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Development of Al–Nb–B master alloy with high Nb/B ratio for grain refinement of hypoeutectic Al–Si cast alloys

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Al–Nb–B master alloy has been regarded as a promising grain refiner that can reduce grain size of hypoeutectic Al–Si casting alloys. However, its grain refinement performance remains to be improved. In this work, the grain refinement efficacy of Al–Nb–B master alloy is significantly enhanced by modifying the Nb/B ratio through thermodynamic calculation. An Al–Nb–B master alloy with optimum Nb/B ratio of ~ 10:1 provides a fully equiaxed structure across the sections of the Al–10Si and commercial Al–9Si–0.08Ti alloys with an average grain size below 220 μm. The phenomenon is attributed to the existence of NbAl3 and the higher number density of NbB2 at the Nb/B ratio of ~ 10:1, which offers sufficient active nucleating sites to promote the formation of smaller grains. Moreover, the segregation behavior of Si atoms and interfacial energies after doping Si are investigated by first-principles calculations, and the results reveal that Si tends to segregate to the NbAl3/α-Al interface, whereas grain refining potency of NbAl3 for Al remains unchanged. This study has implications for strategic design of Al–Si cast alloy with fine and equiaxed grain structure inoculated by grain refiner.
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Development of Al–Nb–B master alloy with high Nb/B
ratio for grain refinement of hypoeutectic Al–Si cast
J. Xu
, Y. Jiang
, and Q. Li
Materials Genome Institute and Shanghai Institute of Materials Genome and State Key Laboratory of Advanced Special Steels and
Shanghai Key Laboratory of Advanced Ferrometallurgy and School of Materials Science and Engineering, Shanghai University,
Shanghai 200444, China
China Science Lab, General Motors Global Research and Development, 56 Jinwan Road, Shanghai 201206, China
Received: 21 April 2019
Accepted: 9 August 2019
Published online:
19 August 2019
ÓSpringer Science+Business
Media, LLC, part of Springer
Nature 2019
Al–Nb–B master alloy has been regarded as a promising grain refiner that can
reduce grain size of hypoeutectic Al–Si casting alloys. However, its grain
refinement performance remains to be improved. In this work, the grain
refinement efficacy of Al–Nb–B master alloy is significantly enhanced by
modifying the Nb/B ratio through thermodynamic calculation. An Al–Nb–B
master alloy with optimum Nb/B ratio of *10:1 provides a fully equiaxed
structure across the sections of the Al–10Si and commercial Al–9Si–0.08Ti alloys
with an average grain size below 220 lm. The phenomenon is attributed to the
existence of NbAl
and the higher number density of NbB
at the Nb/B ratio of
*10:1, which offers sufficient active nucleating sites to promote the formation
of smaller grains. Moreover, the segregation behavior of Si atoms and interfacial
energies after doping Si are investigated by first-principles calculations, and the
results reveal that Si tends to segregate to the NbAl
/a-Al interface, whereas
grain refining potency of NbAl
for Al remains unchanged. This study has
implications for strategic design of Al–Si cast alloy with fine and equiaxed grain
structure inoculated by grain refiner.
Hypoeutectic Al–Si cast alloys have been widely used
in automotive industry due to their low density,
excellent special strength, and adequate castability
[1,2]. Generally, a refined microstructure, which can
be commonly achieved with the addition of grain
refiners, is crucial for improving mechanical proper-
ties, reducing porosity and hot tearing, and modify-
ing secondary phase distribution [3].
The widely used Al–5Ti–1B master alloy [46]is
effective in refining wrought aluminum alloys, but
ineffective in refining the casting aluminum alloys
Address correspondence to E-mail:
J Mater Sci (2019) 54:14561–14576
Metals & corrosion
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
... Recently, it was reported that NbB 2 particles could also serve as nucleation sites for Mg grains during solidification, resulting in grain refinement [15][16][17][18]. Nb-based heterogeneous nuclei were firstly used to refine Si-containing aluminum alloys to avoid the "poisoning" effect [19,20]. Bolzoni et al. investigated the grain refinement effect of Nb-containing particles on Mg-Al alloy. ...
