New intermetallic phases in the NiSiMg ternary system

Abstract The isothermal sections of the NiSiMg ternary phase diagram in the Ni-rich region were studied after slow cooling to room temperature from the homogenization temperature (900 or 1000 °C) as well as after isothermal annealing at 500 and 900 °C followed by water quenching. The phase equilibria observed at room temperature and 500 °C are essentially identical. However, those observed in the Si-rich region at 900 °C are different than phases existing up to 500 °C. This difference seems to be related to the phase transformation occuring around 820 °C in the NiSi binary system in the compositional range in between δ-Ni2Si and NiSi. The three newly discovered ternary phases were arbitrarily designated as η, κ, and ζ. The lattice structures of the ternary single phases except ζ were determined with the help of the X-ray pattern indexing program, TREOR. The lattice structure of η was determined to be a cubic structure with the lattice parameters, a = b = c = 11.28080 A and a = b = c = 11.30834 A using the two near single phase alloys in the η phase region. The compound η belongs to the structure type of Mn23Th6 (also called Mg6Cu16Si7). The lattice structure of the κ phase (stoichiometry: MgNi8Si5.5) is proposed to be a hexagonal structure with the lattice parameters, a = b = 11.6227 A and c = 11.6502 A .