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Evolution of discontinuous/continuous Al3(Sc,Zr) precipitation in Al-Mg-Mn 5083 alloy during thermomechanical process and its impact on tensile properties
Abstract
The evolution of discontinuous and continuous Al3(Sc,Zr) precipitation in an Al-Mg-MnAA5083 alloy during heat treatment and hot rolling was investigated. The results showed that, at a high Sc content (0.43 wt%), a large number of line/fan-shaped structures were formed as discontinuous Al3(Sc,Zr) precipitation during solidification, while no such discontinuous precipitation was observed when the amount of Sc added was low (0.15 wt%). During the three-step heat treatment (275 °C /12 h + 375 °C/48 h + 425 °C/12 h), two types of precipitates — Mn-bearing dispersoids and spherical Al3(Sc,Zr) precipitates — were formed as the main strengthening phases. In the high-Sc alloy, the discontinuous Al3(Sc,Zr) precipitates dissolved partially. However, the quantity of the spherical Al3(Sc,Zr) precipitates in the high-Sc alloy was much lower than that in the low-Sc alloy, which degraded its aging hardening response. During hot rolling, although the discontinuous precipitates were completely dissolved, the number density of the spherical Al3(Sc,Zr) precipitates in the high-Sc alloy was still lower than that in the low-Sc alloy. The tensile properties of the Sc-containing alloys improved significantly compared with those of the base alloy. However, the yield and ultimate tensile strengths of the high-Sc alloy were lower than those of the low-Sc alloy. This indicates that the discontinuous precipitation had a deleterious effect on the mechanical properties of the alloy.
... Conventionally, Al-Mg alloys are classified as non-heat-treatable and can only be reinforced through solid solution strengthening and work hardening [3][4][5][6], thus the attainable strength of these alloys is inherently lower than that of heattreatable aluminum alloys, limiting their further applications. It was been found that the co-addition of Sc and Zr forms Al 3 (Sc, Zr) dispersoids with a core-shell structure [7,8], significantly improving the mechanical properties [9][10][11][12] and thermal coarsening resistance [13,14] of the alloys. The formation of core-shell Al 2 (Sc, Zr) dispersoids with L1₂ structure minimizes the lattice mismatch with the α-Al matrix, resulting in a strong coherent interface [15]. ...
... However, the large eutectic phases cannot be fully dissolved by the one-stage homogenization process, adversely affecting the alloy's performance. Consequently, multi-stage homogenization is also developed for Al-Mg-Sc alloys [12,14,[19][20][21]. Algendy et al. [22] found that as three-step homogenization (430 °C/2 h + 480 °C/2 h + 525 °C/2 h) can efficiently dissolve low-melting-point eutectic phases, however, the Al 3 (Sc, Zr) dispersoids are coarsened, resulting in lower strengthening effect. ...
The evolution of Al3(Sc, Zr) and α-Al(Fe, Mn)Si dispersoids and their influence on the recrystallization behavior and mechanical properties of an Al–Mg–Sc alloy under various homogenization and annealing treatments were investigated by scanning electron microscopy, transmission electron microscopy and tensile testing. The results revealed that the one-step homogenization (OS, 350 °C/6 h) produces higher number density of Al3(Sc, Zr) and α-Al(Fe,Mn)Si dispersoids compared with the three-stage homogenized alloy (THS8, 270 °C/6 h + 350 °C/6 h + 500 °C/8 h), significantly increasing the recrystallization resistance. The one-step homogenization produces evidently higher strength but lower elongation compared to the there-step homogenization when annealing at low temperature (350 ℃/1 h). However, when annealing at 550 °C/1 h, the one-step and three-step homogenized samples exhibit similar strength, suggesting the mechanical property difference can be eliminated by high temperature annealing. The evolution of Al3(Sc, Zr) dispersoids during homogenization and annealing treatment plays a key role in determining the property differences of these samples. Although the OS and THS8 treatments produces distinct distributions of Al3(Sc, Zr) dispersoids, the dispersoids can rapidly coarsen and produce similar dispersoid distributions after annealing at high temperature (550 °C), this difference can only be retained at low temperature annealing (350 °C). Besides, dislocation strengthening is also responsible for the property difference of these alloys. These results provide new information for designing new heat treatment process of Al–Mg–Sc–Zr alloys.
... Al-Mg series alloys have been widely used in various industries, including the automotive, marine, packaging and construction industries, due to their excellent weldability, ductility, toughness, formability, and corrosion resistance [1][2][3][4][5]. However, their strength cannot be enhanced through work hardening [6], only by grain refinement [7][8][9]. ...
Standard AA5083 (ZSE000), AA5083 modified with 0.3 wt.% Zr and 0.3wt.% Sc (ZSE330) and AA5083 modified with 0.3 wt.% Zr, 0.2wt.% Sc and 0.1wt.%Er(ZSE321) sheets were fabricated through a short process (including a simulated twin-belt continuous casting, subsequent direct rolling, intermediate annealing, cold rolling and stress-relief annealing) to systematically investigate the influence of partially substituting Er for Sc on the microstructure, mechanical properties and corrosion resistance of short-processed Al-4.7Mg-0.6Mn-0.3Zr-0.3Sc sheets. The results show that ZSE321 presents the optimal tensile properties (UTS: 541 MPa; 0.2%PS: 469 MPa and EF:7.7%) among the three experimental sheets. This is attributed to significant grain refinement, the inhibition of the recrystallization and promotion on the precipitation of Al3(Sc, Zr, Er) nanoparticles. Furthermore, the corrosion properties of the experimental sheets were also explored in this study, and the short-processed ZSE321 sheet presents the optimum corrosion resistance.
... Our previous studies [10,19] found that adding 0.08% Sc moderately improved the mechanical properties of AA5083 hot-rolled sheets, whereas the addition of 0.15% Sc significantly increased the tensile properties owing to the precipitation of an increased number density of L1 2 -Al 3 (Sc,Zr) nanoparticles. However, further increasing the Sc to 0.43% decreased the tensile properties, mainly because of the negative effect of discontinuous precipitation of L1 2 -Al 3 (Sc,Zr) [25,26]. ...
The mechanical properties, corrosion behavior, and related microstructures of Al–Mg-Mn AA5083 rolled sheets microalloyed with Sc and Zr were investigated under various thermomechanical processing conditions. The results revealed that adding 0.05 wt.% Sc resulted in a minor change in the alloy performance relative to the base alloy. When the Sc content reached 0.1 wt.% it significantly improved the alloy strength and recrystallization resistance, and dramatically lowered the intergranular corrosion (IGC) susceptibility. Compared with the base alloy, the 0.1 wt.% Sc addition (along with 0.08 wt.% Zr) increased the yield strength (YS) by 8, 75, 25, and 33% for the cold-rolled (H18-), annealed (O-), extra-deformed (H116-), and stabilized (H321-tempers) conditions, respectively. In addition, the recovery and recrystallization resistances were significantly improved, as demonstrated by the retained deformed fibrous structure in the O- and H321-tempers. Furthermore, the IGC susceptibility was substantially decreased in the H321-temper due to the prevention of continuous β-Al3Mg2 precipitation along recrystallized grain boundaries. The YS of the final rolled sheets was modeled using constitutive analysis to predict the effects of various microstructural components on the strengthening contribution.
