Mechanism of mechanical alloying in Ni [sbnd]Al and Cu [sbnd]Zn systems

Abstract

Nanocrystalline Al 3Ni, NiAl and Ni 3Al phases in the Ni Al 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, Al 3Ni and AlNi were always ordered, while Ni 3Al was disordered. MA results in a large extension of the NiAl and Ni 3Al phase fields particularly towards Al-rich compositions. The crystallite size was finest ( ∼ 6 nm) when NiAl and Ni 3Al 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 Al 3Ni, 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.