Thermal stability of nanocrystalline Ni silicides synthesized by mechanical alloying

Solid state reactions induced by mechanical alloying followed by isothermal annealing in elemental blends of Ni and Si have been studied over the entire composition range of the Ni–Si system. Above a critical crystallite size, the congruent melting phases, γ-Ni31Si12 and NiSi, become unstable and react with second phase available to form equilibrium non-congruent, β1-Ni3Si and α-NiSi2, after isothermal annealing at 723 and 1023 K, respectively. Formation of equilibrium ε-Ni3Si2 has not been observed in the composition range 33–50at.% Si even after annealing at 1073 K. The results also indicate that the crystallite size of non-congruent melting phases is well above 100 nm even at the stage of their evolution, suggesting that they are probably stable only in the bulk state. The grain growth is faster for the non-congruent phases when compared to that of congruent melting compounds. The interfacial energy of the compounds plays an important role in controlling their stability and grain growth in the nanocrystalline state.