Graphical Abstract
... Rolling temperatures higher than the precipitation temperature may accelerate coarsening of dispersed particles, hence lowering their strengthening effect and recrystallization resistance. Our previous study [11,29] has shown that the L1 2 -Al 3 (Sc,Zr) and Mn-dispersoids in AA5083 progressively coarsened during hot rolling above 500 • C, and has suggested that lowering the hot rolling temperature could improve the alloy strength. ...
The impact of hot rolling temperature on the microstructure evolution and mechanical properties of hot/cold rolled AA5083 alloy containing Sc and Zr was studied. The results revealed that low hot-rolling temperatures (LHRT: 400–425oC) resulted in superior tensile properties compared to high hot-rolling temperatures (HHRT: 500–525 oC). The yield strength (YS) of the LHRT samples reached 450 MPa in the H18-temper and 291 MPa in the O-temper, which were respectively 20% and 39% higher relative to the HHRT samples. Microalloying with Sc/Zr and two-step homogenization promoted the formation of dispersed particles in the form of Mn dispersoids and nanosized Al3(Sc,Zr) precipitates. The rolling temperature was found to have a profound impact on the characteristics of these dispersed particles and on the recrystallization resistance. The HHRT caused the coarsening of both dispersed particles during rolling. The LHRT preserved the deformed elongated grain structure due to a stronger retardation of recovery and recrystallization during hot rolling and post-annealing. The strengthening mechanisms of the Mn-dispersoids and Al3(Sc,Zr) precipitates were quantitatively analyzed based on particle characteristics. The predicted YS contributions in the O-temper were in good agreement with the measured YS difference between the LHRT and HHRT samples.
... Модифицирующее воздействие, как правило, обусловлено образованием интерметаллических соединений (ИМС) типа Al 3 РЗМ, которые при осаждении в виде мелких частиц (в том числе наноразмерных) и равномерном распределении в объеме сплава могут значительно улучшить его свойства. При этом основополагающим в процессе модифицирования является наименьший размер ИМС, а также когерентность с алюминиевой матрицей [10]. В последние годы самым используемым модификатором алюминиевых сплавов из группы РЗМ является скандий [11][12][13]. ...
... In previous research, the primary reason for the impact of Sc and Zr microalloying on the microstructure and properties of aluminum alloys is the formation of Al 3 (Sc x Zr 1−x ) dispersoids [1][2][3][4]. This nanoscale dispersoid is precipitated during the homogenization treatment of Al-Sc-Zr alloy ingots due to the supersaturated solid solution decomposition [5]. ...
Artificial neural networks (ANNs) were established for the homogenization and recrystallization heat treatment processes of 5182-Sc-Zr alloy. Microhardness and conductivity testing were utilized to determine the precipitation state of Al3(ScxZr1−x) dispersoids during the homogenization treatment, while electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were used to observe the microstructure evolution of the alloy. Tensile experiments were performed to test the mechanical properties of the alloy after recrystallization annealing. The two-stage homogenization parameters were determined by studying the changes in microhardness and electrical conductivity of 5182-Sc-Zr alloy after homogenization with the assistance of artificial neural networks: the first-stage homogenization at 275 °C for 20 h and the second-stage homogenization at 440 °C for 12 h. The dispersoids had entirely precipitated after homogenization, and the alloy segregation had improved. A high-accuracy prediction model, incorporating multiple influencing factors through artificial neural networks, was successfully established to predict the mechanical properties of the 5182-Sc-Zr alloy after annealing. Based on the atomic plane spacing in HRTEM, it was determined that the Al3(ScxZr1−x) dispersoids and the Al matrix maintained a good coherence relationship after annealing at 400 °C.
... Dispersoids pin grain boundaries and inhibit grain growth. This approach is realized for 5000-type [27][28][29][30], 6000-type [31][32][33][34], 7000-type [35][36][37], and 2000-type [38,39] alloys. Thermomechanical treatment should provide a uniform distribution of the coarse particles in the aluminum matrix and a high number density of nanoscale-sized dispersoids. ...
This study focused on the microstructural analysis, superplasticity, modeling of superplastic deformation behavior, and superplastic forming tests of the Al-Mg-Si-Cu-based alloy modified with Fe, Ni, Sc, and Zr. The effect of the thermomechanical treatment with various proportions of hot/cold rolling degrees on the secondary particle distribution and deformation behavior was studied. The increase in hot rolling degree increased the homogeneity of the particle distribution in the aluminum-based solid solution that improved superplastic properties, providing an elongation of~470-500% at increased strain rates of (0.5-1) × 10 −2 s −1. A constitutive model based on Arrhenius and Beckofen equations was used to describe and predict the superplastic flow behavior of the alloy studied. Model complex-shaped parts were processed by superplastic forming at two strain rates. The proposed strain rate of 1 × 10 −2 s −1 provided a low thickness variation and a high quality of the experimental parts. The residual cavitation after superplastic forming was also large at the low strain rate of 2 × 10 −3 s −1 and significantly smaller at 1 × 10 −2 s −1. Coarse Al 9 FeNi particles did not stimulate the cavitation process and were effective to provide the superplasticity of alloys studied at high strain rates, whereas cavities were predominately observed near coarse Mg 2 Si particles, which act as nucleation places for cavities during superplastic deformation and forming.
This research examines the influence of Zr additions on the microstructure and properties characteristics of as-cast Al-6.5Mg-0.50Mn-0.25Sc alloys. The casting microstructure evolution, primary phase, mechanical properties, and the fracture morphology of alloys were characterized by optical microscopy (OM), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), Vickers hardness test and universal electronic testing machine. The conclusion reveals that incorporating Zr in the casting Al-6.5Mg-0.50Mn-0.25Sc alloy has an obvious effect on both the microstructure and comprehensive performance. This positive effect is primarily due to the Al3(Sc, Zr) primary phase, which serves as a heterogeneous nucleation agent during solidification, thereby contributing to grain refinement and the second-phase strengthening, specifically, at 0.10 wt.% Zr; as-cast alloy achieves the smallest average crystal size of 63 μm and exhibits the best combination of performance. The ultimate tensile strength (UTS), yield strength (YS), elongation (EL), and microhardness of the alloy reached 327 MPa, 190 MPa, 12%, and 107 HV, respectively.
