Transformations involving crystalline phases containing both substitutional and interstitial elements have been studied theoretically to establish the conditions under which plates of the product phase can form with just partial redistribution of the interstitial element during nonequilibrium nucleation and growth. The rate at which an interface moves depends both on its intrinsic mobility and on the ease with which the interstitial element diffuses ahead of the moving interface. The two processes are coupled so that a stable velocity can be calculated by matching the velocity solutions for diffusion and for interfacial motion. Interfacial mobility is modelled in terms of the dislocation theory of glissile martensitic interfaces. A thermodynamic model incorporating the effect of interfacial dissipation has enabled interfacial compositionsto be deduced for growth involving partial supersaturation. These data are then used to calculate the corresponding diffusion field velocity. As a possible model for bainitic transformation, calculations for an Fe-0.4C wt% alloy show that the maximum rates for both nucleation and growth can occur for incomplete partitioning of solute. At high undercoolings, diffusionless growth can occur, even when nucleation still requires some partitioning of carbon.
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