nd phase; by contrast, after T6 heat treatment, the phase composition of WE43B alloys with and without Zn addition remained mostly the same. A long-period stacking ordered phase was predicted by CALPHAD calculation, but this phase was not observed in either the as-cast or heat-treated Zn-containing WE43B alloys. The optimum temperature and duration of T6 heat treatment were obtained using CALPHAD calculations and hardness measurements. Addition of Zn resulted in a slight reduction in the average grain size of the as-cast and T6 heat-treated WE43B alloys and endowed them with increased corrosion resistance with little effect on their mechanical properties.
... These investigations report enhancement of strength and ductility [10,11] by the addition of different RE elements to the ZK system [12]. Neodymium, with its low solid solubility (3.6 wt % at 549 • C [13]), is an ideal element because relatively low concentrations are necessary to introduce secondary phase particles that further improve strength at elevated temperatures [14,15]. Due to the low cytotoxicity of Nd, the Mg-Nd-Zn system is also considered as a prospective alloy for bio-absorbable implants [16]. ...
Mg-4Nd base alloys with Zn additions of 3, 5 and 8 wt % were investigated with in situ synchrotron radiation diffraction during solidification. This method enabled the investigation of phase formation and transformation in the alloys. The diffraction results were supported with TEM observations on the as-solidified samples. The results show the effect of increased Zn addition on stabilizing the Mg3RE phase (RE—rare earth). The experimental results agree only partially with the theoretical calculations indicating the need to improve the existing thermodynamic database on the alloy system.
... These investigations report further enhancement of elevated temperature strength and ductility [10,11] by the addition of Rare earth (RE) elements to the Mg-Zn-Zr system [12]. Neodymium having a relatively low solid solubility in Mg (3.6 wt.% at 549 • C [13]) is an ideal RE element because high concentrations are not needed to produce second phase particles in order to further improve the elevated temperature strength [14,15]. Furthermore, Nd is not toxic therefore the Mg-Nd-Zn system is under investigations for bio absorbable implant materials [16]. ...
The mechanical properties of as-cast Mg-4Nd-xZn (x = 0, 3, 5 or 8 wt.%) alloys were investigated both in situ and ex situ in as-cast and solution-treated conditions. The additions of 3 or 5 wt.% Zn in the base Mg-4Nd alloy did not improve yield strength in comparison to the binary Mg-4Nd alloy. Mechanical properties were shown to improve only with the relatively high concentration of 8 wt.% Zn to Mg-4Nd. The change in intermetallic morphology from a continuous intermetallic to a lamella-like intermetallic was the primary reason for the decreased mechanical properties in Mg-4Nd-3Zn and Mg-4Nd-5Zn compared with Mg-4Nd and Mg-4Nd-8Zn. The dissolution of intermetallic at grain boundaries following heat treatment further indicated the importance of grain boundary reinforcement as shown in both in situ and ex situ compression testing. Azimuthal angle-time plots indicated little grain rotation most noticeably in Mg-4Nd, which also indicated the influence of a strong intermetallic network along the grain boundaries.
To have a better understand on the change of microstructure via kinetics, the diffusion behavior of Mg alloys is of special interest to researchers. Meanwhile, diffusion coefficients of Mg based alloys can explain and represent their diffusion behavior well. The evolution of experimental and calculated methods for detecting and extracting diffusion coefficients was discussed briefly. The reasonable diffusion data, especially self-diffusion coefficients, impurity diffusion coefficients and inter-diffusion coefficients of Mg alloys, were reviewed in detail serving to design the Mg alloys with higher accuracy. Then the practical applications of diffusion coefficients of Mg alloys were summarized, including diffusional mobility establishing, precipitation simulation and mechanical properties prediction.
In this study, a high-performance Mg-3Nd-3Gd-0.2Zn-0.5Zr alloy is prepared. The precipitate microstructures of the studied alloy during isothermal aging at 225 °C are characterized by HAADF-STEM observations, and the corresponding mechanical properties are also investigated. It is noted that the evolution of the precipitate microstructure is mainly divided into three stages: the precipitation and growth of β″, the transition from β″ to β1, and the ripening of β1. There are apparent differences in the mechanical properties of alloy in these three stages. At the beginning of aging treatment, due to the precipitation and growth of β″, the yield strength of alloy increases rapidly while the elongation decreases. Afterwards, the transition of precipitates from β″ to β1 leads to the simultaneous increment in the yield strength and elongation of the alloy. Finally, as the β1 precipitates ripen, the yield strength of the alloy decreases while its elongation shows a trend of increment owing to the significantly improved dimensions and reduced number density.
The effects of different extrusion ratios (7.6, 12.5 and 26.1) on the microstructure evolution, texture characteristics and mechanical properties of Mg-2.5Nd-0.5Zn-0.5Zr (wt.%) alloys have been systematically studied. The results showed that with the increase of extrusion ratio, the dynamic recrystallization grain size decreased first and then increased, the texture state changed from weakened bimodal texture to strong basal texture with the texture strength increased from 6.7 to 19.4. The growth of grain size was mainly attributed to rise of temperature transformed by the deformation heat with high extrusion ratio and the more intense extrusion deformation. The mechanical properties of the alloy increased first and then decreased with the increase of the extrusion ratio. When the extrusion ratio was 12.5, the alloy exhibited excellent mechanical properties. The higher yield strength results from the interaction of refinement strengthening, dislocation strengthening, precipitation strengthening and texture strengthening. With the increase of extrusion ratio, the proportion of basal slip increased gradually when the alloy was tensile in extrusion direction, and the deformation mechanism was mainly basal slip. When the alloy was tensile along the transverse direction, the proportion of basal slip decreased, and the proportion of prismatic slip increased gradually. The main deformation mechanism were basal slip and prismatic slip.
In this work, a high-quality Mg-2.85Nd-0.25Zn-0.5Zr (wt%) alloy ingot with a diameter of 280 mm was successfully fabricated by direct chill casting. There was no obvious composition segregation along the radial direction of the ingot. The large temperature gradient along the radial direction, accurately the higher cooling rate in the surface, gives rise to the grain size and fraction of eutectic compounds to gradually decrease from the center to the surface. This results in varying mechanical properties at different positions, such as higher strength and ductility in the surface than in other positions. The peak-aged alloy exhibited a higher strength-ductility synergy, with tensile strength of 282 MPa and elongation of 12.5%, than other Mg-Nd based alloys processed by permanent mold casting and sand casting. Furthermore, it is worth noting that the peak-aged alloy manifested significant strain hardening and high heat resistance, as evidenced by liner strain hardening at temperatures between 100 and 250 °C, which is mostly due to stable solute hardening triggered by the large atomic radius of Nd atoms in the α-Mg matrix. These findings provide references for the application and manufacture of large-scale magnesium alloy ingots.
The effect of Sn addition on the microstructure and mechanical properties of Mg-12Gd-3Y-0.5Zn-0.6Zr (wt%) alloys was investigated. The results show that the microstructure of the as-cast Sn-containing alloys consists of α-Mg, Mg24(RE,Zn)5 and Sn3RE5 phases. The Sn3RE5 phases with superior high temperature stability increase with the Sn content. The Young's modulus of Sn3Gd5 and Sn3Y5 compounds are 109.0GPa and 119.2GPa, respectively, which results in the elastic modulus of the alloys increasing with the Sn element. Mg-12Gd-3Y-0.5Zn-0.6Zr–2Sn alloy has the highest elastic modulus of 51.3GPa. The addition of Sn can significantly increase the number density of fine circular β′ phases in peak-aged alloys. Suitable volume fraction of Sn3RE5 phases, fine DRXed grains, weak basal texture and fine precipitates endow the peak-aged Mg-12Gd-3Y-0.5Zn-0.6Zr–1Sn alloy with excellent comprehensive performance, and its elastic modulus, UTS and YTS are 50.4GPa, 452MPa and 368MPa, respectively.
This work realized the texture tailor of Mg-2.6Nd-0.55Zn-0.5Zr (wt.%) alloy sheets via the hot extrusion on the billets subjected to different solution times from 0h to 16h. The solution treatment of 530 °C could effectively dissolve the coarse Mg12Nd particles. But abundant small second phase particles again developed after hot extrusion owing to the solid solution decline of Nd in magnesium matrix and the strain induced precipitation. The occurrence of dynamic recrystallization during hot extrusion improved the microstructure homogeneity and refined the grain sizes to 4.2–5.7μm. Additionally, the activations of basal slip, prismatic slip and pyramidal <c+a> slip together promoted the development of double peak basal texture in the hot-extruded sheets. However, this kind of texture was weakened by the particle-stimulated nucleation recrystallization and its weakening degree gradually decreased with the prolongation of solution time. This made the sheet with the long solution time more likely retain the strong deformation basal texture. Such strong basal texture increased the activation fraction of prismatic slip along the extrusion direction (ED), producing the higher yield stress and the lower uniform elongation. Influenced by this, the anisotropies in the yield stress and the uniform elongation between the ED and the transverse direction (TD) were obviously weakened. Besides, the activation of tension twinning greatly increased the uniform elongation and decreased the yield stress.
In low cycle fatigue, Mg–Nd based alloys exhibit initial cyclic hardening and then softening. The cyclic hardening and softening behaviour of a peak-aged Mg–Nd-based alloy has been studied using transmission electron microscopy. The initial cyclic hardening is attributed to the increase of dislocation density and interaction of dislocations with nano-scale precipitates. The shearing of dislocations through precipitates and the transformation of precipitates in different stages of fatigue are responsible for the cyclic softening. The competing hardening and softening mechanisms present throughout the entire fatigue process. Hardening mechanism dominates in the very beginning of cyclic loading while softening takes over after certain numbers of cycles, say ~ 350 cycles.
This work successfully fabricated the fine-grained Mg-2.6Nd-0.55Zn-0.5Zr (wt%) alloy plates with the average grain sizes of 2.39–3.49μm via the extrusion process. The microstructure evolution during extrusion process and the associated mechanical properties were investigated. Results showed that dynamic recrystallization (DRX) significantly refined the grain size and improved the microstructure homogeneity. Extrusion fragmentation effect on the coarse Mg12Nd particles contributed to the development of fine-grained uniform microstructure, especially at the lower extrusion temperature. Additionally, extrusion process turned the fiber basal texture into the double peak basal texture owing to the sequential deformation mode activation from basal slip and tension twinning to pyramidal <c+a> slip. Subsequent continuous DRX and particle-stimulated nucleation (PSN) recrystallization played a weakening effect on the double peak basal texture. Moreover, decreasing the extrusion temperature increased the activation difficulty of pyramidal <c+a> slip and weakened the effect of PSN on texture modification, leading the basal plane distribution along the extrusion direction to be strengthened. Fine-grained strengthening significantly improved the mechanical properties, but such strengthening capability was severely dependent on the texture. During tension deformation, the weak basal texture easily activated the basal slip whereas the strong basal texture needed to activate the prismatic slip. Hall-Petch analysis indicated that the higher yield stress in the strong basal texture was ascribed to the remarkable deformation mode strengthening (activation of prismatic slip) and the geometrical strengthening (higher Taylor factor). The weak basal texture was beneficial to improve the uniform elongation owing to the higher strain hardening ability, which was more obvious when the tension twinning was activated.
Mg–Nd–Zn–Zr magnesium alloy (JDBM) has been studied widely as biodegradable medical material. To process high quality JDBM wires, effects of annealing on the mechanical properties and degradation behavior after drawing were studied by microscopic observations, tensile and immersion tests. The as-extruded wires with a diameter of 3 mm could be drawn up to 9 passes without annealing until 125% cumulative drawing deformation. Complete recrystallization occurred after annealing at 325 °C for 30 min, 350 °C for 5 min or 450 °C for 3 min, respectively. Room temperature tensile tests and simulated body fluid immersion tests showed that annealing at slightly elevated temperature for short time could obtain better properties due to the finer grain size and more dispersive distribution of precipitates. For this study, annealing at 350 °C for 5 min is the best parameters which can be utilized to further fabricate fine wires.
Mg-2.6Sm-1.3Gd-0.6Zn-0.5Zr (wt.%) alloy is prepared with metal mold casting and followed hot extruded at 400°C with the extrusion ratio of 25:1. Microstructure and mechanical properties of the as-extruded alloy are investigated using optical microscope (OM), scanning electron microscope (SEM) with electron backscatter diffraction (EBSD), X-ray diffraction (XRD) and transmission electron microscope (TEM). The results show that the as-extruded alloy is fully dynamically recrystallized, which develops rare earth (RE) texture component. The as-extruded alloy shows an age-hardening response at 200°C, in which there are numerous nano-scale plate strengthening phase β′ precipitated in the matrix. The as-extruded alloy in the peak-aged condition exhibits ultimate tensile strength, yield strength, and elongation of 332 MPa, 220 MPa and 22.7% at room temperature, respectively. The dominant strengthening mechanisms of the investigated alloy are fine-grain strengthening, solid solution strengthening and precipitate strengthening.
