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Coupled Diffusional/Displacive Transformations



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Mujahid, Shafiq Ahmad 


The displacive transformation of austenite to ferrite in steels containing both substitutional and interstitial elements has been studied. The aim was to establish the conditions under which plates of the product phase can form with a partial redistribution of the interstitial element during nonequilibrium nucleation and growth. An earlier model describing such 'coupled diffusional/displacive transformation' (CDDT) has been applied over a wide range of carbon concentrations, revealing a variety of discrepancies.

It was found that the theory correctly predicts the variation in the martensite-start temperature with carbon concentration, but fails to estimate the corresponding changes in the bainite-start temperatures of the same steels. Thus, the accuracy claimed by the original theory appears fortuitous for bainite. The failure is attributed to the fact that the model does not include any variation in the stored energy as a function of transformation temperature. The nature of the required variation in stored energy with temperature was calculated by fitting against available data and the CDDT model was modified appropriately. The estimated variation in stored energy is consistent with an expectation that when the yield strength is exceeded at a high enough temperature, plastic accommodation of the shape change should lead to a reduction in the stored energy. The modified model predicted a sharper transition from growth involving full partitioning of carbon, to diffusionless growth when applied to a number of alloyed steels. This abrupt transition from paraequilibrium to diffusionless growth is in fact consistent with experiments; Widmanstatten ferrite at all temperatures is known to grow at a rate controlled by the diffusion of carbon in the austenite ahead of the interface, whereas the growth rate of bainite subunits is much larger then might be expected from carbon diffusion-controlled growth. Considerable work is also reported on how bainite transformation might be described by the CDDT model, but significant difficulties remain.

Another model was developed to study the kinetics of the partitioning of carbon from supersaturated ferrite into residual austenite. The time required was estimated analytically and using a finite difference model. It was found that in all the cases investigated, the analytical solution underestimates the diffusion time, the discrepancy increasing at lower temperatures, or when the concentration of substitutional solutes which stabilise austenite is reduced. This is attributed to the fact that the analytical method fails to take account of the coupling of the diffusion fluxes that arise in both the austenite and the ferrite. The results were first discussed in the context of displacive transformations in steels. The model was latter extended to the non-ferrous, Ag-44.9Cd at.% alloy. This alloy undergoes a β2 - α1 transformation which is sometimes called "bainite" by virtue of the fact that the plates appear to be different in composition from the parent phase. The α1 plates could on the other hand, form without diffusion, the cadmium partitioning into the β2 matrix after formation. The results are compared with published data, but they indicate that there is a need for more accurate diffusion data before definitive conclusions can be made on the mechanism of transformation.






Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge