MECHANISM OF MECHANICAL ALLOYING IN NI-AL AND CU-ZN SYSTEMS

Nanocrystalline Al3Ni, NiAl and Ni3Al phases in the Ni-Al system and t he alpha, beta, gamma and epsilon phases in the Cu-Zn system were synt hesized by mechanical alloying of elemental blends in a planetary mill . in the as-milled state, Al3Ni 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 crysta llite size was finest (similar to 6 nm) when NiAl and Ni3Al phases coe xist after prolonged milling. In contrast, in all Cu-Zn blends contain ing 15-85 at.% Zn, the Zn-rich phases were first to form and final cry stallite sizes were coarser (15-80 nm). Two different modes of alloyin g have been identified. In the case of NiAl and Al3Ni, where the ball milled product is ordered and the heat of formation is large (Delta H- f > 120 kJ mol(-1)), a rapid discontinuous mode of alloying accompanie d with an additive increase in crystallite size is detected. In all ot her cases irrespective of the magnitude of Delta H-f, gradual diffusiv e mode of intermixing during milling seems to be the underlying mechan ism of alloying.