The microstructures, texture, damping and mechanical properties of Mg-Nd-Zn-Zr alloy processed by hot extrusion were investigated in this work. The results showed that the microstructures were markedly refined, and uniformly distributed after hot extrusion. The average grain size was refined to 8.1 ± 1.6 μm (extrusion ratio was 7.65, E1) and 6.28 ± 1.7 μm (extrusion ratio was 12.56, E2), respectively, and the yield strength of the alloy was increased significantly. The dislocation density and texture of the alloy increase with the increase in the extrusion ratio. The damping values Q−1 of as-cast (Initial), E1 and E2 alloys were 0.03504, 0.01634 and 0.01539 at the strain of 1 × 10−3, respectively. Mg-Nd-Zn-Zr alloy could be classified as high strength (259 MPa), high plasticity (21.4%) and high damping (0.01539) Mg alloy.
This work was undertaken to investigate the specific role of element Gd on the microstructural evolution and mechanical properties of Mg-3Nd-0.2Zn-0.5Zr alloy. It is noticeable that, as the Gd content increases, the average grain size of as-cast alloys is continuously reduced, accompanied by the main secondary phases changing from Mg12Nd to Mg3Gd. Tensile tests reveal that addition of Gd (1.5–4.5 wt%) could lead to substantial enhancements of alloy strength (25–70 MPa) and an excellent combination of strength and ductility is obtained in the alloy with 4.5 wt% Gd addition (YS = 200 MPa, UTS = 343 MPa, EL = 5.4%). Microstructure characterization indicates that the improved solid-solution strengthening effect originated from increasing Gd additions plays a key role in the significant strength improvements obtained in as-quenched alloys. HAADF-STEM observations suggest that Gd is highly enriched in the dominantly disc-like prismatic β″ phases, leading to the strongly enhanced precipitation kinetics and greatly augmented volume fraction of β″ phase. A quantitative microstructural comparison of peak-aged specimens indicates that the significant strength enhancements should be primarily derived from the denser dispersion of β″ phases with higher aspect radio arising from Gd addition.
Magnesium alloys are widely used in automotive applications in which heat treatment is of primary concern. This article systematically presents the effect of heat treatment when applied on different magnesium alloys. It summarizes the changes occur as a result of heat treatment like microstructure, mechanical properties and corrosion behaviour. The influence of elements such as rare earths, aluminum, zinc, etc applied on magnesium alloys was outlined. Effect of heat treatment and deformation on magnesium alloys are discussed. Challenges as a result of heat treatment were addressed for further research and scope of work.
In this study, the microstructural evolution, mechanical properties and biocorrosion performance of a Mg–Zn–Ca–Mn alloy were investigated under different conditions of heat treatment, extrusion, one pass and two passes of half equal channel angular pressing (HECAP) process. The results showed significant grain refinement of the homogenized alloy after two passes of HECAP process from 345 µm to 2 µm. Field emission scanning electron microscopy (FESEM) revealed the presence of finer Mg6Zn3Ca2 phase as well as α-Mn phase after HECAP process. The results also showed that mechanical characteristics such as yield strength, ultimate tensile strength and elongation of the HECAPed samples improved by ∼208%, ∼144% and ∼100% compared to the homogenized one, respectively. Crystallographic texture analysis indicated that most of the grains at the surface were reoriented parallel to the (0001) basal plane after HECAP process. Electrochemical corrosion tests and immersion results indicated that the sample with two passes of HEACP had the highest biocorrosion resistance confirming that the basal planes had the lowest corrosion rate compared to the non-basal ones. The mechanical behavior and bio-corrosion evaluation demonstrated that the HECAPed Mg–Zn–Ca–Mn alloy has great potential for biomedical applications and a mechanism was proposed to explain the interrelations between the thermomechanical processing and bio-corrosion behavior.
During the past decades, with the increasing demands in lightweight structural materials, Mg alloys with low density and high performance have been extensively investigated and partly applied in some industries. Especially when rare earth (RE) elements are added as major alloying elements to Mg alloys, the alloy strength and creep resistance are greatly improved, which have promoted several series of Mg-RE alloys. This paper reviews the progress and developments of high-performance Mg-RE alloys in recent years with emphasis on cast alloys. The main contents include the alloy design, melt purification, grain refinement, castability, novel liquid casting and semisolid forming approaches, and the industrial applications or trials made of Mg-RE alloys. The review will provide insights for future developments of new alloys, techniques and applications of Mg alloys.
The microstructures evolution, texture characteristics and mechanical properties of Mg-2.5Nd-0.5Zn-0.5Zr alloy processed by high strain rate rolling (HSRR) at different temperature were systematically investigated in the current study. The results showed that the initial material was approximately equiaxed grains (23.8 μm) with a large number of divorced eutectic Mg12Nd phases precipitated along grain boundaries in a continuous network. After HSRRed, the microstructure was obviously refined, and uniform distributed. The average grain size was refined to 3.63 μm (R673), 2.71 μm (R648) and 2.17 μm (R623), respectively, and corresponding percentage of recrystallization were 46.1%, 49.3% and 63.9%, respectively. R623, R648 presented a strong (0002) basal surface bimodal texture in the RD direction but this texture was in the TD direction in R673, all of which preferential orientation was at 66.3–90° region. An excellent combination of ultimate tensile strength (324 ± 2 MPa), the yield strength (298 ± 3 MPa) and the elongation (6.9 ± 1.5%) were achieved in a fine-grained (2.17 ± 1.17 μm) Mg-2.5Nd-0.5Zn-0.5Zr alloy prepared by HSRR at 623 K. The higher yield strength can be obtained by refinement strengthening, dislocation strengthening, precipitation strengthening and texture strengthening.
The addition of Zn to the Mg–Nd system improves the yield strength and creep resistance, however its influence on the intermetallic phases in the ternary system is not yet fully understood. Understanding the sequence of phase-formation and phase-evolution during solidification and processing is essential to microstructure design. The solidification was investigated with in-situ synchrotron radiation-diffraction and tomography during cooling from the molten state to 200°C to investigate the phase-formation and transformation characteristics. The solidification starts with α-Mg followed by two distinct intermetallic phases T2 and T3. The results suggest that Zn stabilizes the Mg 3 Nd phase and accelerates precipitate formation. The dendritic morphology changes during solidification towards coarser shapes, thus impedes feeding and promotes hot tearing.
