Influence of coherency strain and applied stress upon diffusional ferrite nucleation in austenite: Micromechanics approach

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In the quest to develop ultrafine-grained ferrite steels, an external stress is often applied to control the austenite-to-ferrite transformation kinetics. To understand the role of an applied stress in diffusional ferrite nucleation, a micromechanics analysis was performed. It is well known that the austenite-to-ferrite transition is accompanied by a volume increase of up to 9% at absolute zero. The calculation due to the volume change alone shows that a coherent ferrite particle has less strain energy in the Nishiyama-Wasserman (NW) orientation than in the Kurdjumov-Sachs (KS) orientation, and preferred shapes of NW-oriented particles are disk-like, acicular, and spherical in the order. When an applied elastic stress is introduced, two interaction terms arise. The first one is an inhomogeneity term due to the difference in elastic constants between fcc and bcc Fe, and the other is an interaction term between the volume change and the applied stress. The interesting feature of the austenite elastic constants-high bulk modulus but soft shear modulus-combined with the strong elastic anisotropy of ferrite, reveals the diverse influence of applied stress upon the energetics of ferrite formation. In certain applied stress modes, both inhomogeneity and interaction energy terms are found to lower the free energy associated with the ferrite particle, promoting enhanced ferrite nucleation. As an example, coherency strain alone decreases, when compared to a strain-free case, the nucleation rate by an order of 10-21, but its interaction with an appropriate applied stress can increase the rate by a factor of about 30.

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Philosophical Magazine