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