Nanoscale mapping of phase-transformation pathways in medium-Mn TRIP steel by multimodal STEM

Abstract

The mechanical response of third-generation advanced high-strength steels is governed by phase transformations at the nanoscale, yet the coupled evolution of chemistry and crystallography remains poorly resolved. Here we apply a correlative scanning transmission electron microscopy approach that enables simultaneous mapping of lattice structure, crystallographic orientation, and phase distribution at 10 nanometre resolution in a medium-manganese TRIP steel. We combine nano-beam electron diffraction and energy-dispersive X-ray spectroscopy maps to characterize an industrial medium-manganese steel containing 7.15 weight percent Mn. Tensile testing of a rolled steel sample was performed, and lamellae were extracted from deformed and undeformed regions. Manganese-resolved energy-dispersive X-ray spectroscopy provides a chemical fingerprint that, when combined with nano-beam electron diffraction based phase segmentation, enables robust ferrite-martensite separation and phase-resolved lattice-parameter refinement. The phase fractions of ferrite, austenite, and martensite are quantified together with their corresponding lattice parameters, accompanied by measurable shifts in grain-size distributions and crystallographic texture in the deformed regions. Kernel average misorientation maps reveal systematically lower local misorientation in ferrite than in martensite. This multimodal workflow provides a transferable framework for quantitative, phase-resolved analysis of complex multiphase alloys at the nanoscale.