Hot rolling, cold rolling, solution treatment-natural aging, and solution treatment-artificial aging were used to process Al-7 Mg-xZn-0.25Sc alloys (x = 0, 1wt.%, 2wt.%, and 3wt.%). The grain size of these alloys after deformation or heat treatment decreased with increased Zn. Dynamic and static precipitations of T′/T occurred in the alloys with Zn content during deformation and heat treatment, respectively, and the volume fraction of the precipitated phases in the Al-7 Mg-xZn-0.25Sc alloys increased with increased Zn content. The strength of the alloys also increased with increased Zn content at all states, and the yield strength and ultimate tensile strength of the alloy with 3% Zn content after cold rolling reached 403 and 567 MPa, respectively. Compared with the rolled alloys, the heat-treated alloys exhibited much higher ductility due to their higher numbers of Mg solution atoms, and the elongation of all T4 alloys exceeded 30%. The lack of pinning points for the movable dislocations and the high-volume fraction of T phases caused the high Zn content alloy after hot rolling to exhibit higher room-temperature internal friction values than the other alloys. However, the high-Zn concentration Al-7 Mg-xZn-0.25Sc alloy exhibited weak corrosion resistance due to the formation of T grain boundary phases, which increased the electrochemical imbalance.
In order to develop the aluminum sheets suitable for the twin-belt continuous casting and direct rolling, the composition optimization of Al-Mg-Mn alloys evolved from AA5052 (Al-2.5 Mg) alloy was firstly carried out via the synergy between the PANDAT software and the micro–hardness test. Subsequently, the influence of Er and Zr on the microstructure and mechanical properties of the optimized Al-Mg-Mn sheet (Al-1.5 Mg-0.8Mn) was investigated in this work. The results showed that the tensile properties of Al-1.5 Mg-0.8Mn sheets processed by the rolling and the subsequent annealing were improved by the co-addition of the Zr and Er. The optimized tensile properties (UTS: 320.5 MPa; YS: 231.6 MPa; E.L.: 6.75%) were achieved in Al-1.5 Mg-0.8Mn sheets modified by 0.2 wt.% Zr and 0.3 wt.% Er, which is attributed to the finest fibrous grains, the fine, homogeneously distributed residual Al6Mn, Al3Zr and Al3(Zr, Er) second phases and more nano-sized Al3(Zr, Er) precipitations.
In this study, the effect of Sc content on the microstructural characteristics and phase evolution of an Al–Cu–Li alloy under as-cast and homogenization conditions was investigated. The results show the grain refinement effect was distinct when the content of Sc was 0.16 wt%. However, further increased in Sc content resulted in a limited decrease in grain size. Besides, In the 0.16Sc alloy, the flocculent phase with primary Al3(Sc,Zr) phase coexisting with W (Al8-xCu4+xSc) phase appeared, and in the 0.32Sc alloy, the additional blocky primary Al3(Sc,Zr) phase was observed. With the increased of Sc content, the proportion of primary Al3(Sc,Zr) and W phases gradually increased, and the number and size of Al2Cu and Ag-containing Al2CuMg phases at grain boundaries gradually decreased. Two different morphologies of the primary W phase, flocculent and long strip shaped, were revealed within the Sc-containing alloy. The formation of the core-shell structure formed by Al3(Sc,Zr) particles and W phase was generated by the peritectic-eutectic reaction (Liquid + Al3(Sc,Zr)→α-Al + W) and the long strip of W phase was generated by the eutectic reaction (Liquid→α-Al + Al2Cu + W). During the homogenization process, the Al2Cu and Ag-containing Al2CuMg phases were completely re-dissolved after the two previous stages of homogenization, and only a small amount of the spheroidal residual W phase persisted. A subsequent low temperature treatment was employed to allow the Sc atoms that had diffused into the Al matrix to precipitate as Al3(Sc,Zr) dispersoids, resulting in a higher number density of Al3(Sc,Zr) particles for more efficiently utilizing the Sc elements.
5xxx5xxxaluminum alloysAluminum alloys are traditionally considered non-heat-treatable. With the addition of Sc/Zr and multistep heat treatmentHeat treatment, two kinds of dispersoidsDispersoids (AlMn and Al3(Sc,Zr)) were formed. The effect of Sc additions (0.08–0.43 wt.%) on dispersoidDispersoids formation and mechanical propertiesMechanical properties of hot-rolled sheets was investigated. The results showed that tensile properties initially increased with increasing Sc addition. The yield strength (YS) and ultimate tensile strengthTensile strength (UTS) of the alloy with 0.16 wt.% Sc reached 295 and 411 MPa, respectively, showing improvementsImprovement of 28% in YS and 8% in UTS compared to the base alloy. However, with a further increase of Sc, the tensile properties declined owing to the formation of a line/fan-shaped microstructureMicrostructure associated with discontinuous Al3(Sc,Zr) precipitationPrecipitation during solidificationSolidification. The evolution of Al3(Sc,Zr) and AlMn dispersoidsDispersoids during heat treatmentHeat treatment and hot rollingHot rolling was characterized using scanning and transmissionTransmission electron microscopies. Their influence on the mechanical propertiesMechanical properties of hot-rolled AA5083 alloys was discussed.
The properties of 5xxx5xxxaluminum alloysAluminum alloys can be improved with small additions of Sc. When Sc and Zr are added to 5xxx alloys, the alloys become heat-treatable as Al3(Sc,Zr) nanoprecipitates form at 300–400 °C. However, the heat treatmentHeat treatment and thermo-mechanical processingProcessing (TMP) need to be adapted to maximize the value of the Sc addition. In this work, AA5083 slabs were DC cast with and without minor additions of Sc and/or Zr. The room/elevated temperature mechanical propertiesMechanical properties of the cast materials were evaluated after various annealingAnnealing and homogenizationHomogenization treatments. Samples were then hot and cold rolled with different TMP treatments. The microstructureMicrostructure, room temperature mechanical propertiesMechanical properties and corrosionCorrosion performance were evaluated for selected tempers. It was evident that the properties could be improved significantly with small additions of Sc, but that this depends very much on the amount of Sc and on the processingProcessing parameters.
The precipitation behavior of dispersoids in an Al-5% Mg-0.8% Mn alloy after various single-and multistep heat treatments and their effect on rolling performance were investigated. The results show a significant increase in the electrical conductivity and microhardness caused by the precipitation of submicron dispersoids after all heat treatments. In addition, the multistep heat treatments resulted in higher microhardness and a larger fraction of dispersoid zones than single-step treatments. Two types of Mn-bearing dispersoids with cube- and rod-like morphologies were identified. The low-temperature two-step heat treatment (275 °C/12 h + 375 °C /48 h) produced the highest number density of dispersoids with the finest size among all the studied heat treatments but severe defects were created during hot-rolling. The three-step heat treatment (275 °C/12 h + 375 °C/48 h + 500 °C/4 h) provided the best combination of dispersoid characteristics and rolling performance. The mechanical properties of rolled sheets subjected to the three-step heat treatment were improved compared to those treated using industrial homogenization treatment, owing to the higher number density and finer size of the dispersoids.
