Metastable phase state during mechanical alloying

Abstract

The interstitial mechanism of interdiffusion is effective under the conditions of powder treatment in high-energy ball mills. The major arguments to support this point of view are the following: (1) intermetallic phases are formed in sequences strictly determined by the asymmetry of the partial diffusion coefficients, the first to emerge being an intermetallic phase based on the low-mobile component; (2) the experimental evidence demonstrating the similarity of diffusion mechanisms in Mechanical Alloying (MA) and Solid State Reactions (SSR) in the course of annealing of diffusional pairs. The asymmetry of partial diffusion coefficients and the interstitial diffusion mechanism during annealing are determined by thermal activation, while in MA, the same is due to mechanical activation. Finally (3), our approach is based on the theory of Indenbom and Orlov explaining the plasticity mechanisms by the generation and movement of interstitial atoms under large or impact strains at low temperatures. The formation of certain phases during MA can be delayed due to the lack or insufficiency of vacancy mobility, if the thermodynamic driving force is much too low to be able to overcome the energy barriers for nucleation and growth. In the Cr-Ti -system, formation of stable Laves- phase and amorphization occur during post -MA annealing.