The impact of grain-scale strain localization on strain hardening of a high-Mn steel: Real-time tracking of the transition from the gamma -> epsilon -> alpha ` transformation to twinning

Strain partitioning and localization were investigated in a high-Mn steel (17.1 wt.% Mn) during tensile testing by a correlative probing approach including in-situ synchrotron X-ray diffraction, micro- digital image correlation (mu-DIC) and electron microscopy. By combining Warren's theory with the mu-DIC analysis, we monitored the formation of planar faults (stacking faults and mechanical twins) and correlated them with the local strain partitioning behavior within the microstructure. Starting with an initial microstructure of austenite (gamma) and athermally formed epsilon- and alpha'-martensite, strain accumulates preferentially near the gamma/epsilon interfaces during tensile straining. The local microscopic von Mises strain (epsilon(vM)) maps obtained from mu-DIC probing show that these local strain gradients produce local strain peaks approximately twice as high as the imposed macroscopic engineering strain (epsilon), thus locally triggering formation of epsilon-martensite already at early yielding. The interior of the remaining austenite, without such interfacial strain peaks, remained nearly devoid of planar faults. The local strain-driven growth of the epsilon-domains occurs concomitantly with the alpha'-martensite formation. At intermediate macroscopic applied strains, austenite grain size is considerably reduced to a few nanometers and the associated gamma/epsilon interfacial microscopic strain peaks increase in magnitude. This scenario favors twinning to emerge as a competing strain hardening mechanism at engineering strain levels from epsilon = 0.075 onwards. At large tensile strains, the gamma -> epsilon -> alpha' transformation rates tend to cease making both twinning and SFs formation to operate as the main strain hardening mechanisms. The findings shed light on the transformation micromechanisms in multiphase Mn-TRIP steels by revealing how strain localization among the constituents can directly influence the kinetics of the competing strain hardening mechanisms. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. (AU)