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The origin of the difficulty in refining the grain of Al-Si

Time:2020/07/15 丨 source:未知 丨 visit count:

The origin of the difficulty in refining the grain of Al-Si alloy

 
Introduction: Fine grain strengthening can improve the strength and plasticity of the alloy at the same time. It has become a routine process to improve the quality and performance of castings by adding refiners to the aluminum casting industry. However, the traditional Al-5Ti-B thinner is easily poisoned by silicon, making it difficult to effectively refine the cast Al-Si alloy. This is an old problem in the aluminum casting industry. In this paper, using multi-scale (Å~mm) characterization and calculation methods, the interface characteristics between the nucleation particles in the Al-10Si/Al-5Ti-B ingot and the aluminum matrix are studied in detail, and the composition of the refined phase during the solidification process is investigated. The evolutionary law of the proposed new mechanism is different from the traditional understanding, that is, the source of silicon poisoning is the segregation of solute Si to the surface layer of TiB2 particles, not the precipitation and coating of silicide. This has laid a theoretical foundation for solving the engineering problem that Al-Si alloy is difficult to refine.
 
Al-Si alloys are the most widely used cast aluminum alloys. They are widely used in the manufacture of thin-walled automotive body parts, engine parts, transmission system parts, complex-shaped radiators, and oil pipelines. However, without any treatment, the coarse α-Al dendrites and a large number of brittle Al-Si eutectic structures in the Al-Si alloy will greatly weaken the strength and plasticity of the alloy. By adding a fining alloy containing nucleating particles to control the nucleation and growth of α-Al during solidification, refining the solidified structure to improve the strength and plasticity of the material has become a routine process in the aluminum casting industry. However, Al-Si alloys are still a type of aluminum alloy that is more difficult to refine. When the Si concentration is greater than 5 wt.%, the fine graining efficiency of the traditional Al-Ti-B refiner is significantly weakened, which is a Si poisoning effect. For more than 60 years, people have never been able to understand the origin and mechanism of the silicon poisoning effect, which has greatly restricted the development of new anti-toxic aluminum-silicon alloy fine crystal agents and limited the further improvement of the strength and plasticity of the cast aluminum-silicon alloy.
 
The team of Professor Li Qian of Shanghai University cooperated with Dr. Hu Bin of General Motors China Research Institute and Professor Nie Anmin of Yanshan University to use spherical aberration transmission electron microscopy to characterize the nucleation particles (TiB2) and α- in Al-10Si/Al-5Ti-B ingot The atomic structure and element distribution of the interface between Al, for the first time discovered the segregation of Si atoms at the TiB2/α-Al interface. Combined with phase diagram thermodynamic calculations (CALPHAD) and first-principles calculations, the possibility of silicide formation at the nucleation interface was evaluated in detail, and the order of the atomic segregation of the nucleation base and the α-Al epitaxial shape were thoroughly investigated. The impact of nuclear difficulty. The combination of experiments and theoretical calculations has clarified the origin and mechanism of the silicon poisoning effect, and provides a key theoretical basis for the development of anti-silicon poisoning refiners. The above results were published in the top journal "Acta Materialia" in the field of metallurgy and metal materials under the title of "Insight into Si poisoning on grain refinement of Al-Si/Al-5Ti-B system". The first author of the paper is Dr. Li Yang, and the corresponding author is Professor Li Qian.
 
Paper link:
https://www.sciencedirect.com/science/article/pii/S1359645420300641
 

 
Figure1(a) shows the metallographic structure of the Al-10Si ingot (Al-9.98Si-0.08Ti-0.015B) added with 0.1 wt.% Ti Al-5Ti-B refining agent, showing that the α-Al dendrites are coarse , With an average size> 1000 μm. Figure 1(b)(c) is the backscattered electron image of the ingot. It can be seen that TiB2 particles are concentrated in the final solidified Al-Si eutectic region, indicating that they have low nucleation efficiency and are severely poisoned.
 

Fig. 1 (a) Metallographic morphology of Al-9.98Si-0.08Ti-0.015B-0.12Fe; (b) (c) Microscopic morphology of TiB2 particles in the Al-Si eutectic zone backscattered electron image.
 
Fig. 2 is the atomic-level resolution of HAADF and ABF images of the poisoned TiB2/α-Al interface in the Al-Si eutectic zone of the Al-9.98Si-0.08Ti-0.015B ingot. 2~3 layers (112) TiAl3 atomic layer (TiAl3 two dimensional compound (TiAl3 2DC)) are attached to the (0001) TiB2 substrate. The epitaxial growth theory and the edge-to-edge matching model (E2EM) indicate that TiAl3 2DC has a strong nucleation efficiency for α-Al, which significantly improves the fine-grain ability of TiB2 particles. From the differential charge density distribution (Figure 3), the formation of TiAl3 2DC originates from the strong chemical interaction between Ti atoms on the (0001) TiB2 substrate and Ti atoms on the (112) TiAl3 atomic layer.
 

