Synthesis of nanocrystalline alloys and intermetallics by mechanical alloying

Abstract

Nanocrystalline Al3Ni, NiAl and Ni3Al phases in Ni-AI system and the α, β, γ, ε and deformation induced martensite in Cu-Zn system have been synthesized by mechanical alloying (MA) of elemental blends in a planetary mill. Al3Ni and NiAl were always ordered, while Ni3 Al was disordered in the milled condition. MA results in large extension of the NiAl and Ni3Al phase fields, particularly towards Al-rich compositions. Al3Ni, a line compound under equilibrium conditions, could be synthesized at nonstoichiometric compositions as well by MA. The phases obtained after prolonged milling (30 h) appear to be insensitive to the starting material for any given composition > 25 at.% Ni. The crystallite size was finest (∼ 6 nm) when NiAl and Ni3Al phases coexisted after prolonged milling. In contrast, in all Cu-Zn blends containing 15 to 85 at. % Zn, the Zn-rich phases were first to form, and the final crystallite sizes were coarser (15-80 nm). Two different modes of alloying have been identified. In case of NiAl and Al3Ni, where the ball milled product is ordered, as well as, the heat of formation (ΔHf) is large (> 120 kJ/mol), 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 ΔHf, a gradual diffusive mode of intermixing during milling seems to be the underlying mechanism of alloying.