Phase Transformations in Metals and Alloys

Understanding a phase transformation involves an appreciation of the reasons for its occurrence and of the mode or modes by which it takes place. The former will not be explored herein beyond noting that a system can spontaneously undergo a change in phase (or phases) if by so doing its free energy is reduced. Provided a reduction in free energy ensues, the product of the transition need not be that pertaining to the equilibrium state. A total change may yield directly the equilibrium state or traverse a path composed of several tranformations involv­ ing metastable phases. This, combined with the fact that a given change can be accomplished by more than one kinetic mechanism generating modal competi­ tion, endows the study of phase transformations and their consequences with the complexities that render it so intriguing. Phase transitions have received exten­ sive investigation because of their inherent fascination and because they play a particularly important role in property control for material applications. As can be seen from some of the more recent surveys of the field (1-3) it is customary to group together changes that exhibit common characteristics in an effort to classify phase transformations in a systematic manner. Criteria involved in classification are predominantly of morphological character but also include, or imply, mechanistic processes. This process has led to the establishment of accepted group names: 1. continuous precipitation, 2. massive transformations, 3. discontinuous precipitation, 4. martensitic transformations, 5. bainitic trans­ formations, 6. order-disorder transformations, and 7. spinodal decomposition. Because of an initial restriction on the length of this review, order-disorder transformations and spinodal decomposition will not be considered here, but they have been discussed by Wayman (4) in a previous volume of this series. Continuous precipitation is characterized by the formation of grain boundary and intragranular particles of the new phase, which generally has a structure and