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, $γ$, 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 $\varepsilon$ grain formation events were captured with 3DXRD, indicative of the indirect transformation of martensite via the reaction $γ\rightarrow \varepsilon \rightarrow α'$. Using $\varepsilon$ 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 $\varepsilon$-forming $γ$ grains possessed a neighbourhood with increased ferritic/martensitic volume fraction. This finding shows, unambiguously, that $α$/$α'$ promotes $\varepsilon$ formation in neighbouring grains. The minimum strain work criterion model for $\varepsilon$ variant prediction was also evaluated, which worked well for most grains. However, $\varepsilon$-forming grains with a lower stress were less well predicted by the model, indicating crystal-level behaviour must be considered for accurate $\varepsilon$ 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.