Synthesis of stable and metastable phases in the Ni[sbnd]Si system by mechanical alloying

Abstract

Mechanical alloying (MA) of NiSi elemental powder blends was undertaken to synthesize the NiSi and NiSi2 compounds having a very narrow homogeneity range, a difficult situation to achieve by conventional melting and solidification methods. Elemental powder blends corresponding to Ni75at.% Si and Ni80at.% Si compositions, were subjected to MA in a high-energy SPEX shaker mill to study phase evolution as a function of milling time. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) techniques were used to characterize the phases present in the as-milled powders. It was noted that a supersaturated solid solution had formed in the early stages of milling containing up to 18.2at.% Si and 20.5at.% Si in the Ni75at.% Si and Ni80at.% Si blends, respectively. After 2h of milling, the equilibrium intermetallic NiSi phase started to form, and its amount increased with increasing milling time. On continued milling for about 4h, the NiSi phase that had formed earlier and the remaining free Si powder reacted to form the equilibrium intermetallic NiSi2 phase. Although, under equilibrium conditions, a mixture of both NiSi2 and Si phases is expected to be present in both the Ni75at.% Si and Ni80at.% Si compositions investigated, only the NiSi2 phase was present in both the powder blends after 15h of milling. This constitution in the milled powder has been associated with a partial loss of Si content during milling of the powder blends, confirmed by EDS analyses. Reasons for the observed formation of supersaturated solid solutions and the increased homogeneity range for the NiSi2 intermetallic have been explained.