Massive transformation and absolute stability
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I. INTRODUCTION
A MASSIVE transformation is a composition-invariant reaction, i.e., a transformation in which the product and the parent phases have the same composition. In recent years, much work has been undertaken in order to clarify the mechanisms of this phase transformation. There are two major topics in massive transformations that are still debated; (a) the structure of the interface and (b) the precise mechanism leading to composition invariance. A general theory of this transformation must deal with both aspects, but such a model is not yet available. In order to contribute to the understanding of this interesting phase transformation, we propose in this article to disconnect these two topics and to concentrate only on the second aspect, i.e., the composition invariance during growth. Phase transformation theory predicts that a homogeneous transformation product of the same composition as the parent phase may grow under two limiting conditions, which are (a) complete solute trapping due to loss of local equilibrium and (b) steady-state plane-front growth at local equilibrium. In previous experiments, evidence for both mechanisms has been found. Perepezko and co-workers[1,2,3] showed that massive transformations occur under conditions far from local thermodynamic equilibrium of the interface, typical for partitionless transformation. As a consequence, they proposed T0 as the upper limiting temperature. On the other hand, Hillert[4] proposed that the lower solvus temperature (␣ /␣ ⫹ ␥ limit of the ␥ → ␣ transition in Fe alloys) should MILTON LIMA, formerly with the Department of Materials, Swiss Federal Institute of Technology, Lausanne, is Research Metallurgist with the Center for Laser and Applications, IPEN, 05508-900 Sao Paulo, Brazil. WILFRIED KURZ, Professor, is with the Department of Materials, Swiss Federal Institute of Technology, Lausanne, 1015 Lausanne EPFL, Switzerland. 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
be the growth temperature. This implies that local equilibrium may also lead to composition invariance. The main difference between these two approaches is that, in the second case, a concentration spike at the interface (of extent D/V ) is formed, which, due to its limited spatial resolution, might not be detected by a microprobe. Later, Hillert and Schalin[5] examined the effect of solute drag on the massive transformation and found that “without dynamic nucleation the natural limit for massive transformation is the phase boundary for the new phase.” In their isothermal experiments with Fe-Ni alloys, Borgenstam and Hillert[6a,6b] found that at the higher temperatures growth occurred close to the ␣ /␣ ⫹ ␥ limit, and at lower temperatures, the transformation took place inside the two-ph
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