Grain-level effects on in-situ deformation-induced phase transformations in a complex-phase steel using 3DXRD and EBSD

A novel complex-phase steel alloy is conceived with a deliberately unstable austenite, γ , phase that enables the deformation-induced martensitic transformations (DIMT) to be explored at low levels of plastic strain. The DIMT was thus explored, in-situ and non-destructively, using both far-field Three-Dimensional X-ray Diffraction (3DXRD) and Electron Back-Scatter Diffraction (EBSD). Substantial α ′ martensite formation was observed under 10 % applied strain with EBSD, and many ɛ grain formation events were captured with 3DXRD, indicative of the indirect transformation of martensite via the reaction γ → ɛ → α ′ . Using ɛ grain formation as a direct measurement of γ grain stability, the influence of several microstructural properties, such as grain size, orientation and neighbourhood configuration, on γ stability have been identified. Larger γ grains were found to be less stable than smaller grains. Any γ grains oriented with {100} parallel to the loading direction preferentially transformed with lower stresses. Parent ɛ -forming γ grains possessed a neighbourhood with increased ferritic/martensitic volume fraction. This finding shows, unambiguously, that the nearby presence of α and α ′ promotes ɛ formation in neighbouring grains. The minimum strain work criterion model for ɛ variant prediction was also evaluated, which worked well for most grains. However, ɛ -forming grains with a lower stress were less well predicted by the model, indicating crystal-level behaviour must be considered for accurate ɛ formation. The findings from this work are considered key for the future design of alloys where the deformation response can be controlled by tailoring microstructure and local or macroscopic crystal orientations.