... Pure aluminum (99.8 wt%), niobium powders (purity 99.9 wt%, particle size <40 μm) and KBF 4 salt (purity 99.5 wt%) were selected to prepare the Al-NbB 2 master alloy. The results of the phase diagram calculations showed that the reaction of the Al-Nb-B system was favored when the melting temperature was 800-850 • C, accordingly the melting temperature was set to 830 • C [19,20] in the present study. First, the pure aluminum was placed in a graphite crucible at 830 • C to obtain an Al melt. ...
... Based on the thermodynamic calculation results, when the weight ratio of Nb and B in the raw material was 4: 1, sole NbB 2 particles would form in the master alloy. For the purpose of generating as many NbB 2 particles as possible, the weight ratio of Nb and B (Boron) in the raw material was designed to be 4: 1 [20]. During 2 h of holding time before casting, stirring was manually repeated for 30 s every 20 min. ...
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Enhancing strength and ductility simultaneously is a critical issue for Mg alloys, which can be achieved by grain refinement. How to improve the effectiveness of grain refiner is another significant issue to be solved in the Mg alloy foundry industry. In this study, new Al–NbB2 master alloys for AZ91 alloy were prepared by the reaction of halide salt with molten aluminum. The improvement of the refining efficiency of the Al–NbB2 master alloy was achieved by increasing the cooling rate during the preparation process. The results showed that the newly prepared Al–NbB2 master alloy had a remarkable grain refinement effect, and the decrease in the size of NbB2 particles in the master alloy can enhance the refinement efficiency. AZ91 alloy with the minimum average grain size (AGS, 68 μm), which was obtained by adding 0.5 wt.% Al–NbB2 master alloy exhibited enhanced tensile properties. The ultimate strength, yield strength and elongation of refined AZ91 alloy were 204 MPa, 90 MPa and 8.2%, respectively. The matching relationships between NbB2 and α-Mg were characterized with the assistance of the high-resolution transmission electron microscope (HRTEM) and edge-to-edge models (E2EM). There were good matching relationships between NbB2 particle and α-Mg, as [1 2¯1 0]/[12¯1 0] in (101¯0)/(0002) and [1 2¯1 3]/[101¯0] in (10 1¯0)/(10 1¯1), which is the key for grain refinement. This work may provide a new insight into the design of high refining efficiency of grain refiner for Mg alloy.
... It has been reported [22][23][24] that the heat capacity should be around 782 W for efficient cooling. With conformal cooling, even better efficiency can be achieved [25][26][27]. The loss of heat of the liquid metal is done by the mold. ...
... The loss of heat of the liquid metal is done by the mold. As the cooling channels are located close to the mold surface, the mold surface can easily reach back to its required temperature [25][26][27]. On the other hand, Stolt et al. [28] reported that the difference in the thermal expansion coefficient of the die and the AM material could result in possible deterioration of the mold. ...
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Porosity is one of the challenges in high-pressure die casting (HPDC) of aluminum alloys. A typical control of porosity involved controlled solidification which is attained by the cooling of the die. In traditional cooling, this is done by straight cooling channels. However, they can be ineffective due to the limited geometry of these channels. Alternatively, conformal cooling channels can be produced in any geometry by additive manufacturing. In this study, the thermal analysis of the mass production engine crankcase die was performed and the regions where the metal-die heat transfer load is high and limiting the production rate were examined. In order to increase the heat transfer and maximize the cooling rate in the different locations of the mold, conformal cooling inserts were designed and produced in a direct metal laser sintering 3D print machine by additive manufacturing technology. AlSi9Cu3 alloy was used to produce the cast part by HPDC. It was observed that porosity was decreased by 43% (from 251 to 156 mm³) and the cycle duration was decreased by 4 s when conformal cooling was used. Secondary dendrite arm spacing (SDAS) was decreased by 39% to 8.27 mm while eutectic silicon modification was enhanced.
... However, on the slant fracture surface, sudden nucleation and coalescence of micro-voids in a macroscopic shear band can be observed [18]. Despite many experimental advances made in recent years, the understanding of mechanisms of ductile failure in second-phase particle-strengthened Al alloys remains incomplete [19][20][21][22][23][24][25][26]. ...
... With the continued increase in the applied displacement, the voids formed are found to grow at both sides of the particle, as shown in Figs. [23][24][25]. The in-situ experimental results in Section 2 also indicate that as the applied load increases, the micro-voids may nucleate and the impingement of the micro-voids initiated at locations of interfacial debonding or particle breakage further results in fracture or damage of Al matrix. ...