The nucleation and growth process of Al3Sc precipitates in Al-0.55 wt% Sc alloy during isothermal aging was studied by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). It is found at the early aging stage that the solute Sc atoms are enriched on the stacking fault in the matrix then rapidly converted to L12-ordered Al3Sc precipitates without the formation of other transition phases. The matrix and precipitates form a sharp interface at the beginning of phase transition and maintain a completely coherent crystallographic orientation relationship. Due to the influence of the anisotropy of interface energy, the equilibrium shape of the precipitated phase is similar to the spherical polyhedron. In the coarsening stage of precipitates, the interface of α-Al/Al3Sc gradually changes from coherent to semi-coherent, resulting in the gradual loss of symmetry of the non-contrast line, and finally the stress field contrast on the surface of particles and interface dislocations gradually appear.
Sc-containing Al alloys have excellent mechanical properties, which provide the foundation for their applications. The precipitation of the Al3Sc/Al3(Sc, Zr) is generally considered to be a key factor for the performance boost. However, the performances of the alloy in different production batches often exhibit unstable and fluctuation, which may be related to the heterogeneous of scandium. Herein, we investigated the precipitation behaviors of Al3Sc/Al3(Sc, Zr) phases in the Al-0.5Sc and Al-0.5Sc-0.25Zr alloys at 340 °C. Results shown that the high scandium‑zirconium ratio not only lead to the discontinuous precipitation, but also contribute to the abnormal continuous Al3(Sc, Zr) precipitation in Al-0.5Sc-0.25Zr alloy. The morphology of abnormal Al3(Sc, Zr) is similar to the common secondary Al3(Sc, Zr) phase with a coffee-bean like morphology. With the aid of Three-Dimensional Atomic Probe (3DAP), the core-shell structures of Al3(Sc, Zr) phases are quantitatively analyzed. In the shell layer of abnormal Al3(Sc, Zr) phase, the atomic concentration ratio of Sc to Zr is much higher than the common continuous precipitated Al3(Sc, Zr) phase. The abnormal Al3(Sc, Zr) phase may be one of the key factors for the structure heredity of Sc-containing Al alloys.
The study compares the precipitation kinetics of the L12-structured phase, microstructure, and mechanical properties in Al-Mg-Zr and Al-Mg-Sc-Zr alloys with a close atomic concentration of dispersoid-forming elements. Continuous and discontinuous precipitation mechanisms leading to spherical nanoscale and fan-shaped precipitates are revealed in both alloys. Scandium raises the precipitation kinetics of L12-structured dispersoids owing to its high diffusivity in Al. The low effect of two-stage annealing on precipitation and mechanical properties is observed in the Sc-bearing alloy compared to the Sc-free one. Due to heat treatment optimization, the Zr-bearing alloy exhibits higher yield strength as compared with the alloy containing both Sc and Zr. The Sc-free alloy requires a long-term two-step heat treatment to provide increased precipitates number density and high-strength values.
The recrystallization behaviour and Al3(Sc, Zr) particles were studied by annealing the samples with different deformation ways. Two samples were studied, cold rolling (CR) and equal channel angular pressing (ECAP). The ECAP sample has higher dislocation density, leading to the results that the ECAP sample is recrystallized thoroughly whereas the CR sample is only recrystallized partly. Additionally, the morphology of Al3(Sc, Zr) particles are changed after the recrystallization. Almost all the Al3(Sc, Zr) particles in ECAP sample are incoherent and large, losing pinning effect, but those in the CR sample just grow up a bit and pin the grain boundaries (GBs) strongly.
We explain the critical effect of casting processing route (i.e., the solidification cooling rate) on the kinetics of Sc–Zr precipitation upon solidification and, subsequently, on the age-hardening response of cold-rolled/aged Al–3Mg-0.36Sc-0.14Zr sheets. The evolution mechanisms of various types of Al3(Sc,Zr) precipitates are investigated in the as-cast specimens (fabricated via strip casting vs. mold casting techniques), as well as in the subsequently cold-rolled/aged sheet specimens. The Al3(Sc,Zr) precipitates appeared to have formed upon either discontinuous or continuous precipitation during cooling from solidification temperatures; the former leads to the formation of lamellar Al3(Sc,Zr) cells while the latter results in the formation of spherical precipitates, all of which possessing an L12 crystal structure and a coherent interface with the α-Al matrix. It is shown that slower cooling rates via mold casting lead to the occurrence of both continuous and discontinuous precipitation upon cooling from the solidification temperature. In contrast, higher cooling rates via strip casting results in the apparent prevention of all discontinuous precipitation reactions as well as in the suppression of continuous precipitation. The overall effect is the greater preservation of Sc supersaturation within the α-Al matrix of the as-cast specimens fabricated via strip casting which, subsequently, leads to higher precipitation kinetics and thus higher tensile properties upon age-hardening of the cold-rolled sheets.
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An Al–4Cu–Sc alloy bearing the strengthening ternary W- (Al, Cu, Sc) phase has been successfully prepared by molten salt electrolysis (MSE) assisted by ultrasound. Modification of the morphology at W- (Al, Cu, Sc) ternary phase and mechanical properties of the Al–4Cu–Sc alloy are comprehensively studied in this work. Without the presence of ultrasound, a quantity of large-clustered eutectic W-phase precipitate around grain boundaries, accompanied by a small amount of bulk peritectic W-phase nucleating on the substrate of pre-existed Al3Sc. Both the eutectic and peritectic W-phase get significant refinement and dispersion in the presence of ultrasound. The size of W-phase cluster reduces from length ∼150 μm and width ∼30 μm down to diameter about 30 μm. The refining mechanism of W-phase should be attributed to the enhanced nucleation rate for pre-existed Al3Sc as well as the uniform distribution of solute in the presence of ultrasound. The values of Vickers hardness and yield strength of the alloy prepared by synergetic ultrasound assisted electrolysis and solidification process are about 68 H V and 157 MPa, approximately 36% and 67% higher than those of the MSE alloy. It should be due to the ultrasound-enhanced refinement and dispersion of fine W-phase, serving as strengthening phase.
Erbium (Er) is a promising element to replace expensive Scandium (Sc) to improve the mechanical strength of aluminum-based alloys due to the formation of nanoscale dispersoids. However, the effect of Er on precipitation behavior and properties of such alloys is not fully investigated. The paper studied the Er effect on precipitation kinetics of the L12 structured phase by hardness measurements and electron microscopy techniques. Compact continuously-formed dispersoids, which contained similar Zirconium (Zr) and Er concentrations, nucleated homogeneously in a grain body and heterogeneously on dislocations and grain boundaries. Fan-shaped aggregations of the L12 structured Zr-rich Al3(Er,Zr) phase discontinuously precipitated near the grain boundaries. It was found that a two-stage treatment mode provided a maximum hardening effect due to high density of continuously-formed precipitates. The effect of Er and the annealing regimes on the precipitation behavior of the Al-3Mg-0.25Er-0.25Zr alloy were discussed. Mechanical properties, corrosion behavior, and superplasticity of the sheets exposed to simple thermo-mechanical treatments were studied.
