Investigation of the Phase Transformation Kinetics and Texture Evolution in a TRIP Steel under Complex Loads

The martensitic phase transformation kinetics and its relation with texture evolution and deformation/transformation microstructures under complex loading conditions were investigated in a 304L stainless steel that exhibits the transformation induced plasticity (TRIP) effect when strained at ambient temperature. The applied load paths included torsional and biaxial deformation scenarios including simultaneous biaxial torsion/tension and torsion/compression as well as stepwise deformation of tension followed by torsion. Synchrotron x-ray and electron back-scatter diffraction techniques were used to investigate the phase transformation-microstructure-texture evolutions relations.

Under torsional deformation, the inhomogeneous distribution of martensite phase fractions were recorded through the radius of solid cylindrical specimens consistent with the shear strains showing a linear variation along the radial position. The fundamental studies on phase transformation kinetics and texture evolution during torsional deformation were conducted using data close to the surface of the specimens. Texture-transformation relations were also investigated and preferential phase transformation of grains belonging to certain components in the parent austenite phase (namely the C- and B-texture components) was identified.

When the specimens were deformed under biaxial deformation conditions, the resultant phase transformations were enhanced under biaxial torsion/tension and stepwise tension-torsion cases, whereas there was a decrease in transformation kinetics under the biaxial torsion/compression loading. Further analyses were performed with selected subsets of the biaxial loading studies and the changes in phase transformation kinetics were discussed in the context of preferential phase transformation mechanisms and effects of the addition of axial strains on the phase transformation kinetics. Furthermore the results were also discussed from a microscopic perspective including the formation of shear bands and their intersections, and volume changes associated with the martensitic transformation.