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This paper addresses the challenge of reconstructing randomly distributed second-phase particle-strengthened microstructure of AA7075-O aluminum sheet material for computational analysis. The particle characteristics in 3D space were obtained from focused ion beam and scanning electron microscopy (FIB-SEM) and SEM-based Electron Backscatter Diffraction/Energy Dispersive X-ray Spectrometry (EBSD/EDS) techniques. A theoretical framework for analysis of elastic-plastic deformation of such 3D microstructures is developed. Slip-induced shear band formation, void initiation, growth and linkage at large plastic strains during uniaxial tensile loading were investigated based on reconstructed 3D representative volume element (RVE) models with real-distribution of particles and the results compared with experimental observations. In-situ SEM interrupted tension tests along transverse direction (TD) and rolling direction (RD), employing microscopic-digital image correlation (μ-DIC) technique, were carried out to investigate slip bands, micro-voids formation and obtain microstructural strain maps. The resulting local strain maps were analyzed in relation to the experimentally observed plastic flow localization, failure modes and local stress maps from simulations of RVE models. The influences of particle size, shape, orientation, volume fraction as well as matrix-particle interface properties on local plastic deformation, global stress-strain/strain-hardening curves and interfacial failure mechanisms were studied based on 3D RVE models. When possible, the model results were compared with in-situ tensile test data. In general, good agreement was observed, indicating that the real 3D microstructure-based RVE models can accurately predict the plastic deformation and interfacial failure evolution in AA7075-O aluminum sheet.
... In terms of solidification microstructure of hypoeutectic Al-Si alloy, it is mainly composed of primary a-Al and eutectic Si. In order to refine the primary a-Al grain, grain refiners such as Al-5Ti-1B and Al-Nb-B have been very often used [2], [3], [4], while in order to modify the eutectic Si, Sr has been very often used [5], [6]. In terms of the thermal and electrical conductivity of hypoeutectic Al-Si alloy, elucidating effects of alloying elements on the thermal and electrical conductivity is of great importance. ...
... It has been reported by Xu et al. that the presence of Si with a high content can react with Ti elements in the alloy to form Ti-Si, Ti-Al-Si and other intermetallic compounds either in the melt, which reduces the growth restriction factor caused by Ti, or on the surface of nucleation sites (i.e., TiB 2 or TiC), which results in the inactivation of TiB 2 or TiC particles for nucleation. The so-called ''Si poisoning'' phenomenon occurs [3], [16]. Moreover, with increasing Si content, the solidification onset temperature decreases and thus leads to an increase of superheat and intensifies the grain coarsening phenomenon at the constant pouring temperature, which is consistent with the variation of a-Al grain size with Si contents, as shown in Fig. 6. ...
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Effects of Si and Sr on solidification microstructure and thermal conductivity of Al–Si binary alloys and Al–9Si–Sr ternary were investigated, respectively, with a special focus on the relationship between solidification microstructure and thermal conductivity. It was found that (i) in Al–Si binary alloys, with increasing Si content, α-Al grain size increases and then decreases when Si content is over 7 wt%, while the percentage of eutectic Si continuously increases, which significantly decreases the thermal conductivity and electrical conductivity, and (ii) in Al–9Si–Sr ternary alloys, the presence of Sr has no significant effect on α-Al grain, but effectively modifies eutectic Si and significantly improves the thermal and electrical conductivity. On this basis, two theoretical calculation models [the Maxwell model and the Hashin–Shtrikman (H–S) model] were used to elucidate the relationship between solidification microstructure and thermal conductivity. Compared with the Maxwell model, the H–S model fits better with the measured values. The obtained results are very helpful to the precise composition control during alloy design and recycling of Al–Si-based alloys with the aim to further improve the thermal conductivity of Al–Si-based alloys. Graphical abstract
... The introduction of borides such as NbB 2 , VB 2 , TiB 2 as grain refiners is one of the methods of achieving grain refinement and maintaining it during processing and service conditions [28][29][30] . Grain refiners are introduced during casting to achieve better mechanical properties, reduce porosity and hot tearing [31] . Whereas grain size in ball-milled alloys is refined and the challenge lies in retaining it during processing. ...
