Phase equilibria and intermetallic phases in the Ni-Si-Mg ternary system

Abstract

The Ni-Si-Mg ternary phase diagram has been established after homogenization and slow cooling to room temperature. The chemical compositions of the alloys and their phases were obtained using fully quantitative energy dispersive X-ray spectroscopy (EDS) with standard spectrum files created from intermetallic compounds Mg2Ni and Ni2Si. The following intermetallic phases have been observed: (a) four new ternary intermetallic phases, designated as ν, ω, μ, and τ, (b) a ternary intermediate phase Mg(Ni,Si)2 based on the binary MgNi2 phase containing Si; (c) three ternary intermetallic phases, η, κ, and ζ, previously reported by the present authors;[10] and (d) Mg2SiNi3 (Fe2Tb type),[9] previously reported by Noreus et al.[8] The MgNi6Si6 phase, which was also previously reported,[7] was not observed at the corresponding composition in the present work. However, the MgNi6Si6 phase reported as being of hexagonal symmetry (Cu7Tb type),[9] with the lattice parameters a=0.4948 nm and c=0.3738 nm, possibly corresponds to the μ phase (Mg(Si0.48Ni0.52)7) discovered in the present work. The lattice structure of the newly discovered ω phase was determined with the help of the X-ray indexing program TREOR (developed by Werner et al.[13]) to be a hexagonal structure of the Ag7Te4 type ((Mg0.52Ni0.48)7Si4) with the lattice parameters a=1.3511 nm and c=0.8267 nm.