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I.

INTRODUCTION

THE fcc:bcc interface structures have been studied extensively for several decades. Although the initial efforts were focused on austenite:ferrite interfaces in ferrous alloys (where the fcc austenite normally cannot be preserved after quenching to room temperature), many of the results linking precipitate morphology and orientation relationship were found to be equally applicable to transformations in other alloy systems. The invariant line theory developed by Dahmen[2] and Dahmen and Westmacott[3] has come to play a key role in our understanding of many diffusion-controlled transformations. The invariant line preserves a perfect registry between the parent and product phases, leading to a higher growth rate in the invariant line direction and lath- or needle-shaped morphologies.[4] Typical fcc:bcc systems in this category include FeCu,[5,6] Cu-Cr,[7,8] Ni-Cr,[9,10] and dual-phase stainless steels,[11] all of which have an fcc/bcc lattice parameter ratio in the range of 1.25 to 1.27. The fcc and bcc crystals in these systems were found to be close to either the Kurdjumov–Sachs (K–S) orientation relationship (OR)[12] or the Nishiyama-Wassermann (N–W) OR.[13,14] Several other low-index ORs,[15] such as the Bain and Dunkirk[16] and Pitsch relationships,[17] that reflect the high symmetry of the fcc and bcc structures were also identified in some ferrous alloy studies. The general conclusions drawn from these studies are as follows. (a) The close-packed planes in the J.K. CHEN, Postdoctoral Fellow, formerly with the Department of Materials Science and Engineering, McMaster University, is with Exxon Research and Engineering Co., Clinton Township, Annandale, NJ 08801. G.R. PURDY and G.C. WEATHERLY, Professors, are with the Department of Materials Science and Engineering, McMaster University, Hamilton, ON, Canada L8S 4L7. A. KROUPA, Researcher, is with the Institute of Physics of Materials, Academy of Sciences, 616 62 Brno, Czech Republic. This article is based on a presentation made in the symposium ‘‘Kinetically Determined Particle Shapes and the Dynamics of Solid:Solid Interfaces,’’ presented at the October 1996 Fall meeting of TMS/ASM in Cincinnati, Ohio, under the auspices of the ASM Phase Transformations Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A

fcc and bcc phases are usually parallel to or nearly parallel to each other (the exceptions to this rule are found in the Bain and Dunkirk and Pitch ORs.) (b) The invariant line direction is close to the conjugate direction. (c) All of the interfaces consist of parallel structures, either dislocations or ledges, aligned along the invariant line direction.[9] Since the barrier to growth along the invariant line direction is low, the overall development of the precipitate morphology depends upon the ease of nucleating kinks laterally at the parallel growth ledges. This, in turn, varies with the defect population and the strain conditions on the interface.[18,19] Some 30 years ago, Ryan et al.[1] studied the crystallography of carbides and nitrid