Microstructures and mechanical properties, especially the contribution of individual strengthening mechanism, of Mg‐10Gd‐0.4Zr alloy with partial substitution of Gd with Nd were studied. The results showed that Mg12Nd phases appeared in the as‐cast alloys when substituting Gd with Nd, and the volume fraction of secondary phase increased with increasing Nd content. After solution heat‐treatment, secondary phases fully dissolved into α(Mg) matrix except for Mg‐5Gd‐5Nd‐0.4Zr alloy, which was attributed to a high Nd content in that alloy, beyond the maximum solubility of Nd in Mg. Ageing‐hardening Age hardening curves indicated that substituting Gd with Nd can shorten the incubation time and improve the peak hardness. And t The peak‐aged Mg‐7Gd‐3Nd‐0.4Zr alloy showed the improved mechanical properties with the yield strength, ultimate tensile strength and elongation of 195 MPa, 294 MPa, and 3.9 % respectively, which was attributed to the dense β” and β' precipitates formed during ageing. Furthermore, the contributions of individual strengthening mechanisms in Mg‐Gd‐Nd‐Zr alloys after different heat‐treatments were calculated, and the results showed that the substitution of Gd with Nd can significantly enhance the contribution of precipitation strengthening, especially in peak‐aged Mg‐7Gd‐3Nd‐0.4Zr alloy.
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Fuel economy and environmental conservation are the major factors to consider magnesium alloys for automotive industry and aero space and other electronics companies. The key features like high strength to density ratio, moderate damping capacity, recyclability, reduced CO2 emissions are added advantages of magnesium alloys in automotive applications. This article reviews historical trends and near future applications of magnesium alloys in automotive industry. As magnesium loses its strength and creep resistance abilities, alternative magnesium alloys are to be explored to supply automotive components in the industry on demand. The objective of this study is to review and evaluate the applications of magnesium in the automotive industry that can significantly contribute to greater fuel economy and environmental conservation. In this study, the current trends, challenges, technological obstacles and future scope of magnesium alloys in the automotive industry are discussed. The consumption of magnesium in automotive industry with reference to environment is explored. Innovative welding and forming techniques available today are encouraging factors for extended use of magnesium and its alloys in automotive sector. This review offer insights and opportunities to researchers for further study and investigation of challenges in the field of automobile industry.
The microstructure and mechanical properties of Mg-4Y-3Nd-xAl (x = 0, 0.6, 0.9, 1.2, 1.5, 2.0%) alloys were investigated in the present work. The results show that the addition of Al can refine the grain of as-cast Mg-4Y-3Nd alloy significantly. The refining effect increases first and then decreases with increasing Al addition. The grain size of Mg-4Y-3Nd alloy is smallest when the Al addition is 1.5%. The reason for refinement is that the in-situ Al2RE particles reacted Al with RE can serve as for α-Mg matrix. The orientation relationship between Al2RE particles and α-Mg was determined by TEM analysis: [101]Al2RE∥[11¯00]Mg(2¯22)Al2RE∥(0002)Mg. The effective size of Al2RE particle served as heterogeneous nucleation site is 1 ∼12 μm. In addition, the strength and ductility of the alloy increase with the increase of Al addition. The Mg-4Y-3Nd-Al alloy with the higher mechanical properties can be attributed to fine grain strengthening and second phase strengthening.
Current hot-tearing predictions for shape castings are suitable for well-investigated alloys with complete mechanical property databases, but are not applicable to newly developed alloys. This study proposed a simplified hot-tearing criterion for shape casting based on the hot-tearing sensitivity to strain rate during solidification. The criterion is simplified to a specific form: M = LRG1.5 < Mcritical, based on the constrained rod casting method that is used to evaluate the hot-tearing susceptibility of Mg alloys, where L (length of the constrained rod), R (the cooling rate), and G (the temperature gradient) can be calculated by numerical simulation, while Mcritical can be determined by comparison of simulation and experimental results in the constrained rod casting. The prediction of the hot-tearing criterion for a 14-inch (0.35 m) Mg wheel casting was in good agreement with the experimental results, confirming its practicability. The proposed hot-tearing criterion is suitable for newly developed alloys without need of consideration of their mechanical properties.
The effects of settling time up to 8 h on the quality of fluxless Mg–Nd–Zn–Zr melt preparation are discussed in terms of melt chemistry and inclusions’ structural properties. The content of Nd, Zn and Zr lie in the demanding scope throughout settling process. Neodymium content rises slightly at the settling time of 0.25 h, then decreases gradually up to 8 h. Zinc experiences a slight increase after 0.25 h, then levels off. Zirconium drops sharply at 0.25 h mainly due to the subsidence of undissolved zirconium (Zr) particles, then drops slightly at 1 h, and then stabilizes. The inclusions have various morphologies like rod-like, granular and clustered, and different types such as oxides, carbides, undissolved Zr particles, and etc. The effect of settling time on inclusions’ removal is remarkable during early settling stage, from 0 to 0.25 h, and is getting less stark with increasing time. The area fraction, average diameter and maximum diameter of inclusions all decrease rapidly at 0.25 h, then gradually, and finally reach a plateau region. The changes of minimum diameter and number density values are insignificant through whole process. The theoretical analysis on inclusions’ settling and rising behavior agrees with the experimental data.
Fig. 3 The settling distance as a function of settling time for various inclusions of different sizes, (a) 1 × 10⁻⁶, (b) 2 × 10⁻⁶, (c) 4 × 10⁻⁶, (d) 8 × 10⁻⁶, (e) 16 × 10⁻⁶ and (f) 32 × 10⁻⁶ m, in the melt at 1033 K. The dashed line indicates the sample depth, approximately 0.2 m below the melt surface. Fullsize Image
The effect of heat treatment conditions on the microstructure and mechanical properties of Mg-3Nd-3Al and Mg-3Nd-0.5Zr alloys has been comparatively investigated. The experimental results showed that the grain size of Mg-3Nd-3Al and Mg-3Nd-0.5Zr alloys was similar, which was 48 ± 4 μm and 46 ± 4 μm in as-cast condition and 50 ± 3 μm and 50 ± 4 μm in solid solution (T4) and solid solution + peak aging (T6) conditions, respectively. The intermetallic phases in the as-cast Mg-3Nd-3Al alloy were granular Al2Nd and acicular Al11Nd3. The intermetallic phase in the as-cast Mg-3Nd-0.5Zr alloy was Mg12Nd. The Al2Nd phase did not dissolve and the Al11Nd3 phase decomposed into the Al2Nd phase and the Mg12Nd phase completely dissolved into α-Mg during T4 treatment process. After T6 treatment, the amount of precipitation phase of the Mg-3Nd-3Al alloy was less than that of the Mg-3Nd-0.5Zr alloy. Compared with the Mg-3Nd-0.5Zr alloy, the Mg-3Nd-3Al alloy had higher strength and elongation in as-cast condition, lower strength in T6 condition and a weaker age-hardening effect.
