Phase Transformation Crystallography of Lath Martensite
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Phase Transformation Crystallography of Lath Martensite Xiao Ma and R.C. Pond Department of Engineering, The University of Liverpool, Brownlow Hill, Liverpool, L69 3GH, United Kingdom ABSTRACT Our current understanding of martensitic transformations has been based on the Phenomenological Theory of Martensite Crystallography developed in the 1950s. Recently, a Topological Model of martensitic transformations has been presented wherein the habit plane is a semi-coherent structure, and the transformation mechanism is shown explicitly to be diffusionless. This approach is used here to model phase transformation crystallography of lath martensite in ferrous alloys. A range of network geometries is predicted corresponding to orientation relationships varying from Nishiyama-Wasserman to Kurdjumov-Sachs. Experimental observations from the literature of the dislocation and disconnection arrays, habit plane and orientation relationship are in good agreement with the model.
INTRODUCTION For many years the cornerstone of our understanding of martensitic transformations has been based on the hypothesis that the habit plane is an invariant plane of the shape transformation, as developed by WLR [1] and BM [2]. Experimental observations of the crystallography of a range of transformations are consistent with this notion, but several instances, including some ferrous alloys, are not. An alternative approach has been developed recently in terms of interfacial defects [3,4] and is referred to here as the “Topological Model” (TM). The TM is a description of the structure of the parent-martensite interface and the linedefects therein; the transformation proceeds by movement of transformation dislocations, or disconnections [5] as they are known, across the interface. This defect motion produces the transformation shear and can be shown explicitly to be diffusionless [6]. The objective of the present work is to account for the formation of lath martensite in ferrous alloys. The principles of the TM are set out in the next section, and subsequently applied to ferrous alloys. Finally, experimental observations of transformations in ferrous systems previously reported in the literature are compared with the present modelling.
TOPOLOGICAL MODEL A fundamental step in the TM of a transformation is to identify a candidate interface between the phases that exhibits coherency; this is refered to as a terrace plane. Feasible terrace planes in stiff engineering materials are expected to have relatively modest coherency strains. Once a terrace plane has been identified, the set of LID, (b, 0), and glissile disconnections, (b, h), that can arise therein can be determined using the theory of interfacial defects [7].
The coherency strains arising at a terrace plane must be relieved by arrays of interfacial defects. An array of appropriately oriented and spaced glissile disconnections can be one of these sets, and synchronous motion of this set can thereby provide the dual function of effecting the transformation and partially relievin
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