Phase fields of nickel silicides obtained by mechanical alloying in the nanocrystalline state

Solid state reactions induced by mechanical alloying (MA) of elemental blen ds of Ni and Si have been studied over the entire composition range of the Ni-Si system. A monotonous increase of the lattice parameter of the Ni rich solid solution, Ni(Si), is observed with refinement of crystallite size. N anocrystalline phase/phase mixtures of Ni(Si), Ni(Si)+Ni31Si12, Ni31Si12+Ni 2Si, Ni2Si+NiSi and NiSi+Si, have been obtained during MA, over the composi tion ranges of 0-10, 10-28, 28-33, 33-50, and > 50 at. % Si, respectively. The results clearly suggest that only congruent melting phases, Ni31Si12, N i2Si, and NiSi form, while the formation of noncongruent melting phases, Ni 3Si, Ni3Si2, and NiSi2, is bypassed in the nanocrystalline state. The phase formation during MA has been discussed based on thermodynamic arguments. T he predicted phase fields obtained from effective free energy calculations are quite consistent with those obtained during MA. (C) 2000 American Insti tute of Physics. [S0021-8979(00)00412-6].