Mechanism of mechanical alloying in Ni-Al and Cu-Zn systems

Abstract

Nanocrystalline Al₃Ni, NiAl and Ni₃Al phases in the NiAl system and the α, β, γ and ε phases in the Cu-Zn system were synthesized by mechanical alloying of elemental blends in a planetary mill. In the as-milled state, Al₃Ni and AlNi were always ordered, while Ni₃Al was disordered. MA results in a large extension of the NiAl and Ni₃Al phase fields particularly towards Al-rich compositions. The crystallite size was finest ( ~6 nm) when NiAl and Ni₃Al phases coexist after prolonged milling. In contrast, in all Cu-Zn blends containing 15-85 at.% Zn, the Zn-rich phases were first to form and final crystallite sizes were coarser (15-80 nm). Two different modes of alloying have been identified. In the case of NiAl and Al₃Ni, where the ball milled product is ordered and the heat of formation is large (ΔH_f > 120 kJ mol^-1), a rapid discontinuous mode of alloying accompanied with an additive increase in crystallite size is detected. In all other cases irrespective of the magnitude of ΔH_f, gradual diffusive mode of intermixing during milling seems to be the underlying mechanism of alloying.