Processing Control for Solid State Amorphization of CoZr by Reaction Ball Milling

Abstract

The processing control based on the formation of multilayers and the kinetics of a solid state amorphizing transformation by mechanical alloying(MA) of Co₈₅Zr₁₅ is proposed, from which one can optimize the conditions for an efficient and high quality production of amorphous powders using a rotating-arm reaction ball mill with a relatively high rotational speed(R) above 400 rev min^-1. For R=400 and 450 rev min^-1, the use of a higher flow rate of cooling water(C_w) above 830 ?? h^-1 leads to the occurence of a plateau of attrition tempeature, although it is short, prior to an exothermic reaction; this MA process is almost conducive to a two-stage amorphization with Johnson-Mehl-Avrami exponent(n) of n₁=1.4 and n₂=2.6. While, the MA with C_w<150 ?? h^-1 without the plateau shows an apparently single value of n=2.0; this process is denoted as a mixed stage amorphization which includes the formation of a crystalline solid solution. A rotational speed change method consisting of the ball milling by the end of the plateau using R=300 rev min^-1 and an exothermic reaction by MA using of 400<R<500 rev min^-1 leads to the two-stage amorphization even for a lower C_w of 20 ?? h^-1. With increasing R from 300 to 450 rev min^-1, the amorphization rate is found to decrease. The activation energy for the amorphization of Co₈₅Zr₁₅ by MA using R=400 rev min^-1 is derived as 2.0±0.5 eV; this value is slightly higher than 1.7 eV for with R=300 rev min^-1.