In order to design a high strength sand cast Mg–Gd–Zn alloy, the microstructures and mechanical properties of cast Mg–12Gd–xZn–0.4Zr (x = 0, 0.4, 0.8)(wt%, signed as GZ1200K, GZ1204K and GZ1208K)alloys were studied. Zn addition changes the constitution of the strengthening phases in Mg[sbnd]12Gd alloy. Small amount of Zn addition leads to the formation of basal precipitates of γ″ and γ′, accelerated-precipitation of the β 1 phases and lower number density of the prismatic β′ phases, and modifies the morphology of the prismatic β′ precipitates. Compared with the Zn-free alloy, the Zn-containing alloys (GZ1204K & GZ1208K)having higher yield strength in peak-aged and over-aged conditions are mainly due to the extra prismatic β 1 precipitates, which do not precipitate in the Zn-free alloy. Under sand casting condition, GZ1208K alloy strengthened by the co-precipitates of the prismatic β′ and β 1 phase indicates a good combination of strength and ductility at room temperature: yield strength of 270 MPa, ultimate tensile strength of 348 MPa and elongation of 2.6%.
Strain-controlled fatigue characteristics of peak-aged and over-aged Mg96.47Nd2.9Zn0.21 magnesium alloys containing 0.42Zr, including stress response, strain resistance, hysteresis loops, strain–life and corresponding low-cycle fatigue life prediction model, were studied. In the peak-aged state (T61: 540 °C × 8 h + 200 °C × 14 h), the alloy shows higher cyclic stress response, but lower ductility than the alloy in the over-aged state (T62: 540 °C × 8 h + 200 °C × 400 h). The yield strength and ultimate tensile strength of the alloy under T61- and T62-treated conditions are close. Compared with T61-treated alloy, the steady stress amplitude occurred in T62-treated alloy is due to higher ductility and more homogenous deformation. In T61 state, the fatigue cracks in the alloy first initiate along the cracked persistent slip bands and then propagate in the trans-granular mode, while in the T62 state, the fatigue cracks initiate along grain boundaries and then propagate in the inter-granular mode.
The Center for Predictive Integrated Structural Materials Science (PRISMS Center) is creating a unique framework for accelerated predictive materials science and rapid insertion of the latest scientific knowledge into next-generation ICME tools. There are three key elements of this framework. The first is a suite of high-performance, open-source integrated multi-scale computational tools for predicting microstructural evolution and mechanical behavior of structural metals. Specific modules include statistical mechanics, phase field, crystal plasticity simulation and real-space DFT codes. The second is the Materials Commons, a collaboration platform and information repository for the materials community. The third element of the PRISMS framework is a set of integrated scientific “Use Cases” in which these computational methods are linked with experiments to demonstrate the ability for improving our predictive understanding of magnesium alloys, in particular, the influence of microstructure on monotonic and cyclic mechanical behavior. This paper reviews progress toward these goals and future plans.
Magnesium alloys based on Nd and Zn are promising materials for both aviation industry and medical applications. Superior mechanical properties of these materials can be achieved by thermomechanical processing such as extrusion or rolling and by aging treatment, which can significantly strengthen the alloy. The question remains especially about the connection of texture strength created in the alloys based on the specific conditions of preparation. This work focuses on the Mg–3Nd–0.5Zn magnesium alloy prepared by hot extrusion of the as-cast state at two different temperatures combined with heat pre-treatment. Extrusion ratio of 16 and rate of 0.2 mm/s at 350 and 400 °C were selected for material preparation. The structures of prepared materials were studied by scanning electron microscopy and transmission electron microscopy. The effect of microstructure on mechanical properties was evaluated. Obtained results revealed the strong effect of thermal pre-treatment on final microstructure and mechanical properties of extruded materials. The Hall–Petch relation between grain size and tensile yield strength has been suggested in this paper based on the literature review and presented data. The observed behavior strongly supports the fact that the Hall–Petch of extruded Mg–3Nd–0.5Zn alloys with different texture intensities cannot be clearly estimated and predicted. In addition, Hall–Petch relations presented in literature can be sufficiently obtained only for fraction of the Mg–3Nd–0.5Zn alloys.
This paper examines deformation behavior of WE43 alloy in direct-chill as-cast (as-cast-WE43) and rolled heat-treated T6 (WE43-T6) conditions with an emphasis on fracture mechanisms. Unlike many Mg allows, as-cast-WE43 and WE43-T6 exhibit no tension/compression asymmetry in their yield stress. WE43-T6 material shows more anisotropy in yield stress than as-cast-WE43, which is attributed to their respected initial crystallographic textures. Both WE43-T6 and as-cast-WE43 exhibit some anisotropy in strain hardening due to texture evolution and deformation twinning. Both materials show a small elongation to fracture of approximately 6% in tension. In contrast, strain to fracture in compression is large. Crystallographic texture evolves substantially in compression, where crystals are slowly reorienting their crystallographic c-axis parallel to the loading direction with plastic strain. Both materials fracture by a typical shear fracture in compression. Fractographic analysis of fractured surfaces in compression for WE43-T6 reveals evidence of transgranular facets that are much larger than grain size with minor content of microvoid coalescence. Although elongation to fracture in tension is small with no necking, detailed analysis of fracture surfaces reveals evidence of ductile microvoid coalescence. However, the intergranular fracture character, especially in the central high stress triaxiality region of the samples, limits the ductility of the material.
