Al-Nd intermetallic phase stability and its effects on mechanical properties and corrosion resistance of HPDC Mg-4Al-4Nd-0.2Mn alloy

Abstract

Microstructural stability is a crucial but controversial issue for Mg-Al-RE alloys. It is important to study the thermal stability of Al-RE intermetallic phase with individual RE element. In this work, Mg-4Al-4Nd-0.2Mn (AlNd44, wt%) alloy with individual RE element Nd was prepared by high-pressure die-casting (HPDC). The microstructure, microstructural stability and its effects on tensile properties and corrosion behavior were investigated. The results show that the HPDC AlNd44 alloy has two intermetallic phases, i.e., acicular Al11Nd3 and particulate Al2Nd, which are distributed at grain/dendritic boundary regions and form reticular structure. According to the quantitative X-ray diffraction phase analysis, the content of Al11Nd3 and Al2Nd phases is 2.5 wt% and 1.8 wt%, respectively. Both the two phases appear to be thermally stable at 473 K. While the acicular Al11Nd3 phase has a tendency to decompose at temperature up to 573 K, and would decompose to particulate Al2Nd phase thoroughly at 673 K. Moreover, Al2Nd phase would also partly decompose at temperature above 573 K. The alloy in as-cast state, with an intermetallic skeleton at grain/dendritic boundary regions, exhibits relatively high tensile properties and corrosion resistance. In contrast, the alloy after annealing at 673 K has the worst tensile properties and corrosion resistance due to the obvious phase transition and microstructure change.