Some aspects of the massive transformation

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A number of issues of current importance to achieving a fundamental understanding of the mechanisms of the massive transformation at the atomic level are identified and further considered. Re-analysis of published growth kinetics data for the massive transformation in six different alloy systems indicates that growth by the ledge mechanism is a reasonable possibility in all of them. Variations in the ratio of the inter-ledge spacing to the ledge height with boundary orientation and in the temperature-dependence of growth kinetics with boundary orientation also seem likely. Plateaux in plots of thermal arrest temperature vs a function of cooling rate are suggested to arise from changes in massive morphology with decreasing transformation temperature, derived from variations with boundary orientation of temperature-dependent growth kinetics. Solute "burial" during the massive transformation is concluded usually to be feasible only when the driving force for transfer back to the matrix of the atoms undergoing burial is much less than that for solvent atoms diffusing from the matrix to the product phase. Invariant plane strain surface reliefs can be generated during massive transformations when reaction occurs at sufficiently large undercoolings so that markedly anisotropic growth, involving comparatively large areas of partially coherent interphase boundary with a constant boundary orientation, is feasible. Massive transformation in a two-phase field below T0 is probably viable when the volume diffusivity in the matrix is too low relative to the trans-interphase boundary diffusivity to permit appreciable solute partition during growth. © 1988.

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Acta Metallurgica