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

Abstract

A novel complex-phase steel alloy is conceived with a deliberately unstable austenite, $\gamma$, 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 ${\alpha }^{\prime }$ martensite formation was observed under 10% applied strain with EBSD, and many $\epsilon$ grain formation events were captured with 3DXRD, indicative of the indirect transformation of martensite via the reaction $\gamma \to \epsilon \to {\alpha }^{\prime }$. Using $\epsilon$ grain formation as a direct measurement of $\gamma$ grain stability, the influence of several microstructural properties, such as grain size, orientation and neighbourhood configuration, on $\gamma$ stability have been identified. Larger $\gamma$ grains were found to be less stable than smaller grains. Any $\gamma$ grains oriented with {100} parallel to the loading direction preferentially transformed with lower stresses. Parent $\epsilon$-forming $\gamma$ grains possessed a neighbourhood with increased ferritic/martensitic volume fraction. This finding shows, unambiguously, that $\alpha$/${\alpha }^{\prime }$ promotes $\epsilon$ formation in neighbouring grains. The minimum strain work criterion model for $\epsilon$ variant prediction was also evaluated, which worked well for most grains. However, $\epsilon$-forming grains with a lower stress were less well predicted by the model, indicating crystal-level behaviour must be considered for accurate $\epsilon$ 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.

20 pages, 3 supplementary pages, 14 figures, 3 supplementary figures. Preprint submitted to Acta Materialia. Updated to correct Figure 14