Comments Concerning Some Features of Phase Diagrams and Phase Transformations

Abstract

Several interesting features in the study of stabilities of phases, and in phase transformations, are discussed. It is proposed that symmetry considerations related to the presence of magnetism in iron suggests that the respective phases, BCC alpha and FCC gamma, have in fact lower symmetries than cubic. A proposal is made that the symbol beta used in the past for the designation of the paramagnetic BCC iron should perhaps be returned as a feature in phase diagrams. The importance of the new concept of a ‘pseudogap’ in the electronic band structure, as a stabilizing electronic feature, is discussed in the light of the Hume-Rothery electron concentration rule. It is proposed that since the thermal activation is a major feature in the behavior of isothermal martensites, a more suitable designation for these types of phase transformations might be “thermally activated martensites”, or TAMs. Massive transformations are discussed briefly and it is emphasized that they present a specific example of an idiomorphic transformation process, not requiring the need for orientation relationships (ORs) between the parent and product phases.