Al-xV alloys (x = 2 at.%, 5 at.%, 10 at.%) with nanocrystalline structure and high solid solubility of V were produced in powder form by high-energy ball milling (HEBM). The alloy powders were consolidated by spark plasma sintering (SPS) employing a wide range of temperatures ranging from 200 to 400°C. The microstructure and solid solubility of V in Al were investigated using X-ray diffraction analysis, scanning electron microscope and transmission electron microscope. The microstructure was influenced by the SPS temperature and V content of the alloy. The alloys exhibited high solid solubility of V―six orders of magnitude higher than that in equilibrium state and grain size < 50 nm at all the SPS temperatures. The formation of Al3V intermetallic was detected at 400°C. Formation of a V-lean phase and bimodal grain size was observed during SPS, which increased with the increase in SPS temperature. The hardness and elastic modulus, measured using nanoindentation, were significantly higher than commercial alloys. For example, Al-V alloy produced by SPS at 200°C exhibited a hardness of 5.21 GPa along with elastic modulus of 96.21 GPa. The evolution of the microstructure and hardness with SPS temperatures has been discussed.
... The nearly normal size distribution indicates that the overall precipitates in the matrix tend to be uniform and homogeneous which is in line with experimental observations in Al-Zn-Mg-Cu alloys [19]. To avoid the innate error in establishing size distribution by sampling in TEM micrograph, a skew size distribution (Since the inevitable fact that 26 there must be a large number of small-sized precipitates in the matrix that are difficult to be observed by TEM, the setting of skew size distribution is reasonable [82,83]) is designed and input into the growth model. The evolutions of the designed skew size distribution calculated by the modified growth model are present in Fig. 12(g-i). ...
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Zn content is one of the most concerned factors in the development of next generation ultra-strength Al-Zn-Mg-Cu alloys owing to its essential role in precipitation strengthening. In the present work, the underlying functions of Zn content in precipitation evolution and strengthening function of Al-Zn-Mg-Cu alloys were systematically investigated by combining multiple experiments and an integrated internal-state-variable model. The experimental results indicated that the increased Zn content in Al-Zn-Mg-Cu alloys would promote the development of precipitates and enhance aging hardening. The diffusion flux of soluble Zn and the coordination of Mg atom controlled the crystallographic microstructures evolution during precipitates nucleation, growth and transition processes. By integrating precipitation development with electrical resistivity and hardness evolutions, an improved internal-state-variable physical model was then developed for the aging responses of Al-Zn-Mg-Cu alloys. The unified model considered the intrinsic characteristics of precipitates such as crystallographic orientation, morphology, component, and distribution. The specific improvements were to balance the combined functions of Zn element and Mg element and consider the plate-like morphology and directed growth as indicated by experiments. This model was also adaptive to heat-treatment variables and chemical compositions, and owned the notable advantages to simultaneously rationalize the observed microstructural characteristics, mechanical and electrical properties following artificial aging of Al-Zn-Mg-Cu alloys. In addition, a preliminary model framework between electrical resistivity and hardness for Al-Zn-Mg-Cu alloys was established.
Creep mechanism was well-known to be mainly dominated by the dislocation sliding and climbing during creep deformation. Here we study the creep deformation of an Al-Cu-Li alloy with the assistance of electropulsing and subsequent microstructural observations. We find that creep strain increased drastically under electropulsing and was almost twelve times as much as that of the non-pulsed sample. Microstructural observations confirmed that dislocation reconfiguration happens via electropulsing, namely helical dislocations being opened rapidly. This opened dislocation structure can possess a much higher mobility than the initial helical dislocation, which mostly responsible for the greatly increased creep strain. Our results revealed a new mechanism accountable for the distinctly electroplastic creep deformation.
Grain refinement of Al–Si alloys has been a long subject of study with various theories still being discussed. The efficiency of grain refiners, fading effect, poisoning effect, and selection of the optimum ratios of particularly Ti/B ratio has been the focus of many researchers. In this work, different grain refiners were added to Al11Si alloy and statistical analysis of tensile properties was conducted by means of Weibull analysis and survivability plots. It was aimed to compare the reproducibility and reliability of mechanical properties of different grain refiners. Ti-free B, Nb, and chemically introduced MTS were investigated. It was found that the DAS and SDAS were not dramatically changed but the reliability of the tensile properties was significantly varied. 0.03wt% Nb added alloy revealed the lowest reproducibility with lowest tensile values whereas 0.1wt % Nb and MTS exhibited the most reliable and highest tensile properties.
Grain refinement is critical to surpassing the bottlenecks of inherent hot tearing of high-strength aluminum alloys fabricated by additive manufacturing (AM). In this study, a synergistic grain-refining strategy including heterogeneous nucleation, solute-driven growth restriction and nanoparticle-induced growth restriction was introduced to control the microstructure of Al-Zn-Mg-Cu alloys during the laser powder bed fusion (LPBF) process. Crack-free Al-Zn-Mg-Cu alloys with significantly refined grains were safely fabricated via LPBF by coincorporation of TiC and TiH2 particles. In-situ L12-Al3Ti particles were produced to promote the heterogeneous nucleation. The grain growth was restricted by adding Ti solute, while introduced TiC nanoparticles (NPs) improved the density of heterogeneous nucleation sites and blocked grain growth physically. The resultant elimination of columnar grains and hot cracks in the (1 wt.%) TiC- and (0.8 wt.%) TiH2-modified Al-Zn-Mg-Cu alloy resulted in excellent ultimate tensile strength (UTS) of 593 ± 24 MPa, yield strength (YS) of 485 ± 41 MPa and elongation (EL) of 10.0% ± 2.5% under the T6 condition. This study provides new insights into improving the grain microstructure and mechanical properties of high-strength aluminum alloys during LPBF.
Hybrid reinforced aluminum-based composites with a silicate component with pozzolanic activity and silicon carbide ceramic particles as reinforcement phases can form an iron-poor friction transfer film during the friction process, thereby making the aluminum-based composites exhibit a high friction coefficient and a low wear rate characteristics. The phase composition in the friction transfer film was investigated utilizing focused ion beams, transmission electron microscopy, and energy-dispersive X-ray analysis. The results showed that the friction transfer film was mainly composed of nanosized aluminum, ultra-fine aluminum, and crushed silicate particles, which interwove into a nano-network structure to protect the matrix and improve the friction performance.
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It is a practically significant issue to overcome the Si poisoning for the grain refinement of commercial Al-Si casting alloys. By understanding the anti Si-poisoning mechanism of Nb, a solution to greatly reduce the grain size of α-Al from over 1000 μm to <100 μm is obtained. Specifically, the Si content for the fine-grained hypoeutectic Al-Si-Nb alloys should be made at ≤12 wt% Si. During the melting step, sufficient stirring and/or short holding duration should be applied to prevent the inoculant NbAl3 substrates being neutralized by the in-situ formed NbSi2 shells.
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The grain refining efficacy of titanium, aluminum, and niobium borides, as well as niobium aluminides, introduced via commercial and lab made master alloys on Al-Si alloys was investigated. Significant grain refinement is achieved via the introduction of these heterogeneous nuclei regardless of their chemistry, stoichiometry of the master alloy, and addition rate. However, the grain refinement is affected by variable such as contact time, as the inoculating particle may sediment or be poisoned, and cooling rate. Specifically, a faster cooling rate generally leads to finer grains due to the lower time for grain growth. In the case of borides, the chemical inoculation efficiency is greatly affected by their thermodynamic stability in molten Al-Si alloys. Conversely, the grain refining potency of properitectic Al3Nb remains unaffected. The underlying grain refining mechanism was finally investigated using current models based on the growth restriction factor Q to simultaneously consider the effect of nucleant potency and alloy chemistry. Among Ti-, Al-, and Nb-based borides with similar particle size and distribution, the latter are the most efficient to grain refine Al-Si alloys.
