Retained Austenite Transformation Kinetics in Third Generation Advanced High Strength Steels as a Function of Strain Path

Abstract

The central objective of this thesis is to determine the strain path dependance of retained austenite (RA) transformation kinetics in an Fe-0.2C-6.4Mn-1.5Si-1.0Al-0.5Cr (wt.%) medium- Mn third generation advanced high strength steel (3G AHSS). A combination of macro- and micro-scale strain paths including uniaxial tension, plane strain tension and compression, three-point (3-pt.) bending, V-bending, and in-situ SEM 3-pt. micro-bending were applied along the rolling direction – normal direction (RD–ND) plane to isolate the effects of strain path on RA transformation kinetics. V-bending along with in-situ SEM 3-pt. micro-bending were conducted in both the RD-ND and transverse direction – normal direction (TD-ND) to assess crystallographic texture effects. Digital Image Correlation (DIC) was employed to capture localized strain distribution, while X-ray Diffraction (XRD), Electron Backscatter Diffraction (EBSD), and Transmission Electron Microscopy (TEM) were used to assess RA stability and transformation kinetics through the analysis of deformation mechanisms. The results clearly demonstrate that RA transformation kinetics are a strong function of the imposed strain path. Tension-dominated modes, particularly uniaxial tension and plane strain tension, exhibited the most rapid transformation kinetics and complete RA depletion, following nearly identical transformation trajectories. In contrast, plane strain compression and both bending modes showed more gradual RA transformation, attributed to elevated hydrostatic pressures and multiaxial strain states that inhibit the volumetric expansion required for the martensitic transformation. As expected, three-point and V-bending displayed intermediate transformation behavior, falling between the extremes of uniaxial/plane strain tension and plane strain compression. EBSD mapping highlighted the role of crystallographic orientation in RA transformation kinetics in V-bending. RA grains oriented along <111> and <100> in the RD direction exhibited accelerated RA depletion due to higher Schmid factor activation, confirming orientation dependent transformation kinetics. Fracture surface analysis further revealed damage modes commonly associated with 3G AHSS, including interface decohesion, shear-driven fracture, and crack initiation around Al- and Mn-rich internal oxides formed during continuous annealing. The tensile surface displayed a mixed-mode fracture with clear evidence of localized shearing, while the compressive side showed a more ductile, transgranular fracture morphology. In-situ SEM micro-bending experiments further revealed that strain localization was highly dependent on loading direction, highlighting the role of crystallographic texture in local deformation behavior. Complementary high-resolution TEM analysis identified key RA transformation and deformation mechanisms including high dislocation activity, ε-martensite formation, and deformation twinning concentrated in high strain regions. These observations confirm that RA transformation is not only strain path dependent but also sensitive to local microstructural conditions, emphasizing the complex interplay between deformation mode, crystallographic orientation, and RA stability through the deformation. These findings demonstrate that RA transformation and damage evolution in med-Mn 3G AHSSs are determined by the combined effects of strain path, local hydrostatic stress, and microstructural features such as crystallographic orientation and phase distribution. The results provide valuable insights into the underlying deformation mechanisms of medium-Mn 3G AHSS and support the development of more predictive models for formability and failure in advanced forming operations.