Lath boundary spacing and orientation dominate damage and micro-/macro-shear banding in high-strength martensitic steel

Ultra-High Strength martensitic Steel (UHSS) often lacks the formability to undergo bending without cracking. We show that UHSS produced from heavily pancaked prior-austenite grains has a much lower bendability than UHSS from equiaxed prior-austenite, even though pancaked martensite has much more favorable habit plane orientations than equiaxed martensite, suggesting higher bendability through homogeneous deformation by in-habit-plane slip. Here, we analyze this contradiction using Electron Backscatter Diffraction (EBSD) - Scanning Electron Microscopy - Digital Image Correlation-based bending experiments. The tests yield microstructure-correlated strain maps, precisely aligned to EBSD-based (prior austenite) grain reconstructions, enabling pixelwise quantification of in-habit-plane versus out-of-habit-plane slip activity using the novel SSLIP framework. Besides plasticity, micro-damage is analyzed, revealing boundary cracking as the dominant micro-damage mechanism, most prevalent in pancaked martensite due to a combination of (1) fine auto-tempered martensite with high boundary density, originating from quenching deformed austenite, (2) habit plane orientations that only seem favorable (Schmid Factor > 0.3) but do not precisely match the optimal micro-shear band formation angle of 45°, and (3) a dense network of micro-shear bands. These micro-shear bands intersect with inter/intra-granular boundaries, resulting in micro-damage, and accelerated global macro-shear band formation and fracture. In contrast, equiaxed austenite results in coarser martensite laths, in which micro-shear bands form with fewer boundary intersections, resulting in localized plasticity without damage, explaining a 3–4 times better bendability despite a much stronger strain heterogeneity. Overall, the ability of UHSS to accommodate strain localizations without boundary cracking results in delayed global macro-shear band formation and increased formability, providing clear guidelines for further UHSS optimization through the size/shape of austenite, and resulting martensite lath orientation/coarseness.