亚共晶Al-Si合金晶粒细化的“硅毒化”效应及“抗硅毒化”机制

Grain refinement is a fundamental method to improve the strength and toughness of Al-Si casting components for automobiles. However, commercial Al-5Ti-B master alloy has poor efficacy to refine hypoeutectic Al-Si casing alloys when Si content is higher than 5 wt.% Si. Meanwhile, the grain size of α-Al refined by Al-Nb-B master alloys haven’t reached the accepted level (< 220 μm) in the cast aluminum industry. The absent of understanding on Si-poisoning effect on heterogeneous nucleation sites inoculated by Al-Ti/Nb-B master alloys hinders the solution of these technical problems. In this work, the thermodynamic condition and kinetic mechanism for the formation of silicides during the casting process will be clarified with the aid of CALPHAD (CALculation of PHAse Diagram)-type thermodynamic calculations, solid/liquid reaction kinetic analysis and in-situ synchrotron X-ray powder diffraction characterization. The nucleation potencies of the silicides will be evaluated in terms of crystallographic misfit and interface energy through edge-to-edge matching analysis and DFT (Density Functional Theory) calculation, respectively. Furthermore, the solidification behavior of hypoeutectic Al-Si alloy during grain refining process will be revealed by in situ X-ray tomographic imaging combined with numerical modeling. Finally, the mechanism of silicon influence on the grain refinement efficacies of Al-Ti/Nb-B master alloys will be proposed. This is of theoretical significance for designing high efficient anti Si-poisoning grain refiners for hypoeutectic Al-Si casting alloys.

细晶强化是提高汽车用Al-Si合金零部件强度与韧性的重要手段。商用 Al-5Ti-1B 细化剂不适用于硅含量大于5 wt.%的亚共晶Al-Si合金，而且Al-Nb-B细化剂的细晶效果未达晶粒小于220微米的行业标准，其根源是未厘清Al-Ti/Nb-B细化质点的硅致失效机理。本项目关注铝硅合金铸造凝固过程中硅化物的形成与转变，运用相图热力学计算、液-固反应动力学模型分析，结合原位同步辐射表征，阐明硅化物的形成热力学条件及生成动力学机理；围绕硅化物与α-Al间界面的结构与能量性质，运用高分辨透射电镜等手段，结合错配度分析及第一性原理计算，认识硅化物的异质形核效能；针对高硅含量下α-Al的形核及长大过程，运用高温原位成像技术，结合凝固组织数值模拟，揭示高硅亚共晶Al-Si合金凝固组织的形成机制。最后提出硅对Al-Si合金晶粒细化的影响机理，为开发出高效抗硅毒化Al-Si合金细化剂提供科学依据。

晶粒细化是同时提高铝合金强韧性的重要手段，但商用Al-Ti-B细化剂却不适用于硅含量大于5wt.%的亚共晶Al-Si合金，其主要科学问题是Al-Ti-B细化剂的硅致失效机理不清楚。本项目结合细化剂/铝基体的界面结构分析、细化相和含硅毒化相的热力学形成条件和动力学形成机理分析、α-Al的异质形核长大机理分析，揭示Al-Ti-B细化剂的硅致失效机理，设计适用于高硅铝合金的高效异质形核剂并阐释其细晶机理，为开发高强韧铝合金提供了科学依据。主要结论如下：.①实验测定了细化相(MAl3、MB2，M=Ti,Nb)、硅化物相(二元M-Si，三元M-Al-Si)的相平衡，第一性原理计算细化相、硅化物相的稳定结构和生成焓，优化建立了CALPHAD型Al-Si-Ti-Nb-B五元系热力学数据库。.②热力学计算分析了α-Al形成前MAl3转变为硅化物的临界Si含量为9.5~10.5wt.%，Ti、Nb共掺杂扩大了MAl3稳定存在的温度及成分范围。通过原位同步辐射测定了MAl3-MB2的恒温转变动力学机理和MAl3、MB2的凝固相转变机理，MAl3转变为τ2或MSi2的过程符合扩散控速的三维长大机制，750℃的反应时长为5~15 min。.③分析了颗粒团聚效应对形核潜能和沉降行为的影响规律，基于自由生长模型推导了α-Al晶粒尺寸与细化剂颗粒尺寸、过冷度等因素之间的关系，建立铸态铝合金晶粒尺寸预测新模型，厘清了不同熔铸工艺下细化相对α-Al晶粒形核生长动力学的作用机制。.④考虑细晶效能、颗粒密度及成本，设计开发了新型Al-Ti-Nb-B细化剂，将Al-10Si合金细化至125μm；推广应用于Al-10Mg合金，细化效果提升2.5倍，延伸率提升125%。设计开发的La-Sr-B细化变质剂，解决了Sr变质剂和含B细化剂毒化问题，同时达到细化变质效果，提高Al-10Si延伸率100%。