Discontinuous reaction of nano-Al3Sc particles in Al-Sc alloys is investigated in detail, with a goal of developing high-level Sc-containing Al alloys used for aerospace and aeronautics industry. This reaction is of interest because it generally has a deleterious effect on the mechanical property of materials. Requirements for the occurrence of discontinuous precipitation in this Al-Sc alloy are distinct from the prior investigations of other Al-Sc alloys. It was found that discontinuous precipitation of nano-Al3Sc particles occurred widely when the alloys were aged at the intermediate temperature ranges of 300–400∘C, while did not occur at relatively low (250∘C) and high (500∘C) temperatures. The discontinuous precipitation of nano-Al3Sc particles can be suppressed by continuous precipitation through controlling the heat treatment process, which helps to control the microstructures of the Al-Sc alloys. Furthermore, the effects of both continuous precipitation and discontinuous precipitation of nano-Al3Sc particles on the hardness of Al-Sc alloys were discussed. Finally, the morphologies of these nano-Al3Sc particles were also analyzed.
Sc and Zr were added to Al-Mn-Mg 3004 alloy to form two populations of strengthening particles (50–70 nm-sized α-Al(Mn,Fe)Si dispersoids and 6–8 nm-sized Al3(Sc,Zr) precipitates), and their strengthening effects on the mechanical properties and creep resistance at ambient and elevated temperatures were studied. The results showed that the microhardness and yield strength at ambient temperature greatly increased upon the addition of Sc and Zr. The creep resistance at 300 °C significantly improved due to the precipitation of fine Al3(Sc,Zr) particles and reduction of the particle-free zone. However, the yield strength at 300 °C remained constant even though the Sc and Zr content increased. The combined effects of α-Al(Mn,Fe)Si dispersoids and Al3(Sc,Zr) precipitates on the yield strengths at 25 °C and 300 °C were quantitatively analyzed based on the Orowan bypass and dislocation climb mechanisms. The analytically predicted yield strengths are in good agreement with the experimental data.
Increasing the recrystallization resistance and the mechanical properties of aluminum-based alloys is possible due to the formation of a nanoscale L12-structured Al3Zr phase. Treatment conditions and alloys composition affect the size and density of dispersoids and their final properties. In this work we analyze the decomposition of the supersaturated solid solution in the as-cast Al-3%Mg-0.25%Zr alloy for different annealing modes to understand the precipitation kinetics of the Al3Zr phase in the presence of Mg. We found that both discontinuous and continuous precipitation mechanisms of the Al3Zr phase are possible in the studied low-alloyed material. One-step annealing leads to the formation of coarse (17 nm) spherical precipitates of a coherent L12-structured Al3Zr phase and discontinuously formed fan-shaped aggregations of the same phase. Two-step annealing provided for the maximum precipitation hardening with the formation of high-density nanoscale (7 nm) dispersoids of the Al3Zr phase. This study highlights the importance of the annealing mode of the as-cast material for achieving a high density of the fine L12 structured Al3Zr phase and the maximum hardening effect.
Mechanical properties, exfoliation corrosion behavior and microstructure of Al-5.98Mg-0.47Mn and Al-6.01Mg-0.45Mn-0.25Sc-0.10Zr (wt%) alloy sheets under various homogenizing and annealing processes were investigated comparatively by tensile tests, electrochemical measurements, X-ray diffraction technique and microscopy methods. The as-cast alloys mainly consist of Fe and Mn enriched impurity phases, Mg and Mn enriched non-equilibrium aluminides and Mg3Al2 phases. During homogenization treatment, solvable intermetallics firstly precipitate and then dissolve into matrix. The optimized homogenization processes for removing micro-segregation and obtaining maximum precipitation strengthening of secondary Al3(Sc, Zr) particles are 440 °C×8 h and 300 °C×8 h, respectively. Sc and Zr additions can make the yield strength of Al-Mg-Mn alloy increase by 21 MPa (6.9%), 120 MPa (61.2%) and 127 MPa (68.3%), when annealed at 270 °C, 300 °C and 330 °C, respectively, indicating that Orowan precipitation strengthening caused by secondary Al3(Sc, Zr) nano-particles is much greater than grain boundary strengthening from primary Al3(Sc, Zr) micro-particles. Increasing homogenization and annealing degrees and adding Sc and Zr all can decrease corrosion current density and improve exfoliation corrosion resistance. The exfoliation corrosion behavior is dominant by anodic dissolution occurring at the interface between intermetallics and a{script}(Al) matrix. After homogenizing at 440 °C for 8 h and annealing at 300 °C for 1 h, yield strength, ultimate strength, elongation to failure and exfoliation corrosion rank are 196 MPa, 360 MPa, 20.2% and PA (slight pitting corrosion) in Al-Mg-Mn alloy, and reach to 316 MPa, 440 MPa, 17.0% and PA in Al-Mg-Mn-Sc-Zr alloy, respectively, revealing that high strength, high ductility and admirable corrosion resistance of Al-Mg-Mn alloys can be achieved by the synergetic effects of Sc and Zr microalloying and heat treatment.
The effect of Sc addition on microstructure and mechanical properties of as-cast Al–Mg alloys was investigated based on optical microscopy (OM), scanning electron microscopy (SEM), electron back-scatter diffraction (EBSD) and properties testing. The results showed that a remarkable grain refining effect was observed for Sc content greater than 0.4 wt.%, changing typical dendritic microstructure into fine equiaxed grains. Such effect was found to be related to the formed primary particles, which hindered the undesirable diffusion and subsequent crystal growth and acted as potent heterogeneous nucleation sites for α-Al matrix during solidification. In addition, these particles could cause a significant refinement strengthening effect. Atomic force microscopy (AFM) and SEM-elemental mapping were employed to investigate the internal structure and elemental distribution of these particles. They could be identified as the eutectics consisted of Al3Sc and α-Al phases rather than individual Al3Sc phase, which exhibited an ‘Al3Sc + α-Al + Al3Sc ⋯’ multilayer structure.
In this study the precipitation behavior of an Al–Mg–0.4Sc–0.15Zr alloy produced by strip casting (FLEXCAST) technology at Novelis is investigated. As-cast, as well as different cold rolled specimens, were heat treated in a single-stage aging process, and a double-stage aging process was examined. Conventional mechanical properties and microstructural measurements were performed on both the as-cast and processed materials. In addition, TEM studies were performed at different stages of heat treatment. The mechanical properties results show that peak aging at 300 °C is dependent on the prior amount of cold rolling, hence the dominant effect is recovery especially at the short period of aging. The results also indicate that even at the relatively long time of aging, i.e. 200 h, the structure is not dominated by the formation of precipitates. Observations indicate that only at a relatively high aging temperature, i.e. 450 °C, substantial amounts of non-coherent precipitates are present in the microstructure.