Figure 2 Atomic structure of (0 0 0 1) TiB2/α-Al interface in Al-9.98Si-0.08Ti-0.015B-0.12Fe ingot: (a) HAADF; (b) ABF-STEM
 

Figure 3 (a) Secondary differential charge density diagram at the TiB2/TiAl3 interface without Si solution (red indicates electron accumulation, blue indicates electron loss, and the isosurface value is ±0.05 e/Å3); (b) at the interface The shape of the electron accumulation region; for simplicity, only the atoms at the interface are shown in Figures (a) and (b); (c) (d) Ti atoms in the vicinity of the TiB2/two-dimensional TiAl3 interface without Si solid solution Wave density map, figure (c) shows the Ti atom under investigation
 
Figure 4 is the atomic level resolved EDS element distribution diagram of the above TiB2/α-Al interface. It is found that Si segregates on the (0 0 0 1) TiB2 surface and dissolves into the TiAl3 2DC layer with a solid solution amount of 7~20 at.%Si. After the DFT structure relaxation of the atomic structure at the interface, it was found that the solid solution of Si atoms into the TiAl3 2DC layer would disturb the ordering of the atoms (Figure 5). The differential charge density distribution diagram shows that covalent bonds are formed between Si atoms and Ti atoms (Fig. 6(a)), causing Ti atoms to deviate from the original lattice site, resulting in distortion of the overall lattice structure (Fig. 5(b)). At the same time, the Ti-Si covalent bond weakens the chemical interaction between Ti atoms at the TiB2/TiAl3 2DC interface (Figure 6(b)), and the chemical interaction between Ti and Al atoms at the TiAl3 2DC/α-Al interface. Eventually, the possibility of Al atoms epitaxially nucleating on TiAl3 2DC is reduced, and the fine graining efficiency of TiB2 particles is weakened.
 

Fig. 4 (a) (0 00 1) High resolution HAADF image at TiB2/α-Al interface; (b) Distribution of Si at (0 0 0 1) TiB2/α-Al interface; (c) (0 0 0 1 ) Composition curve of TiB2/α-Al interface
 

Figure 5 Relaxation structure of interface supercells: (a) (c) does not contain Si; (b) (d) contains Si
 

Figure 6 (a) Differential charge density at the TiB2/two-dimensional TiAl3 interface (1 1 2) on the TiAl3 surface after Si solution; (b) Secondary differential charge density at the TiB2/two-dimensional TiAl3 interface after Si solution (Red indicates electron accumulation, blue indicates electron loss; the isosurface electron concentration is ±0.05 e/Å3); for simplicity, only the atoms at the interface are shown in Figures (a) and (b); (c) After the solid solution of Si, the partial wave state density diagram of Ti atoms near the TiB2/two-dimensional TiAl3 interface, the position of Ti atoms is the same as in Fig. 3(c)
 
Figure 7 shows the phase diagram thermodynamic calculation results of Al-10Si/Al-5Ti-B system. Different from the previous opinion that silicide is the source of poisoning, the thermodynamic calculation of phase diagram shows that no silicide (τ2) phase will be formed during solidification. At the same time, the researchers used multi-scale characterization methods (XRD, SEM and TEM) to analyze the phase composition of Al-9.98Si-0.08Ti-0.015B and Al-9.47Si-3.94Ti-0.79B ingots, and no siliconization was found. 物有 exists. Therefore, the precipitation of silicide is not the cause of Al-5Ti-B poisoning.
 

Fig. 7 (a) Vertical cross-sectional phase diagram of Al-Si-Ti-B system; (b) (c) Al-9.98Si-0.08Ti-0.015B phase change in equilibrium (b) and Scheil (c) solidification
 
In summary, the researchers studied in detail the atomic structure and element distribution at the interface between the nucleation particles (TiB2) and α-Al in the Al-10Si/Al-5Ti-B ingot, and found that Si would segregate on the surface of TiB2 particles. And dissolved into TiAl3 2DC which plays a key nucleation role. The strong interaction between Si atoms and Ti atoms disturbs the crystal structure of TiAl3 2DC to a certain extent, and weakens its chemical interaction with epitaxial α-Al, which eventually leads to weakened nucleation efficiency and poisoning. These understandings have an important role in optimizing the composition of the refiner alloy and formulating a new casting process to avoid the segregation of Si on the surface of the TiB2 particles, improve the Si poisoning, and solve the problem of the difficulty in refining the Al-Si alloy.
 
The article is from website
 
 
 
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