SYNTHESIS OF NANOCRYSTALLINE ALLOYS AND INTERMETALLICS BY MECHANICAL ALLOYING

Nanocrystalline Al3Ni, NiAl and Ni3Al phases in Ni-Al system and the a lpha, beta, gamma, epsilon 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 Ni3Al was disordered in the milled condition. MA results in large ext ension of the NiAl and Ni3Al phase fields, particularly towards Al-ric h compositions. Al3Ni, a line compound under equilibrium conditions, c ould be synthesized at nonstoichiometric compositions as well by MA. T he phases obtained after prolonged milling (30 h) appear to be insensi tive to the starting material for any given composition > 25 at.% Ni. The crystallite size was finest (similar to 6 nm) when NiAl and Ni3Al phases coexisted after prolonged milling. In contrast, in all Cu-Zn bl ends containing 15 to 85 at.% Zn, the Zn-rich phases were first to for m, and the final crystallite sizes were coarser (15-80 nm). Two differ ent 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 for mation (Delta H-f) 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 Del ta H-f, a gradual diffusive mode of intermixing during milling seems t o be the underlying mechanism of alloying.