Thermally assisted solid state amorphization in rod-milled Al 50Hf 50 composite

Abstract

New amorphous Al 50Hf 50 alloy powders have been synthesized by thermally 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 Al 50Hf 50 alloy has been investigated by means of differential thermal analysis, X-ray diffraction, scanning electron microscopy and transmission electron microscopy. During the first few kiloseconds of the mechanical deformation via the rod milling technique, multi-layered composite particles of Al and Hf are intermixed and form an amorphous phase when heated at about 931 K. After 86 ks of rod milling, the maximum heat of formation of the amorphous Al 50Hf 50 alloy ( ΔH a) was measured directly and was found to be −5 kJ mol −1. The crystallization temperature ( T n) and the enthalpy change of crystallization ( ΔH x) of the amorphous phase formed via the TASSA process were found to be 1000 K and −20 kJ mol −1 respectively. For comparison with the TASSA process, an amorphous Al 50Hf 50 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 ΔH x of the MDSSA Al 50Hf 50 alloy powders were found to be 1100 K and −60 kJ mol −1 respectively. The role of the amorphization of the Al 50Hf 50 alloy powders via TASSA and/or MDSSA has been discussed.