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

Nanocrystalline Al<SUB>3</SUB>Ni, NiAl and Ni<SUB>3</SUB>Al 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, Al<SUB>3</SUB>Ni and AlNi were always ordered, while Ni<SUB>3</SUB>Al was disordered. MA results in a large extension of the NiAl and Ni<SUB>3</SUB>Al phase fields particularly towards Al-rich compositions. The crystallite size was finest ( ~6 nm) when NiAl and Ni<SUB>3</SUB>Al 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<SUB>3</SUB>Ni, where the ball milled product is ordered and the heat of formation is large (ΔH<SUB>f</SUB> &gt; 120 kJ mol<SUP>-1</SUP>), 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<SUB>f</SUB>, gradual diffusive mode of intermixing during milling seems to be the underlying mechanism of alloying