仿空间条件下Mg-25at%Zn亚包晶合金的快速凝固组织与微观力学性能

The rapid solidification mechanism of Mg-25at%Zn hypoperitectic alloy under simulated space conditions was studied by the drop tube technique. The microstructure of near-equilibrium solidification was characterized by primary α-Mg dendrite and interdendritic (α-Mg+MgZn) eutectoid structure. In the case of freely falling alloy droplets, theoretical calculations revealed that both cooling rate and liquid undercooling increased exponentially with the decrease of droplet diameter, and their maximums attained 9.78×10³ K s⁻¹ and 154 K (0.23T _L), respectively. As liquid undercooling increased, the nucleation and growth of the primary α-Mg phase and the following eutectoid transition were both suppressed, leading to the formation of metastable Mg₅₁Zn₂₀ peritectic phase. The nanoindentation measurements indicated that both hardness and Young’s modulus of (α-Mg+MgZn) eutectoid structure showed an increasing trend with the enhancement of liquid undercooling. The hardness of the eutectoid structure rose monotonously from 2.8 to 3.5 GPa, whereas that of Mg₅₁Zn₂₀ phase decreased slightly below 3.9 GPa. The Vickers hardness of alloy particles was enhanced from 203 to 283 HV with the rise of undercooling, which was attributed to the increased volume fraction of metastable Mg₅₁Zn₂₀ peritectic phase. Such a metastable phase with high hardness and low Young’s modulus can only be derived from rapid solidification which is quite senseful for improving mechanical properties.