Crystallographic theories, interface structures, and transformation mechanisms
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The structure and properties of an idealized planar interface that traverses a single crystal of the parent phase are first discussed. A (macroscopic) coherent interface is defined in terms of a relatively coarse-grained Larch6-Cahn network related to the observed shape deformation. Reconstructive and displacive transformations are distinguished, and a new category of "diffusional-displacive" transformations is introduced. The crystallographic theory of martensite requires that the habit plane interface is atom conserving (or "glissile"), but nonconservative ("epitaxial") interfaces may form in some diffusional-displacive transformations. A modified Eshelby procedure is used to discuss the strain energy of particles of a new phase forming, by any mechanism, inside a constraining matrix. It is shown that the effective Burgers vector of a step (or ledge) in a fully or partly coherent interface is dependent on the parameters of the shape deformation and increases with the ledge height. Multiple height ledges ("superledges') should only be observed if their fields have been effectively neutralized, either by averaging over displacement directions that are spatially distinct but crystallographically equivalent or by combining with lattice dislocations through processes essentially equivalent to emissary slip or climb. In the latter case, the shape discontinuity is effectively transferred from the interface into the matrix or to a surface. The use of invariant line theories and the concepts of growth, structural, and misfit ledges are also examined.
I.
INTRODUCTION
THIS article is an attempt to establish common ground among scientists interested in the growth of plate-shaped particles during reconstructive or displacive phase transformations in the solid state. Attention is focused on the nature of the interface in relation to the phenomenological theory of martensite crystallography (PTMC), transformation mechanisms, and observable shape changes. Many controversies in this field reflect linguistic rather than scientific difficulties, and to avoid this, I have inserted in the text a series of interlocking, dogmatic statements, each of which may be accepted or rejected by the reader. Most of these statements are resuits or theorems that, subject to specified approximations, are derivable from the basic assumptions of Euclidian geometry and Newtonian mechanics. The statements also include some definitions of terms to which other authors may have given different meanings. Thus, I hope that the statements contain only welldefined concepts; many are "obvious" results, but others may be challenged.* *The statements are here given in a slightly different order, and some have been slightly amended since the conference. A reader who immediately accepts (or rejects) the whole list as obvious (or trivial) will not need to read the intermediate material.
I initially called these statements "axioms," intending this simply to mean statements that command general
J.W. CHRISTIAN, Emeritus Professor, is with the Department of Materi
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