Displacive and elastic driving forces in the bainitic transformation: A multiphase-field study with experimentally observed chemically heterogeneous austenite

The bainitic transformation in steels is a complex process involving phase transformations, carbon redistribution, elastic interactions, and other coupled thermodynamic effects, making its prediction a long-standing challenge in computational materials science. Grounded in the displacive and diffusionless theory of bainitic sub-unit growth, a multiphase-field approach is adopted to study the bainitic transformation. Electron backscatter diffraction maps the overall phase distribution, while combined correlative scanning transmission electron microscopy, transmission Kikuchi diffraction, and energy-dispersive X-ray spectroscopy reveal strong manganese gradients in the final microstructure. Atom-probe tomography quantifies these manganese heterogeneities to define input parameters for the simulations to isolate the effect of chemical heterogeneity on bainitic transformation behavior. The transformation is modeled as a sequence of diffusionless growth events of supersaturated sub-units followed by carbon rejection. Displacive driving forces are formulated using Calphad data, while elastic driving forces are derived from balance equations on singular surfaces and the Hadamard jump conditions. Simulations of sheaf formation through successive sub-unit addition show that heterogeneous manganese enrichment can locally reduce the driving force, leading to earlier growth arrest. Additionally, a pure shear transformation strain is compared with a shear-dominated transformation strain that includes a dilatational strain component. Following sub-unit growth, carbon partitioning is described within a chemo-mechanically fully coupled phase-field model using a carbon-constrained equilibrium, and its effect on the driving forces is assessed via a combined chemo-mechanical growth criterion. By linking experimentally observed chemical heterogeneities and phase-field simulations, this work advances towards a more consistent description of bainitic transformation under chemically heterogeneous conditions.