Effect of Ni content on the microstructure, microhardness and fracture toughness of magnesium silicide ternary alloy

Abstract

The microstructure, microhardness and fracture toughness of the as-solidified intermetallic Mg 2Si alloyed with the Ni range from 1 to 80 at.% were systematically studied. Three microconstituents were found in the microstructure of the alloys with up to 60 at.% target Ni, i.e. Mg 2Si, Mg(Ni, Si) 2 and the Mg-rich eutectic (Mg Mg 2Si). The microstructure of Mg 2Si-80 at.% Ni alloy consists of a phase mixture of (Mg, Si)Ni 2 and Ni(Si) (solid solution). The Mg(Ni, Si) 2 and (Mg, Si)Ni 2 phases seem to have the same crystal structure as the MgNi 2 intermetallic and both the Mg and Ni elements in the MgNi 2 phase can be partly replaced by the Si element. Both the microhardness and the indentation fracture toughness of the Mg 2Si phase slightly increase with increasing Ni content in the alloys up to 40 at.% of target Ni, which is mainly due to the increase of Ni content in the Mg 2Si phase. The chevron-notch beam fracture toughness (four-point bending) increases with increasing Ni content in the as-solidified ternary alloys. Scanning electron microscopy fractographic observation of the fracture surface of chevron-notch specimens showed that all the phases fractured in a cleavage mode. No general plasticity is observed in either compression or four-point bending of Mg 2Si Ni alloys. The fracture properties of the alloys are also discussed in terms of the microcracking development.