During the homogenisation treatment of Al alloys the Al3Zr phase is known to form heterogeneously in interdendritic areas where the Zr supersaturation is low. Several types of clusters were observed in the present Al–Cu–Li alloy. Although some clusters resemble the shape of the θ′-Al2Cu lath-shaped particles, it is explained here that there is no direct nucleation on these particles and neither is Zr contained in them at amounts detectable via TEM-EDX. These planar arrays of Zr dispersoids were established to form via repeated precipitation on dislocations. Nucleation on dislocations was the dominant mechanism for individual Al3Zr dispersoids also in the dendrite centre. This was explained on the grounds of the large atomic size misfit between Zr and the Al matrix which leads to segregation of the former atoms to dislocations and was verified experimentally by EDX. It is noteworthy that although Zr did not interact with the θ′ phase, it did so with the equilibrium θ phase and produced two different types of particles, one containing only Zr, and another having both Zr and Mn. It was also seen to be contained within Al20Cu2Mn3 dispersoids in agreement with previous findings.
Two experimental Al-15 vol.% B4C composites, S40 with 0.4 wt.% Sc and SZ40 with 0.4 wt.% Sc plus 0.24 wt.% Zr, were prepared to examine the mechanical properties during long holding periods and the age hardening responses at elevated temperatures. Alloying with Sc or Sc and Zr displayed a significant precipitation strengthening in Al–B4C composites. Results showed that maximum peak hardness with a long plateau was achieved at 300–350 °C for the SZ40 composite, while the peak aging with a maximum hardness was reached at 300 °C for the S40 composite. The addition of Zr in the composite containing Sc delayed the overaging by ∼100 °C. The mechanical properties of SZ40 were considerably stable after 2000 h annealing at 300 °C, while the S40 composite exhibited a good long-term thermal stability at 250 °C. At all temperatures investigated, the Zr addition enhanced the strengthening effect and the mechanical properties of the SZ40 were higher than those of the S40 composite. The reduction of the mechanical properties of composites during prolonged annealing at higher temperatures was dominated by the precipitate coarsening.
The morphology of discontinuous precipitation of the Ïâ² phase is investigated in very high Li concentration Al-Li-Zr alloys using transmission electron microscopy. Contrary to previous observations, the reaction was found to initiate during quenching following solution heat treatment. Furthermore, the reaction was found to be very sensitive to the cooling rate from the solution treatment temperature since this could change the solute distribution in the vicinity of a grain boundary. It is demonstrated that the initiation of discontinuous precipitation in Al-Li alloys can not be explained in terms of recrystallization pressures that the grain boundaries are repelled by developing discontinuous Ïâ² colonies leading to gross distortion of the grain boundaries. The reaction mechanisms involved, and existing and new models for the discontinuous precipitation of Ïâ², are discussed in the light of these observations.
Al–0.3Sc–0.15Zr alloy was cast using copper die, insulated alumina mould, and conventional investment shell mould to obtain a wide range of cooling rates. A novel method of quenching the investment shell mould along with the liquid metal in oil was also used which resulted in a significant increase in the cooling rate. The order in increasing average cooling rate is 0.16, 0.78, 1.28, 5.93, 7.69°C/s. The as-cast samples were aged isothermally at 300°C and various temperatures for 2h. Slow cooled samples (in alumina-insulated mould) showed the presence of as-cast primary precipitates as well as rod shaped discontinuous precipitates with high density of interfacial dislocation. The amount of as-cast precipitates decreased with increase in the cooling rate. These as-cast precipitates grew at the expense of Sc in solid solution reducing the number of precipitates formed during ageing process. This results in lower increment in hardness on ageing.
Aluminium-rich alloys from the Al-Sc system were examined to determine the form of the equilibrium phase diagram and to obtain information on age hardening of chill cast alloys. Samples containing up to 8.75wt% Sc were examined using thermal analysis and optical microscopy. This work indicated a eutectic type of phase diagram with a eutectic temperature of about 665° C and a eutectic composition of about 0.6wt% Sc. The scandium-rich primary phase was found to be ScAl3 which is f c c with a lattice parameter of 0.4105nm. Chill cast samples of a 1 wt% Sc alloy were examined for their age hardening behaviour over the temperature range of 225 to 360° C. A maximum hardness of 77 VHN was obtained after ageing at 250° C for 3 days. This hardness was retained after ageing for a total of at least 12 days. The hardening precipitates were ScAl3 which were observed to form via a discontinuous precipitation mechanism. The ScAl3 precipitates were observed to have a parallel orientation relationship with the matrix.
The analysis of convergent-beam electron diffraction patterns for foil thickness is easily carried out in the two-beam approximation to dynamical diffraction theory. The effects of this approximation on the accuracy of foil thickness determinations are considered, as well as the range of thicknesses which can be measured with the technique. Inaccuracies due to multiple-beam effects are minimized by avoiding diffracting conditions involving low-order reflections. For 100 keV electron energy, errors of less than 2% in thickness determination are possible in Al, Cu and Au, if the diffraction vector g is equal to or larger than 200, 220 or 311 respectively. The minimum thickness measurable for these materials diminishes as |g| increases, but is approximately 20 nm for the lower-order reflections. The maximum thickness measurable is limited by absorption, and is estimated for Al, Cu and Au to be approximately 600, 130 and 40 nm respectively.
A considerable part of the available literature on scandium in aluminium alloys is reviewed. Experimental data and assessments of the binary Al–Sc phase diagram, a wide range of ternary Al–Sc–X phase diagrams and a few higher order phase diagrams are accounted for, with emphasis on the aluminium rich part of the diagrams. The phase which is in equilibrium with Al, Al3Sc, can form by several different mechanisms, all of which are described. The precipitation kinetics of Al3Sc in binary Al–Sc alloys are discussed, and an overview of the reported influences of ternary alloying elements on the precipitation of Al3Sc is given. The Al3Sc phase particles can serve as a grain refiner in the Al melt, a dispersoid for controlling the grain structure of the alloy and a strengthening precipitate. Several examples of these three effects are mentioned, both in binary Al–Sc alloys, and in more complex alloys. The reported effects of Sc on the precipitation behaviour in Al–Cu, Al–Mg–Si, Al–Zn–Mg and Al–Li alloys are also revised. A brief account of the effects of Sc additions on the corrosion behaviour of Al and Al-alloys is given. Finally, some views on the current and future use of Sc-containing Al alloys are given.
Discontinuous precipitation results in the formation of a two-phase lamellar structure behind moving grain-boundaries. The reaction is of interest because it generally has a deleterious effect on the mechanical, physical, and chemical properties of many commercial alloys. The combination of heterogeneous boundary precipitation and concurrent migration is complex and has resulted in a diverse range of proposed reaction mechanisms, several models of the growth kinetics, and many empirical observations of the effects of ternary additions and lattice strain. This review summarizes existing theories and attempts to reconcile them with the latest experimental data for reactions which take place from supersaturated solid solutions (eutectoid decomposition is specifically excluded). It is considered that fuller understanding requires increased application of the latest electron -optical techniques such as high-resolution microanalysis and in situ experiments in the high-voltage microscope. Furthermore, to understand the effects of grain-boundary structure on initiation and growth kinetics “investigations should be confined to well characterized boundaries, such as bicrystals.