The influences of pressure and aging treatment on microstructures and mechanical properties of rheo-squeeze casting (RSC) Mg–3Nd–0.2Zn–0.4Zr alloys were studied. It was found that the nucleation rate, solid solubility of Nd and Zn in the α-Mg matrix, and dislocation density were increased with increasing applied pressure. After aging treatment, the amount of the Zn2Zr3 phase was increased with increasing pressure; β″ phase and β′ precipitates were observed in the RSC alloy and finer β′ precipitates formed in the permanent mold casting (PMC) alloy. The mechanical properties of as-cast alloys were initially increased and then decreased with increasing pressure, while the properties of T6-treated alloys were increased continuously. Due to the larger grain boundary strengthening contribution, the T6-treated RSC sample showed higher mechanical properties than the PMC sample, and the yield strength, ultimate tensile strength, and elongation could reach 165 MPa, 309 MPa, and 5.7%, respectively.
Magnesium alloys, having high specific strength, with a density only 2/3 of that of aluminum and 1/4 of carbon steels, have become ideal materials for low mass applications such as automobiles and electronic devices. It was dealt with the state of the art in developing cost effective, low mass, high ductility and high creep resistance magnesium alloys that are suitable for structures and power train applications.
A uniform, fine α phase microstructure enhances the mechanical properties of Al–Si alloys; however, it is an open question as to how the α phase affects the crack growth behavior. This paper addresses the effects of the morphology and distribution of α phase on the fracture behavior in a model dual-phase Al–7% Si alloy with different microstructures. The influences of microstructural factors on crack growth behavior are examined using in situ experiments. The results show that a globular α phase microstructure produces a straight crack growth path, whereas a dendritic, orientational α phase microstructure leads to a deflected crack profile. Finite-element modeling is performed to simulate the fracture behavior, and to rationalize the observed phenomena. The near-tip J-integral-based fracture criterion is used to predict the fracture path. Numerical results indicate that a variation in the morphology and distribution of α phase changes the symmetry and intensity of the near-tip stress, strain and displacement fields due to the strong mismatch in elastic–plastic properties of the α phase and eutectic phase, which have major influences on both crack growth direction and crack tip driving force.
The effects of heat treatment on the properties and microstructures of the extruded Mg-Nd-Zr alloy have been studied. The results show that the rare earth-containing phase in the extruded Mg-Nd-Zr alloy is Mg12Nd phase. After ageing treatment, plenty of Mg12Nd phase particles precipitated, and the tensile strength and proof strength increased. After quenching and ageing treatments, the grains coarsened, and the proof strength decreased significantly. Both the ageing and the ageing followed quenching decrease the plasticity of the alloy.
Magnesium is the lightest of all metals used as the basis for constructional alloys. It is this property which entices automobile manufacturers to replace denser materials, not only steels, cast irons and copper base alloys but even aluminium alloys by magnesium based alloys. The requirement to reduce the weight of car components as a result in part of the introduction of legislation limiting emission has triggered renewed interest in magnesium. The growth rate over the next 10 years has been forecast to be 7% per annum. A wider use of magnesium base alloys necessitates several parallel programs. These can be classified as alloy development, process development/improvement and design considerations. These will be discussed briefly and followed by some examples of the increasing uses of magnesium and future trends.
Regularities of the constitution and properties of the Mg alloys containing two rare-earth metals belonging to different subgroups are considered. Both rare-earth metals decrease solubility of each other in solid magnesium, but each of them changes kinetics of the precipitation of other from the supersaturated Mg solid solution. The latter effect can promote strengthening. It is explained by solubility of one rare-earth metal in the precipitates of other.
Microstructure and mechanical properties of Mg–4Y–4Sm–0.5Zr alloy during heat treatments were investigated. The eutectic phase dissolved into the matrix and there was no evident grain growth after solutionized at 798K for 8h. The alloy showed very fine scale precipitates in the grains and along the grain boundaries after ageing at 473K for 16h. And the ultimate tensile strength of 348MPa, yield strength of 217MPa and elongation of 6.9% were attained in this under-aged state. The elongation of alloy greatly decreased, and the ultimate tensile strength decreased respectively with increasing the ageing time, for increasing amount and coalescence of the precipitates along the grain boundaries. The mechanical properties of the alloy could hold up to 473K, and decreased steeply at temperatures higher than 523K, for the softening of the precipitates and activation of non-basal slip systems.
Heat treatments consisting of solution at 798K for 8h and artificial ageing at 448, 473 and 498K were carried out on Mg–4Y–4Sm–0.5Zr alloy. Characterization of phases in Mg–4Y–4Sm–0.5Zr alloy during heat treatments has been investigated, using transmission electron microscopy and thin foil energy dispersive X-ray spectroscopy. The eutectic phase with composition of Mg5(Sm0.6Y0.4) has an fcc crystal structure (a=2.326nm). The quadrate phase with an fcc crystal structure (a=0.5581nm) was found near grain-boundaries in the solution-treated state. The fine plate-shaped precipitate phase which was found to play an important role in the peak-aged state was determined as β′. The ultimate tensile strength of 337MPa, yield strength of 220MPa and elongation of 3.7% are attained in the peak-aged state.
The precipitation sequence of a Mg–2.1Gd–0.6Y–0.2Zr (at.%) alloy at 200°C was investigated by transmission electron microscopy (TEM), and the chemical compositions of the precipitates observed in different stages of the sequence were analyzed by the three-dimensional atom probe (3DAP) technique. The precipitation sequence is described as supersaturated solid solution (SSS) → β″ (D019 Mg3X) → β′ (bco Mg15X3) → β phase (Mg5X). There are structural similarities in each phase and the phase evolution occurs continuously by decreasing the rare earth content when β″ transforms to β′.
The precipitation behaviour of a binary Mg-0.5 at.% Nd alloy has been studied by electrical resistivity and electron microscope techniques. A general scheme of precipitation is proposed, and the conditions defined under which each phase precipitates. G.P. zones are shown to exist, their formation being dependent on the quenched-in vacancy population as in aluminium alloys; in addition, there are two further non-equilibrium phases, and the equilibrium precipitate. The morphology and structure of each of these phases has been investigated, and related to the good elevated temperature strength shown by this alloy.