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Al-Ti-B based master alloys have been widely used for grain refining of Al-alloys in industry for many decades. However, the effectiveness of such grain refiners is severely compromised when a few hundred ppm of Zr is present in the Al melt, and this phenomenon is referred to as Zr poisoning in the literature. So far the exact mechanisms for Zr poisoning are not clear albeit significant research effort on the subject in the last few decades. In this work we investigated the mechanism for Zr poisoning through extensive examinations of the Al/TiB2 interface using the state-of-the-art electron microscopy and ab initio molecular dynamics simulations. We found that the presence of Zr in Al melts leads to (i) the dissolution of the Al3Ti 2-dimensional compound (2DC) formed on the (0 0 0 1) TiB2 surface during the grain refiner production process; and (ii) the formation of an atomic monolayer of Ti2Zr 2DC on the (0 0 0 1) TiB2 surface, which replaces the original Ti-terminated TiB2 basal surface. This monolayer of Ti2Zr not only has large lattice misfit (4.2%) with α-Al, but also is atomically rough, rendering the TiB2 particles impotent for heterogeneous nucleation of α-Al. This work, in combination of our previous work, demonstrates that heterogeneous nucleation can be effectively manipulated, either enhanced or impeded, by chemical segregation of selected alloying/impurity elements at the liquid/substrate interface.
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Hypoeutectic Al–xSi–0.45Mg (x = 6.5, 7.5, 8.5, 9.5) alloys were refined by Al5Ti1B containing TiB2 and Al3Ti3B containing TiB2 and AlB2, respectively. With increasing Si, Si poisoning on TiB2 results in the obvious coarsening of primary α-Al in Al5Ti1B-refined alloys from 350 ± 40 to 400 ± 50, 475 ± 50 and 560 ± 80 μm, and the competition between Si promotion on AlB2 and Si poisoning on TiB2 leads to the slight coarsening of primary α-Al in Al3Ti3B-refined alloys from 215 ± 30 to 265 ± 35, 265 ± 30 and 315 ± 25 μm. After T6 heat treatment, with increasing Si, the yield strength (YS) of Al5Ti1B-refined alloys increases from 294 ± 2 to 299 ± 2, 304 ± 1 and 309 ± 2 MPa, and the elongation first increases from 3.5 ± 0.8 to 4.5 ± 1.0 and 7.8 ± 1.4%, after decreases to 5.5 ± 1.2%, while the YS of the Al3Ti3B-refined alloys increases from 300 ± 1 to 305 ± 2, 312 ± 1 and 317 ± 2 MPa, and the elongation increases from 6.1 ± 1.1 to 8.5 ± 1.2, 11.8 ± 1.5 and 12.1 ± 1.6%. The increase in the secondary phase and precipitation strengthening results in the increase in strength with increasing Si. With increasing Si, the decrease in porosity formation by decreasing solidification interval and increasing fluidity is superior to the increase in porosity formation by slightly coarsening grain size, which leads to the increase in ductility in the Al3Ti3B-refined alloys, while the competition between porosity decreasing and increasing factors leads to the inverted ‘V’-shaped evolution of ductility in the Al5Ti1B-refined alloys.
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The design of chemical compositions containing potent nuclei for the enhancement of heterogeneous nucleation in aluminium, especially cast alloys such as Al-Si alloys, is a matter of importance in order to achieve homogeneous properties in castings with complex geometries. We identified that Al3Nb/NbB2 compounds are effective heterogeneous nuclei and are successfully produced in the form of Al-2Nb-xB (x = 0.5, 1 and 2) master alloys. Our study shows that the inoculation of Al-10Si braze alloy with these compounds effectively promotes the heterogeneous nucleation of primary α-Al crystals and reduces the undercooling needed for solidification to take place. Moreover, we present evidences that these Nb-based compounds prevent the growth of columnar crystals and permit to obtain, for the first time, fine and equiaxed crystals in directionally solidified Al-10Si braze alloy. As a consequence of the potent heterogeneous particles, the size of the α-Al crystals was found to be less dependent on the processing conditions, especially the thermal gradient. Finally, we also demonstrate that the enhanced nucleation leads to the refinement of secondary phases such as eutectic silicon and primary silicon particles.
The in-situ Al3Nb-NbB2-NbC/Al inoculant (Al-5Nb-0.75B4C) ingots and ribbons were prepared with Al-Nb and B4C powers to refine the Al-Cu-Mn alloy. The Al3Nb-NbB2-NbC/Al inoculant ribbons showed excellent inoculant effect, which modified the dendrite crystals to equiaxed crystals and refined the grains from ~100 µm to ~30 µm. The transmission electron microscopy (TEM) images of the Al-Cu-Mn alloys reveal that the θ′ phases were refined from ~83 nm to ~40 nm and their number densities were also increased significantly. The selected area electron diffraction (SAED) patterns indicate that there are orientation relationships (ORs) between α-Al and Al3Nb, NbB2 and NbC, which suggest that these particles possess nucleation potency to α-Al. After inoculated by 1% Al3Nb-NbB2-NbC/Al ribbons, the ultimate tensile strength and elongation of the Al-Cu-Mn alloy were improved from 435 MPa and 8.4% to 514 MPa and 11.5%, respectively. The welding microstructure and property of the Al-Cu-Mn alloy inoculated by 1% Al3Nb-NbB2-NbC/Al inoculant were also improved significantly.
