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

TL;DR

This study uses advanced multimodal STEM techniques to map and analyze phase transformations at the nanoscale in medium-Mn TRIP steel, revealing detailed chemical and crystallographic evolution during deformation.

Contribution

It introduces a correlative STEM approach combining nano-beam diffraction and EDX to quantitatively analyze phase and lattice changes in complex steel alloys.

Findings

Quantified phase fractions and lattice parameters of ferrite, austenite, and martensite.

Identified systematic differences in misorientation between ferrite and martensite.

Demonstrated shifts in grain size and texture in deformed regions.

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.