Phase transformation kinetics and the assessment of equilibrium and metastable states

Abstract

The general characteristics of phase transformation kinetics during cooling and heating are introduced. It is demonstrated that the cooling process always depresses the phase boundaries away from equilibrium and towards lower temperatures; similarly, the heating process always shifts the phase boundaries to higher temperatures. Moreover, the larger the cooling or heating rate, the larger the discrepancy. According to this observation, the reliability of cooling and heating data (irrespective of the methods used to determine them: electrical resistance, dilatometry, etc.) for phase diagram assessments is discussed. The principle for correct assessment of metastable phase information is also briefly introduced. It is pointed out that each kind of metastable phase has its own transformation-start temperature, Ts (Ms, etc.), during cooling. More specifically, each kind of martensite has its own Ms. Examples are shown for Fe-Ni, Fe-Mn, and Ti-Cr alloys. © 1993 ASM International.