ALLOY METASTABILITY DURING NUCLEATION-CONTROLLED REACTIONS

The initial stage of a first order transformation reactions is control led by a nucleation process. In fact, the kinetic constraints provided by nucleation and growth difficulties of the new phase provide the ne cessary condition to expose metastable reaction pathways. The resultin g hierarchy of metastability is viewed in terms of the severity of the imposed kinetic constraints. The identification of nucleation control has been extended to cover solid state reactions such as those involv ed in solid state amorphization reactions (SSAR) of so-called driven s ystems. Some of the fundamental characteristics of alloy metastability under kinetic constraints are illustrated by experimental observation s and model analyses that highlight the current understanding.