A new flux was developed specially for recycling of scrap magnesium alloy AZ91 with high iron content. JDMJ in the flux could effectively remove inclusions from the recycled magnesium alloy and its proper addition was about 2.0 wt%. Excessive addition of the flux would result in flux inclusion in the recycled magnesium alloy. B2O3 in the flux made the iron concentration in the scrap magnesium alloy decrease from 0.044 wt% to about 0.002 wt% during recycling and its optimal addition was 0.3 wt% by Gaussian Fitting. The tensile properties of the recycled magnesium alloy were greatly improved by about 35%. Weight loss measurement, potentiodynamic study and pitting morphology examination revealed that the corrosion resistance of the recycled magnesium alloy was also greatly improved. The mechanisms of inclusion removing and iron reducing in the scrap magnesium alloy during recycling were discussed thermodynamically and formation of FeB was confirmed as the main reason for iron reducing in the recycled magnesium alloy AZ91 by XRD analysis.
The present work involves the examination of the age-hardening response at 200 °C of Mg-3 wt% Nd alloys, with systematic additions of Zn, and characterisation of precipitate microstructures in peak-aged samples of the alloys using transmission electron microscopy. It is found that, while ternary additions of 0.5 wt% Zn to Mg-3 wt% Nd alloy lead to little change in the maximum increment in hardness, further additions of Zn up to 1.35 wt% result in an appreciable reduction in the maximum hardness increment. The reduced hardening increment seems to be attributable to the formation of precipitate plates that form on the basal plane of the matrix phase.
This study on deformation mechanisms of a γ″ (DO2 structure) strengthened nickel base alloy (Inconel 718) has shown a new type of precipitate shearing mechanism. When the size of γ″ particles exceeds a critical value (~ 1onm), these precipitates are sheared by the passage of true crystallographic deformation twins which do not destroy the ordered atomic arrangements within precipitate crystals. For smaller precipitates, shearing occurs by the movement of a group of dislocations which enables restoration of order. Consequent to the change in the precipitate shearing mechanism the work hardening rate drops to a lower value (work hardening exponent changing from ~0.8 to ~0.5) as the deformation twinning mode becomes operative for precipitates with radii larger than about 10 nm. Strengthening due to precipitation has been estimated as a function of the precipitate size, corresponding to différent precipitate shearing and precipitate bypassing mechanisms and these results have been compared with the experimental data.
The structure of cast magnesium alloys (grain size and precipitate morphology and size) affects the properties of the products and the scope for use of the alloys. The structure can be controlled by minor additions of inoculants, which are largely determined on the basis of the composition of the alloy concerned. The present paper reviews the scientific background of structural refinement by inoculation and its application to Mg–Zn, Mg–Al, and Mg–Al–Si alloys.
This paper describes the crystallographic features of a platelet-shaped precipitate that forms in the early stage of age hardening of Mg alloys containing rare-earth elements in the Ce subgroup. Transmission electron microscopy combined with computer simulation revealed that the precipitate, the habit plane of which is a {1100} matrix prism plane, is morphologically analogous to Guinier-Preston zones in Al-Cu alloys, that is two dimensional, but possesses an ordered structure based on the D019 superlattice. The precipitate also contains periodically arranged atomic steps that are similar to antiphase boundaries in ordered alloys. These periodic steps result in a split of superlattice reflections similar to that observed in diffraction patterns obtained from a long-period superlattice such as CuAu II.
The precipitation sequence in a Mg–10Gd–3Y–0.4Zr (wt.%) alloy during isothermal ageing at 250 °C has been investigated using transmission electron microscopy. It is found that the precipitation sequence involves super-saturated solid solution (S.S.S.S.) → β″(D019) → β′(cbco) → β1(fcc) → β(fcc), which is similar to that of WE54 alloy but different from previously reported three-stage sequence: S.S.S.S. → β″(D019) → β′(cbco) → β (bcc). The metastable β′ phase, which plays an important role for the age hardening of the alloy, has a shape of convex lens and is very thermally stable at 250 °C. Formation of β1 phase appears to take place via an in situ transformation from a decomposed β′ phase.
Intermetallic phases can be found in almost every magnesium alloy. These intermetallic compounds play a very important role in optimizing the microstructure and mechanical properties. The present paper reviews the effects of intermetallics in magnesium alloys mainly based on their stabilities: dissolvable intermetallics at low temperatures and thermal stable intermetallics at elevated temperatures. The effects of intermetallics are discussed in the age hardenable and creep resistant magnesium alloys, separately. Finally, the further investigations are remarked on the intermetallics, including their precipitation processes, crystal structures and crystallographic orientation relations with magnesium matrix. The aim is to supply useful information in developing new wrought and creep-resistant magnesium alloys which will be used in the powertrain at elevated temperatures.
Precipitated phases in a Mg–Dy–Gd–Nd alloy during ageing were studied by using transmission electron microscopy. Observations were performed along some orientations parallel to 〈0 0 0 1〉α and/or zone axes of the matrix. Results show that the alloy has a rapid ageing response, which can be attributed to the rapid formation of β″ precipitates, occurring in three planes with DO19 structure; the decreasing of hardness may result from the forming of β1, which forms in three plates, and the orientation relationship between β1 and the matrix may be described by , 〈1 1 0〉β1//〈0 0 0 1〉α; further ageing leads to the formation of β precipitates, which also forms as plates parallel to , and the orientation relationship between β and the matrix phase is identical to that observed between β1 and matrix phase.
The microstructural evolution in a Mg–15Gd–0.5Zr (wt.%) alloy during isothermal aging at 250 °C, has been investigated using transmission electron microscopy. The decomposition of α-Mg supersaturated solid solution (S.S.S.S., cph) in the alloy with increasing aging time is as follows: β″ (D019) → β′(cbco) → β1(fcc) → β(fcc), which is similar to that of Mg–Gd–Y, Mg–Gd–Nd and Mg–Y–Nd alloys, but different from previously reported three stage sequence: S.S.S.S. → β″ (D019) → β′(cbco) → β(fcc). It is found that the metastable β″ and β′ phases coexist in the matrix at the very early stage of aging. Peak age-hardening is attributed to the precipitation of prismatic β′ plates in a triangular arrangement. At the over-aged stage, β1 phase appears to take place via an in situ transformation from a decomposed β′ phase but grows in a direction different from the previous one of β′ phase. Continued aging makes the β1 phase transform in situ to the equilibrium β phase and the orientation relationship between the precipitate and matrix phases is retained through the in situ transformation of the β1 phase.
The effect of microstructure on the deformation mode and fracture behavior of an age-hardened Al-Li alloy 8090 were investigated. The deformation behavior could be modified by varying the aging conditions. It was observed that with an increase of tensile strength, the fatigue strength increases, and the fatigue strength/tensile strength ratio also increases. The electron transmission microscope (TEM) results revealed that the deformation mode of the alloy is controlled by the interaction of dislocations and precipitates.