Al-Sc and Al-Sc-Zr alloys containing 0.05, 0.1 and 0.5 wt.% Sc and 0.15 wt.% Zr were investigated using optical microscopy, electron microscopy and X-ray diffraction. The phase composition of the alloys and the morphology of precipitates that developed during solidification in the sand casting process and subsequent thermal treatment of the samples were studied. XRD analysis shows that the weight percentage of the Al(3)Sc/Al(3)(Sc, Zr) precipitates was significantly below 1% in all alloys except for the virgin Al0.5Sc0.15Zr alloy. in this alloy the precipitates were observed as primary dendritic particles. In the binary Al-Sc alloys, ageing at 470 degrees C for 24 h produced precipitates associated with dislocation networks, whereas the precipitates in the annealed Al-Sc-Zr alloys were free of interfacial dislocations except at the lowest content of Sc. Development of large incoherent precipitates during precipitation heat treatment reduced hardness of all the alloys studied. Growth of the Al(3)Sc/Al(3)(Sc, Zr) precipitates after heat treatment was less at low Sc content and in the presence of Zr. Increase in hardness was observed after heat treatment at 300 degrees C in all alloys. There is a small difference in hardness between binary and ternary alloys slow cooled after sand casting. (C) 2009 Published by Elsevier Inc.
The non-isothermal formation of Sc- and Zr-containing dispersoids in Al–Zr–Sc ternary alloys has been investigated by atom probe tomography (APT). In the early stages of precipitation, a high number density of Sc-rich clusters form. These clusters subsequently transform into Al3Sc particles with a L12 structure. When Zr diffusion becomes significant, Zr atoms are found to segregate to Al3Sc/α-Al matrix interfaces. Further annealing at 748 K gives rise to a duplex core/shell structure.
Specimens of an Al–0.2 wt.% Sc alloy were solution heat treated and isothermally annealed in the temperature range of 190–530 • C, and the precipitation reaction was followed with electrical conductivity measurements. Some specimens were also examined by using light microscopy and transmission electron microscopy. The conductivity measurements were used to characterise the kinetics of the transformation, and a TTT (time–temperature-transformation) diagram was constructed. Two transformation time minima are observed, one at about 310 • C, which is associated with continuous precipitation and the other at about 410 • C which is associated with discontinuous precipitation. Conductivity measurements from the coarsening stage were used for estimating the solvus line of the phase diagram and the interface energy between Al and Al 3 Sc. Good solvus estimates were achieved in the temperature range of 370–530 • C. The estimates of interface energy spread from approx. 20 to 300 mJ/m 2 .
The role of severe plastic deformation on the second-phase stability in a 6082 Al-Mg-Si alloy was studied using differential
scanning calorimetry (DSC) and transmission electron microscopy (TEM) techniques. The alloy was fully annealed prior to undergoing
up to six equal channel angular pressing (ECAP) passes using route C. The Orowan strengthening mechanism was calculated on
the basis of TEM inspections for the two hardening second-phase precipitates: Mg2Si and Si. The former had a major tendency to be cut and fragmented by dislocations, while in the latter, a dissolution process
was induced by severe plastic deformation. Accordingly, the second-phase Si particles became progressively less effective
with increasing deformation (i.e., additional ECAP passes). The increase in hardness with the ECAP passes was mostly due to the grain refining mechanism and
to dislocation tangles within the newly formed grains. The expected, though if limited, contribution of second-phase hardening
was prevalently accounted for by the Mg2Si particles.
The relationship between the processes of the nanostructure evolution and the strain-induced dissolution of phases upon severe
plastic deformation is studied. The known mechanisms of these phenomena are analyzed, and new ones are proposed. The extended
metastable and equilibrium phase diagram used for the analysis of the deformed nanostructured solid solution allows for the
relationships of the equilibrium states of the system of bulk phases with the equilibrium and metastable states of the systems
of planar, linear, and point defects.
A systematic description is given of cellular reactions with special account to the discontinuous precipitation. The following
special aspects of this reaction are discussed: The constitutional, nucleation, and other conditions for its occurrence; the
role of grain boundaries, dislocations, and vacancies for the mechanism; the effect of continuous precipitation before discontinuous
precipitation; multiple discontinuous reactions; effects of grain boundary structure, third elements, and external stress;
reactions in highly defect crystals and amorphous solids. It is shown that all phenomena can be understood either by effects
on driving force or mobility of the reaction front.
Minor additions of Sc are effective in controlling the recrystallization resistance of 5xxx, 2xxx, and 7xxx aluminum. The addition of Sc to aluminum results in the rapid precipitation of homogeneously distributed Al3Sc dispersoids, which are coherent with the matrix and have the L12 structure. The presence of Al3Sc dispersoids increases the recrystallization resistance of wrought alloys. The higher coarsening rate of Al3Sc compared to that of Al3Zr may limit its applications as a single ancillary addition. When both scandium and zirconium are used in the same alloy,
Al3(Sc1-x
, Zr
x
) dispersoids form. These dispersoids are more effective recrystallization inhibitors than either Al3Sc or Al3Zr. The Al3(Sc1-x
, Zr
x
) dispersoids precipitate more rapidly than Al3Zr but have a slower coarsening rate than Al3Sc. Furthermore, the distribution of Al3(Sc1-x
, Zr
x
) is significantly more homogeneous than Al3Zr. It was also established that alloys containing up to 3.5Mg showed improvement in recrystallization resistance when both
Sc and Zr were present. Several morphologies of Al3Sc and Al3(Sc1-x
, Zr
x
) were also observed.
The solidification behaviour of dilute Sc containing Al alloys has been investigated. In binary Al–Sc alloys, Sc additions greater than the eutectic composition (0.55 wt%) were found to produce a remarkable refinement in the grain size of aluminium castings, of two orders of magnitude, due to the formation of the primary Al3Sc intermetallic phase during solidification. The refinement in grain size only occurred in hypereutectic compositions and was shown to be far greater than can be achieved by conventional Al grain refiners. Grain refinement by the addition of Sc is accompanied by a change in growth morphology from dendritic, in the large unrefined grains, to fine spherical grains with a divorced eutectic appearing on the grain boundaries in the refined castings. Similar levels of refinement were observed in Al–Sc–Zr and Al–Cu–Sc alloys. In the latter, a change in the segregation behaviour of Cu was observed, from a strongly interdendritic segregation pattern to a more homogeneous distribution. The supersaturated Al–Sc solid solution can decompose via a discontinuous precipitation reaction to form coherent rod-like precipitates of the L12 Al3Sc phase.