By using density functional theory based on the first-principles method, the interfacial adhesion, stability and bonding nature of Al(1 1 1)/NbB2 (0 0 0 1) were studied to investigate the heterogeneous nucleation potential of α-Al grains on NbB2 particles. For Al(1 1 1)/NbB2 (0 0 0 1) interface, there are six different models that are Nb-terminated and B-terminated interfaces with different stacking sequences (top-site, hollow-site, and bridge-site), respectively. The research show that B-terminated-hollow-sited interface with the largest work of adhesion and smallest interfacial energy is the most stable and preferred among six different models, and interfacial energy of the model is lower than that of α-Al/Al melt, 0.15 J/m². Furthermore, the difference charge density and partial density of states are presented to discuss bonding nature of the interface. B-terminated-hollow-sited interface have more covalent features than that of the others. For Nb-terminated-hollow-sited interface, Nb–Al metallic bonds are formed across interface. As a result, α-Al grains is inclined to form nucleus on the B-terminated-hollow-sited NbB2 (0 0 0 1).
Addition of Al-5Ti-1B alloy to molten aluminum alloys can refine α-Al grains effectively and thereby improve their strength and toughness. TiAl3 and TiB2 in Al-5Ti-1B alloy are the main secondary-phase particles for refinement, while the understanding on the effect of their sizes on α-Al grain refinement continues to be fragmented. Therefore, Al-5Ti-1B alloys with various sizes and morphologies of the secondary-phase particles were prepared by equal channel angular pressing (ECAP). Evolution of the secondary-phase particles during ECAP process and their impact on α-Al grain refinement were studied by X-ray diffraction and scanning electron microscope (SEM). Results show that during the ECAP process, micro-cracks firstly appeared inside TiAl3 particles and then gradually expanded, which resulted in continuous refinement of TiAl3 particles. In addition, micro-distribution uniformity of TiB2 particles was improved due to the impingement of TiAl3 particles to TiB2 clusters during deformation. Excessively large sizes of TiAl3 particles would reduce the number of effective heterogeneous nucleus and thus resulted in poor grain refinement effectiveness. Moreover, excessively small TiAl3 particles would reduce inhibitory factors for grain growth Q and weaken grain refinement effectiveness. Therefore, an optimal size range of 18–22 μm for TiAl3 particles was suggested.
The Al/TiB2 interface is of significant importance in controlling the mechanical properties of Al-B4C composites and tuning the heterogeneous nucleation of Al/Si alloys in industry. Its stability and bonding conditions are critical for both purposes. In this paper, the interfacial energies were investigated by first-principles calculations, and the results support the reported grain refinement mechanisms in Al/Si alloys. Moreover, to improve the mechanical properties of the interface, Mg and Si were doped at the interface, and our simulations show that the two interfaces will both weaken after doping Mg/Si, thus the formation of TiB2 is inhibited. As a result, the processability of the Al-B4C composites may be improved. Our results provide a theoretical basis and guidance for practical applications.
TiCx contained Al–Ti–C is a kind of grain refiner for Al alloys. In this work, the influence of C/Ti stoichiometry, i.e. the x value in TiCx on grain refinement efficiency was investigated. TiCx particles have been obtained in five Al–5Ti–mC (m = 0.1, 0.5, 0.8, 1, 1.25) master alloys and the x values were measured to be 0.72, 0.75, 0.79, 0.81and 0.8, respectively. It was found that the refinement performance of the master alloys had a close relationship with the x value of TiCx. The Al–5Ti–mC alloy with lower-x TiCx shows better refinement efficiency and anti-fading capability. It is supposed that TiCx particles with lower x are more preferred to release Ti atoms during nucleating process and have a better Ti-absorbing capability. This contributes to the Ti-rich zone formation at TiCx/melt interface, thus enhancing the refinement and anti-fading capability.