Critical limit for massive transformation
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I. INTRODUCTION
THE massive transformation is defined as the reaction by which a one-phase alloy transforms into a new phase by the growth of blocky or massive grains, such that the whole volume of material may transform. The early work was reviewed in 1968 by Massalski,[1] and recent work to determine where in a phase diagram the massive transformation may be observed was reviewed in 2000 by Borgenstam and Hillert.[2] This aspect will be further discussed in the present article, and it will be related to phenomena occurring in the advancing phase interface or in front of it. The massive shape requires that sidewise diffusion be negligible and the growth velocity does not vary much with time or direction and is not much affected by crossed grain boundaries. Thus, it is generally agreed that the massive transformation is partitionless, at least after an initial transient period, i.e., the growing phase (␣) inherits the initial composition of the parent phase (␥), and it takes place by the migration of an incoherent phase interface. However, it has been emphasized by Aaronson et al.[3] that the new phase may have a crystalline orientation related to the parent phase as a memory of the nucleation process. Such cases should be allowed within the definition if the velocity is sufficiently independent of direction to yield massive crystals. On the other hand, it is evident that under the right conditions, any type of interface can result in a partitionless transformation. One extreme case is the martensitic interface, which is glissile and can result in very high growth velocities, so high that there is no time for individual atomic movements. That transformation will be not only partitionless but even diffusionless. However, it will not be classified as massive because the velocity will depend strongly on direction, and an interface may even stop without impinging on other units. The other extreme would be a perfectly incoherent interface which advances with the same velocity in all directions and stops only by impingement. Its mobility is much lower, and there may be time for local diffusion of atoms inside the migrating interface and in a spike in front of the advancing interface. One may classify the result as the “ideal massive MATS HILLERT, Professor Emeritus, is with the Department of Materials Science, KTH, SE-10044 Stockholm, Sweden. 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
transformation.” Between the two extremes, one can imagine a whole series of partly coherent interfaces, all of which give rise to partitionless transformations. The shape of the interface may be smoothly curved, jagged, or acicular. It does not make much sense to try to define exactly what deviation from “smooth” would be allowed within the definition of the massive transfor
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