Mechanisms of the massive transformation
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INTRODUCTION
THE massive transformation (MT) is here defined as a phase transformation taking place by diffusional nucleation and growth during which the matrix phase is converted to a product phase with the same composition but a different crystal structure by means of diffusional jumps across areas of massive:matrix boundaries whose structure permits such jumps to be accomplished. Three groups of issues have long formed the core of debates about this transformation. The first is whether the MT can take place below To (the temperature at which the free energies of the matrix and product phase are equal in they absence of strain energy[1]) within the appropriate two-phase region.[2–5] The second concerns the role, if any, played by crystallography in the nucleation and growth processes involved in the MT.[3,5–9] When the homologous temperature region under consideration is sufficiently high for a transformation involving only transinterphase boundary diffusion to occur at detectable rates, the third issue is whether the existence of a MT can be deduced from the existence of a phase transformation in a pure element or from the configuration of a phase diagram.[10] Following consideration of these issues, a synthesis of them will be used to offer an overall view of how the MT takes place—particularly in a two-phase region below To, where competition from precipitation of the equilibrium phase sharply limits the assumptions allowable as to the transformation mechanisms involved. Consideration of the phase diagram configurations under whose aegis the MT can occur will be delayed until near the end of this review in order to take advantage of recent improvements in the understanding of this transformation. Similarly, because recent studies on massive:matrix interfacial structure will be seen to have provided new insight into H.I. AARONSON, R.F. Mehl University Professor Emeritus, is with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213-3890, and Visiting Professor, School of Physics and Materials Engineering, Monash University, Victoria 3800, Australia. Contact e-mail: [email protected] This article is based on a presentation made at the symposium entitled “The Mechanisms of the Massive Transformation,” a part of the Fall 2000 TMS Meeting held October 16-19, 2000, in St. Louis, Missouri, under the auspices of the ASM Phase Transformations Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
the nature of this structure, consideration of the nucleation of the MT will be delayed until after growth aspects of this transformation have been reviewed. II. OCCURRENCE OF THE MT WITHIN TWOPHASE REGIONS A. Thermodynamic and Kinetic Bases That occurrence of the MT within a two-phase field at temperatures below To is at least thermodynamically permissible has long been recognized.[5] If volume diffusion kinetics in this temperature range are very slow, as in Fe-Ni alloys, precipitation of the equilibrium (or a metastable equilibrium) product phase will not be able to comp
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