(1. 清华大学 新材料国际研发中心，北京100084；
2. 清华大学 机械工程系，北京100084；
3. 清华大学 航天航空学院，北京100084)
摘 要: 运用固体与分子经验电子理论(EET)的计算结果分别建立Al-Si、Mg-Al两种合金熔体的剩余化学键的局域价电子结构基本单元即残余键结构模型，从压力对熔体结构作用的层面，分析热力学平衡相图中最大固溶点和共晶点发生偏移的本质原因，并指出压力作用下热力学平衡相图的变化是合金熔体微观结构信息变化的宏观反映。结果表明：压力作用抑制了合金熔体中Si-Si、Al12残余键结构的扩散速率，使其不容易“捕捉”到周围的残余键结构或游动原子，因而形成偏聚的可能性大为减小；而处于“悬空”状态的Al键会以周围Mg原子为“桥”将Al12残余键结构连接起来形成包含Mg17Al12结构的原子集团，为自发产生β(Mg17Al12)相凝固核心提供“有效晶胚”，从而使Mg-Mg、Mg-Al原子集团内Al原子的数量相对减少。
关键字: Al-Si合金；Mg-Al合金；压力；残余键结构模型；熔体结构；非平衡相图
（1. Advanced Materials International R&D Center, Tsinghua University, Beijing 100084, China;
2. Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China;
3. School of Aerospace, Tsinghua University, Beijing 100084, China）
Abstract:By using the calculated results of EET theory, the local valence electron structure elements of residual chemical bonds of Al-Si and Mg-Al alloy melts, that is, the residual bond structure models, were established. From the viewpoint of effect of pressure on melt structure, the essential reason of deviation phenomenon for maximum solution point and eutectic point in thermodynamics equilibrium phase diagram was analysed. Moreover, under pressure condition, the change of thermodynamics equilibrium phase diagram was macroscopic reflection of microstructure change of alloy melt. The results show that the pressure function reduces the diffusion rates of Si-Si and Al12 residual bond structures in alloy melt, making them not easy to capture the other residual bond structures and wandering atoms around them, so the possibility of segregation is decreased greatly. However, taking Mg atoms as “bridge”, the “dangling” Al bonds will connect with Al12 residual bond structures, forming the atomic clusters including Mg17Al12 structures, which supply “effective crystal embryo” for forming β(Mg17Al12) phase spontaneously, and reduce the Al quantity in the Mg-Mg and Mg-Al atom clusters.
Key words: Al-Si alloy; Mg-Al alloy; pressure; residual bond structure model; melt structure; non-equilibrium phase diagram