The diffusion coefficients of several transition elements (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) and a few non-transition elements (Mg, Si, Ga, and Ge) in fcc and liquid Al are critically reviewed and assessed by means of the least-squares method and semi-empirical correlations. Inconsistent experimental data are identified and ruled out. In the case of the elements, for which plentiful experimental data are available in the literature, the least-squares analysis gives rise to the activation energies and pre-exponential factors in an Arrhenius equation. For the elements with limited experimental data or no data at all, the diffusion parameters are estimated from two semi-empirical correlations. In one correlation, the logarithmic pre-exponential factors are plotted against the activation energies for various elements in Al. In the other correlation, the activation energies are shown as a function of valences relative to Al. The diffusion coefficients calculated by using the evaluated diffusion parameters agree reasonably with the reliable experimental data. The proposed semi-empirical correlations are used to predict the diffusion coefficients of a few elements in liquid Al. A satisfactory agreement between the predicted and measured diffusion coefficients is obtained.
The microstructure, mechanical properties and weld hot cracking behaviour of a cast Al–Mg–Sc alloy containing 0.17 wt.% Sc were compared with those of a Sc-free alloy of similar chemical composition. Although this level of Sc addition did not cause grain refinement, the dendritic substructure appeared to be finer. There was a significant increase in the yield and tensile strength and the microhardness of the Al–Mg–Sc alloy relative to its Sc-free counterpart. A discontinuous precipitation reaction was observed at the dendritic cell boundaries. Microchemical analysis revealed segregation of Mg and Sc at these interdendritic regions. No improvement was observed in the resistance of the alloy to weld solidification cracking or heat affected zone (HAZ) liquation cracking. This is explained in terms of the inability of this level of Sc addition to refine the solidification structure and to influence the liquation of solute-enriched dendritic cell boundaries of the cast material.
Precipitation of the Al3Sc (L12) phase in aluminum alloys, containing 0.1, 0.2 or 0.3 wt% Sc, is studied with conventional transmission and high-resolution (HREM) electron microscopies. The exact morphologies of the Al3Sc precipitates were determined for the first time by HREM, in Al–0.1 wt% Sc and Al–0.3 wt% Sc alloys. The experimentally determined equilibrium shape of the Al3Sc precipitates, at 300°C and 0.3 wt% Sc, has 26 facets, which are the 6 {100} (cube), 12 {110} (rhombic dodecahedron), and 8 {111} (octahedron) planes, a Great Rhombicuboctahedron. This equilibrium morphology had been predicted by first principles calculations of the pertinent interfacial energies. The coarsening kinetics obey the (time)1/3 kinetic law of Lifshitz–Slyozov–Wagner theory and they yield an activation energy for diffusion, 164±9 kJ/mol, that is in agreement with the values obtained from tracer diffusion measurements of Sc in Al and first principles calculations, which implies diffusion-controlled coarsening.
This study investigates the mechanical properties of ternary Al(Sc,Zr) alloys containing 0.27–0.77 vol.% of Al3(Sc,Zr) precipitates with an average radius . Microhardness values at ambient temperature follow predictions of the Orowan dislocation bypass mechanism, with a transition to the precipitate shearing mechanism predicted for 〈r〉 larger than 2 nm. Addition of Zr to binary Al(Sc) alloys delays the onset and kinetics of over-aging at 350 and 375 °C, but has little influence on the magnitude of the peak microhardness. Creep deformation at 300 °C is characterized by a threshold stress, which increases with 〈r〉 in the range 2–9 nm, in agreement with prior results for binary Al(Sc) alloys and a recently developed general climb model considering elastic interactions between dislocations and coherent, misfitting precipitates. At constant 〈r〉 and precipitate volume fraction, Zr additions do not significantly improve the creep resistance of Al(Sc) alloys.
Atom-probe tomography (APT) and high-resolution transmission electron microscopy are used to study the chemical composition and nanostructural temporal evolution of Al3(Sc1−xZrx) precipitates in an Al–0.09 Sc–0.047 Zr at.% alloy aged at 300 °C. Concentration profiles, via APT, reveal that Sc and Zr partition to Al3(Sc1−xZrx) precipitates and Zr segregates concomitantly to the α-Al/Al3(Sc1−xZrx) interface. The Zr concentration in the precipitates increases with increasing aging time, reaching a maximum value of 1.5 at.% at 576 h. The relative Gibbsian interfacial excess of Zr, with respect to Al and Sc, reaches a maximum value of 1.24 ± 0.62 atoms nm−2 after 2412 h. The temporal evolution of Al3(Sc1−xZrx) precipitates is determined by measuring the time dependence of the depletion of the matrix supersaturation of Sc and Zr. The time dependency of the supersaturation of Zr does not follow the asymptotic t−1/3 law while that of Sc does, indicating that a quasi-stationary state is not achieved for both Sc and Zr.
Yield strength at ambient temperature and creep resistance between 225 and 300°C were investigated in dilute Al(Sc) alloys containing coherent Al3Sc precipitates, which were grown by heat-treatments to radii in the range 1.4–9.6 nm. The dependence of the ambient-temperature yield stress on precipitate size is explained using classical precipitation strengthening theory, which predicts a transition from precipitate shearing to Orowan dislocation looping mechanisms at a precipitate radius of 2.1 nm, in good agreement with experimental data. At 300°C creep threshold stresses are observed and found to be much lower than the yield stresses, indicative of a climb-controlled bypass mechanism. The threshold stress increases with increasing precipitate radius, in qualitative agreement with a climb model taking into account stiffness and lattice mismatches between matrix and precipitates [1].
The effects of Mg alloying on the temporal evolution of Al3Sc (L12 structure) nanoscale precipitates are investigated, focusing on the morphology and coarsening kinetics of Al3Sc precipitates in an Al–2.2 Mg–0.12 Sc at.% alloy aged between 300 and 400 °C. Approximately spheroidal precipitates are obtained after aging at 300 °C and irregular morphologies are observed at 400 °C. The coarsening behavior is studied using conventional and high-resolution transmission electron microscopies to obtain the temporal evolution of the precipitate radius, and atom-probe tomography is employed to measure the Sc concentration in the α-matrix. The coarsening kinetics are analyzed using a coarsening model developed by Kuehmann and Voorhees for ternary systems [Kuehmann CJ, Voorhees PW. Metall Mater Trans A 1996;27:937]. Values of the interfacial free energy and diffusion coefficient for Sc diffusion in this Al–Mg–Sc alloy at 300 °C are independently calculated, and are in good agreement with the calculated value of interfacial free energy [Asta M, Ozolins V, Woodward C. JOM 2001;53:16] and the experimental diffusivity obtained for the Al–Sc system [Marquis EA, Seidman DN. Acta Mater 2001;49:1909; Marquis EA. Ph.D. Thesis. Materials Science and Engineering Department, Northwestern University, 2002; Fujikawa SI. Defect Diffusion Forum 1997;143–147:115].
Scandium in aluminium alloys
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