Workshop on first-order displacive phase transformations: Review and recommendations

Abstract

The development of technologically useful microstructure-property relationships in materials almost invariably involves phase transformations, and successful optimization depends on a detailed understanding of the transformation mechanisms [1-4]. There are two primary phase transformations in crystalline solids: replacive and displacive [5-8]. Replacive transformations involve the local and/or long-range thermally activated diffusional rearrangements of atoms on lattice sites [5, 6]; these processes are well understood and are applied with consistent reliability. Displacive transformations proceed by means of small cooperative displacements of atoms from their lattice sites (i.e. less than an interatomic distance, but much larger than the amplitude of lattice vibrations) to alter crystal symmetry without altering the atomic order or composition [5-7]. Our understanding of these processes is limited in comparison [8-11 ]. In metals, alloys and ceramics, the important displacive transformations are first-order transformations [5-7]. They range from being weakly to being strongly first order, where this variation has a marked effect on their characteristic transformation behavior, and hence, requires different approaches to analyze the different mechanisms. Now more than ever, a definitive understanding of these mechanisms is vital since these displacive transformations figure prominently in many new advanced materials (e.g. pertaining to the stability of semiconductor and metallic heterostructures or to the structural transformations of the high T~ oxide superconductors [12]). The workshop program covered the latest experimental observations and theoretical thinking related to first-order displacive transformations in a wide variety of metallic and non-metallic materials. The central issue, as emphasized above, was on the mechanisms, i.e. how the transformations initiate and proceed. Naturally these questions can only be answered if the properties of both parent and product phases are known, particularly in proximity to the transformations themselves. The program thus included sessions dealing with the calculation of the free energies of structures and with their experimental determination. The remaining sessions, constituting the bulk of the program, dealt with precursor phenomena and their potential relevance to transformation mechanisms. The following is a review of the significant conclusions and recommendations of the workshop.