The Nd-Mg system was studied using differential thermal analysis (DTA), X-ray examination, metallography, and microprobe analysis.
The following intermetallic compounds were found to exist and their crystal structures confirmed or determined: NdMg (cubic,
cP2 CsCl type, melting point 800 °C), NdMg2 (cubic, cF24 MgCu2 type, peritectic formation ∼755 °C), NdMg3 (cubic, cF16 BiF3 type, melting point 780 °C), and Nd5Mg41 (tetragonal, tI92 Ce5Mg41 type, decomposes peritectically at 560 °C). The NdMg2 phase undergoes a eutectoidal decomposition at 660 °C. Three eutectic equilibria were observed to occur at 42.5 at. pct Mg
and 775 °C, 64.5 at. pct Mg and 750 °C, and 92.5 at. pct Mg and 545 °C, respectively. In the Nd-rich alloys, previously determined
data[15] concerning the Mg solubility in α-Nd (8.2 at. pct Mg, ≈550 °C) were accepted. The Mg solubility in β-Nd was evaluated as
34 at. pct Mg at 775 °C. The β-Nd phase was observed to decompose eutectoidally at 17 at. pct Mg and 545 °C. Moreover, in
the Mgrich alloys, a metastable NdMg12 phase (tetragonal, tI26 ThMn12 type) was observed in samples quenched from the liquid. The general properties of the Nd-Mg phases are compared with those
of the R-Mg compounds and briefly discussed.
The mechanical properties of Mg-Al alloys are mainly determined by the microstructure, i.e., the amount and morphology of the phases, but also by the presence of defects arising from the melt handling and casting
process. In order to obtain information about the isolated effect of the microstructure, it is, therefore, necessary to minimize
the amounts of defects. In this study, this has been achieved by remelting and solidifying the alloys in a gradient furnace.
The drawing rate was varied from 0.3 to 6 mm/s, which yielded a wide variety of microstructures. Three samples were produced
for each parameter set, in order to have a statistical basis for the evaluation. The results showed that homogeneous and reproducible
samples could be produced, and that the tensile properties obtained showed a very small scatter. The effects of microstructural
parameters such as grain size, secondary dendrite arm spacing (SDAS), eutectic fraction, and eutectic morphology on the yield
strength, ultimate tensile strength (UTS), fracture elongation, and hardness has been investigated.
Mg–8Gd–0.6Zr–xNd–yY (mass%) alloys which containing different Nd:Y mass ratio of 3:0, 2:1, 1:2 and 0:3 with a constant x + y = 3 were prepared by metal mould casting method, and the microstructure, aging behaviour and tensile properties have been investigated. The fibrous eutectic areas along the boundaries enlarge clearly in the as-cast alloys containing Y element, and the fine grain boundaries and dispersed precipitation are observed in the aged alloys. The Mg–8Gd–0.6Zr–2Nd–Y alloy exhibits notably age-hardening behaviour and the highest mechanical property. The ultimate tensile strength and yield strength of Mg–8Gd–0.6Zr–2Nd–Y alloy in the peak aged hardness are 293 and 221 MPa at room temperature, 248 and 191 MPa at 230 °C. The improvement of age-hardening response and tensile properties is mainly attributed to the quadrate-like stable Mg5RE precipitate, which forms readily and orderly in aged Mg–8Gd–0.6Zr–2Nd–Y alloy.
Microstructure and mechanical properties of Mg–10Gd–2Y–0.5Zr (wt.%) alloy in a series of tempers, including as-cast, cast-T4, cast-T6 and extruded-T5 conditions, have been investigated. The evolution of the microstructure from as-cast to cast-T4 to cast-T6 involves solid solution + eutectic compound → supersaturated solid solution + cuboid-shaped compound → solid solution + β′ precipitates + cuboid-shaped compound. Zirconium cores exist in all these conditions. Effective grain refinement is attained by hot extrusion with a small extrusion ratio (∼9.3). A good combination of high strength and sufficient ductility at room temperature is achieved for the cast-T6 alloy by optimizing the heat treatment parameters and for the extruded-T5 alloy extruded at 673 K, whose ultimate tensile strengths, tensile yield strengths and elongations are 362, 239 MPa and 4.7%, and 403, 311 MPa and 15.3%, respectively. Moreover, the strengths decrease gently from room temperature to 200 °C with a gradual increase of elongation. Existing traditional strengthening theories, together with data from microstructural characterization and mechanical properties are used to determine the magnitude of individual contribution. Strengthening due to precipitation is the largest contribution to alloy strength, either in cast-T6 condition or in extruded-T5 condition. The grain boundary strengthening also contributes significantly after hot extrusion.
Thermo-mechanical treatments consisting of hot extrusion at 573 or 673 K and artificial aging at 473 K were carried out on a Mg-4Y-3RE alloy. The material extruded at 573 K, followed by aging for 2 h, showed fine needle-like precipitates and the high density of a dislocation cell. This material exhibited a high strength of 370 MPa at room temperature. The material extruded at 673 K exhibited a very small grain size of 1.5 μm and fine spherical precipitates. This material showed not only a high strength of about 300 MPa from room temperature to 473 K, but also superplastic behavior at a high strain-rate of 4×10−1 s−1 and at 673 K. Furthermore, a good combination of high strength and high ductility at room temperature was attained for the material extruded at 673 K. These qualities can likely be attributed to the very small grain size.
Thermo-mechanical treatments, through combined action of cold or hot work and heat treatment, were performed on a Mg–6Gd–2Nd–0.5Zr alloy to investigate their effect on the microstructure and mechanical properties of the alloy. Cold deformation of 5 or 10% between homogenization and aging produced high density of twins and dislocations in as-quenched alloys, and caused acceleration in age-hardening response at 200 °C. Cold-deformed and peak-aged alloys exhibited higher strength and lower elongation than those of peak-aged cast alloy. Alloys subjected to hot extrusion at 450 or 350 °C showed very fine grains, and meanwhile maintained a considerable supersaturation in matrix, leading to a significant age-hardening response at 200 °C. Hot extrusion and aging brought about an overall improvement in mechanical properties, and the best combination of strength and ductility was achieved for alloy after extrusion at 350 °C and aging at 200 °C for 24 h, namely ultimate tensile strength = 381 MPa, tensile yield strength = 273 MPa and elongation = 17.6%.
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