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

Abstract

Solid state reactions induced by mechanical alloying (MA) of elemental blends 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. Nanocrystalline phase/phase mixtures of Ni(Si), Ni(Si)+Ni₃₁Si₁₂, Ni₃₁Si₁₂+Ni₂Si, Ni₂Si+NiSi and NiSi+Si, have been obtained during MA, over the composition 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, Ni₂Si, and NiSi form, while the formation of noncongruent melting phases, Ni₃Si, Ni₃Si₂, and NiSi₂, is bypassed in the nanocrystalline state. The phase formation during MA has been discussed based on thermodynamic arguments. The predicted phase fields obtained from effective free energy calculations are quite consistent with those obtained during MA.