Phase-field simulation of distribution of dislocation density and internal stress in as-quenched martensite in low-carbon steel

A phase-field simulation of martensitic transformation (MT) in a polycrystalline structure is performed to reveal the distribution of dislocation density and internal stress in the as-quenched martensite of Fe–0.1 mass%C alloy. The simulation is started by seeding a nucleus of the martensite (α’) phase in the austenite (γ) phase near the grain boundary. A multivariant structure comprising three Bain variants of α’ phase is formed in all γ-phase grains, and the volume fraction of the retained γ phase is less than 1% after MT. Mean dislocation density ( ρ ¯ ) in the α’ phase increases sharply at the onset of MT and demonstrates marginal change thereafter during MT. In contrast, ρ ¯ in the γ phase gradually increases during MT, and its magnitude reaches a level comparable to that in the α’ phase at the end of MT. The histogram of dislocation density in an as-quenched martensite spreads over a wide range of dislocation densities. The sign and magnitude of the hydrostatic stress ( σ h ) depend on the position of the as-quenched martensite. The variance of σ h in the retained γ phase and prior austenite grain boundary (PAGB) are larger than that in the α’ phase. The mean hydrostatic stress ( σ ¯ h ) in the α’ phase fluctuates around zero during MT. In contrast, σ ¯ h in the γ phase and PAGB are negative (compressive stress) and positive (tensile stress), respectively and increase significantly in magnitude during MT.