Growth kinetics and mechanism of the massive transformation
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INTRODUCTION
TRANSITIONS between phases in the solid state can occur by a number of different mechanisms. One apparently simple class includes those transformations such as polymorphic phase changes, martensitic reactions, orderdisorder transitions, and massive transformations which involve changes in crystal structure without changes in composition. During a massive transformation a new structure forms during heating or cooling and grows through a noncooperative transfer of atoms across relatively high energy incoherent interfaces. With this process a single massive grain can grow at speeds exceeding 1 cm per second and absorb several parent phase grains during transformation. Since only a few jumps may be sufficient to transfer each atom from one structure to another, the relevant atomic mobility is limited to the interface region. Moreover, massive transformations exhibit nucleation and growth characteristics which are thermally activated. To portray the principal features of the massive transformation as related to growth kinetics, it is useful to identify the phase relations and thermodynamic conditions that have been determined to be compatible with the occurrence of a massive reaction. These features are of importance because the usual treatments that induce a massive reaction can also yield other transformation products. During continuous cooling or heating the resulting product microstructure is often a composite of the various possible competing transformations including the massive reaction. The governing phase relations and thermodynamics can be used effectively to rank the massive transformation and growth characteristics in terms of the relative driving free energy and to identify the stable and metastable equilibrium conditions prevailing during reaction. This basis is of further value in assessing a general nature of the atomistic events involved in interface transfer during the dynamic state of transformation. J. H. PEREPEZKO is Professor, Department of Metallurgical and Mineral Engineering, The University of Wisconsin-Madison, 1509 University Avenue, Madison, WI 53706. This paper is based upon a presentation made at a symposium on The Massive Transformation, held at the Pittsburgh meeting of The Metallurgical Society of AIME and the Materials Science Division of ASM, October 9, 1980, under the sponsorship of the MSD Phase Transformations Committee. METALLURGICALTRANSACTIONS A
The important result of a massive transformation is a microstructural constituent which is unique. In fact, the term massive as it was used originally was derived from the appearance of the product structure (i.e., blocky or massive). 2 However, in current usage the term massive growth is often applied also for rapid growth and has even been utilized to describe partitionless solidification. 3 In the present discussion consideration will be limited only to solid state massive reactions. In most early work the main features of growth were deduced from the morphological appearance. These features have been defined more cl
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