Formation of invariant plane-strain and tent-shaped surface reliefs by the diffusional ledge mechanism
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INTRODUCTION
AN invariant plane-strain (IPS) surface-relief effect formed where a plate-shaped transformation product intercepts a free surface polished fiat prior to transformation has long been accepted as a defining characteristic of a shear or martensitic phase transformation. ~-4] Diffusional transformations have also been found, however, to be capable of producing IPS reliefs. Changes in long-range order and/or in composition (the latter particularly in substitutional alloys) relative to the matrix phase taking place continuously throughout the transformation process are usually acceptable criteria for diffusional transformationsJ 5'6] A prototypical example of a drastic change in long-range order across the transformation interface accompanying an IPS surface relief is represented by the formation of AuCu II plates from a disordered equiatomic Au-Cu matrix. 17] Counterpart examples of a marked change in composition formed in conjunction with an IPS relief are the precipitation of 3/ (later 3,) A1Ag2 plates from a AI-Ag solid solutions t8,91 and of a~ Cu-Zn plates from ordered fl Cu-Zn. It~ There do not appear to have been any questions raised about the long-range order change in AuCu II or about the composition change in 3' or 3/ A1Ag2. However, the question as to whether or not a~ Cu-Zn plates inherit the composition of their ordered fl matrix has been under active dispute in the literature for some time. In these proceedings, though, Hamada e t al. I~~ have evidently brought to an end the long controversy about the composition of aL plates at early growth times by demonstrating that even when these plates are only 3-nm thick, their composition differs significantly from that of their matrix phase. Also in these proceedings, Muddle t~~j confinns that the transformation crystallographies of M.G. HALL, Lecturer, is with the School of Metallurgy and Materials, University of Birmingham, Birmingham, B 15 2TP, United Kingdom. H.I. AARONSON is with Geo-Centers, Inc., Ft. Washington, MD, and is stationed in the Physical Metallurgy Branch, Code 6320, the Naval Research Laboratory, Washington, DC 203755343. This article is based on a presentation made at the Pacific Rim Conference on the "Roles of Shear and Diffusion in the Formation of Plate-Shaped Transformation Products," held December 18-22, 1992, in Kona, Hawaii, under the auspices of ASM INTERNATIONAL's Phase Transformations Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
AuCu II, 1' A1Ag2, and a~ Cu-Zn plates are all in good agreement with the predictions of the phenomenological theory of martensite crystallography (PTMC). ~ Hence, the presence of an IPS surface-relief effect no longer serves to identify uniquely the operation of a shear transformation mechanism. A further problem that has arisen in connection with the use of surface reliefs to identify the atomic mechanism of plate formation is the appearance of surface reliefs that are not an IPS but that are nonetheless composed of tilted planar surfaces (rather than the curved surfaces implied
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