Solid-state amorphization reaction in mechanically deformed AlxHf100-x multilayered composite powders and the effect of annealing

Single phase amorphous AlxHf100-x alloys with a wide amorphization range (33 less than or equal to x less than or equal to 75) were synthesized by the solid-state interdiffusion of pure polycrystalline Al and Hf powders at room temperature using a rod-milling technique. The mechanisms of metallic glass formation and competing crystallization processes in the mechanically deformed composite powders were investigated by means of X-ray diffraction, differential thermal analysis, scanning electron microscopy and transmission electron microscopy. The numerous intimate layered composite particles of the diffusion couples that formed during the first and intermediate stages of milling (0-173 ks) are intermixed to form amorphous phase(s) upon heating to about 980 K by so-called thermally assisted solid-state amorphization (TASSA). The amorphization heat formation for the binary AlxHf100-x system via TASSA, Delta(a)(TASSA), was measured directly as a function of the milling time. Homogeneous amorphous alloys were also fabricated directly without heating the composite multilayered particles after milling the particles for a longer milling time (360-720 ks). This amorphization reaction is attributed to mechanical driven solid-state amorphization (MDSSA). The maximum heat formation of amorphization for the binary AlxHf100-x system via MDSSA, Delta H-a(MDSSA), was estimated. The crystallization characteristics indexed by the crystallization temperature, T-x, and the enthalpy change of crystallization, Delta H-x, were measured for the amorphous alloys formed either by the TASSA (T-x(TASSA) and Delta H-x(TASSA)) or the MDSSA (T-x(MDSSA), and Delta H-x(MDSSA)) processes. The roles of amorphization and crystallization in each process are discussed. (C) 1999 Elsevier Science S.A. All rights reserved.