An in situ investigation of neighborhood effects in a ferrite-containing quenching and partitioning steel: Mechanical stability, strain partitioning, and damage

Abstract

Understanding the micromechanical consequences of transformation-induced plasticity (TRIP)-assisted steels is challenging, due to the presence of multiple phase constituents and difficulties in spatially and temporally resolving the transformation of sub-micron scale retained austenite. Local stress or strain partitioning among constituents is considered important in determining the stability of retained austenite, which often competes with other effects such as composition or crystal orientation. In this work, we study the neighborhood effects on micromechanical behaviors of multi-phase TRIP steel using a ferrite-containing quenching and partitioning steel as a model system by employing in situ SEM/EBSD and in situ synchrotron X-ray diffraction tensile tests, phase-specific nanoindentation, and atom probe tomography. Our results reveal that retained austenite, bainite, and tempered martensite clusters behave similarly due to underlying strengthening mechanisms, resulting in the early transformation of retained austenite surrounded by bainite and tempered martensite matrices. Conversely, in large ferrite grains, the intragranular strain distribution develops heterogeneously, which retards the transformation of some embedded retained austenite grains. The in situ microstructure-based strain mapping also demonstrates that the local strain increment of the retained austenite inside ferrite grains decreases as the deformation level increases, leading to a non-linear strain partitioning behavior at high strain levels. Another neighborhood-induced effect is observed in ferrite grains. When ferrite grains are neighbored by hard fresh martensite, their lateral contraction is inhibited, causing a plane strain tension path locally in ferrite, even when the globally applied strain path is uniaxial tension.