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1. Materials Science and Engineering, University of Liverpool, Liverpool, L69 3GH, UK. 2. Dept. of Materials, University of Oxford, Oxford, OXI 3PH, UK 3. IRC in Materials, University of Birmingham, Birmingham, B15 2TI, UK. 4. Dept. of Metallurgy and Materials Eng., University of Connecticut, Storrs, CT 06269, USA. ABSTRACT The topological theory of interfacial defects and the associated flux analysis are reviewed. It is shown that the shears and diffusive fluxes associated with the motion of disconnections can be determined directly from their crystallographic characteristics, and that the effects of changes in chemical composition, interfacial misfit and ordering can be incorporated into the analysis. The special conditions are identified for which there is conservation of atomic sites during the motion of disconnections. It is shown that, under these circumstances, disconnection motion may result in mixed-mode displacive-diffusive transformations whereby diffusion is required for the transformation to proceed but the interfaces exhibit crystallographic characteristics which one would normally associate with a martensitic transformation. It is shown that the growth of y lamellae in TiAl-based alloys is an example of such a mixed-mode transformation. INTRODUCTION Although phase transformations in metals are classified conventionally as either martensitic (i.e. diffusionless) or diffusional, there are many instances of transformations which exhibit some characteristics of both modes. For example, many precipitates which must form by diffusional processes (since a pronounced change in chemical composition is produced) also exhibit a lattice correspondence of the type which one might expect for a martensitic product [1,2]. This issue has been addressed in some detail recently [3-5], and it has been shown that, under some circumstances, a lattice correspondence can be retained during a diffusional transformation giving a product phase orientation and a surface relief which are consistent with the predictions of the phenomenological theory of martensite crystallography (PTMC) [6,7]. Such mixed-mode transformations have been described as "displacive-diffusive" by Christian [4] who highlighted the as-yet unresolved question of why the PTMC is applicable to diffusional transformations wherein a glissile interphase boundary structure is not needed. When considering the role of the interfacial defects in such transformations it is necessary to recall that interfacial defects are characterised by two parameters, the Burgers vector, b and the step height, h (e.g. [8]). The former is associated with deformation but both b and h are relevant to diffusive processes. These characteristics are encompassed in the topological theory of interfacial defects developed by Pond and co-workers in which defects are characterised by combinations of symmetry operations, one from each of the adjacent crystals [9,10]. Those defects for which both b and h are finite are described as disconnections [11] and the diffusive fluxes associated wit