THERMALLY ASSISTED SOLID-STATE AMORPHIZATION IN ROD-MILLED AL50HF50 COMPOSITE

New amorphous Al50Hf50 alloy powders have been synthesized by thermall y assisted solid state amorphization (TASSA) in mechanically deformed rod-milled AlHf composite particles, using a mechanical alloying (MA) process. The mechanism of TASSA for the formation of amorphous Al50Hf5 0 alloy has been investigated by means of differential thermal analysi s, X-ray diffraction, scanning electron microscopy and transmission el ectron microscopy. During the first few kiloseconds of the mechanical deformation via the rod milling technique, multi-layered composite par ticles of Al and Hf are intermixed and form an amorphous phase when he ated at about 931 K. After 86 ks of rod milling, the maximum heat of f ormation of the amorphous Al50Hf50 alloy (DELTAH(a)) was measured dire ctly and was found to be -5 kJ mol-1. The crystallization temperature (T(x)) and the enthalpy change of crystallization (DELTAH(x)) of the a morphous phase formed via the TASSA process were found to be 1000 K an d -20 kJ mol-1 respectively. For comparison with the TASSA process, an amorphous Al50Hf50 alloy has been formed directly after a longer rod milling period (720 ks), without heating the composite particles. Thus , the amorphization process is attributed to mechanically driven solid state amorphization (MDSSA). The thermal stability characterized by T (x) and DELTAH(x) of the MDSSA Al50Hf50 alloy powders were found to be 1100 K and -60 kJ mol-1 respectively. The role of the amorphization o f the Al50Hf50 alloy powders via TASSA and/or MDSSA has been discussed .