Scanning Electron Microscopy/Electron Backscatter Diffraction–Based Observations of Martensite Variant Selection and Slip Plane Activity in Supermartensitic Stainless Steels during Plastic Deformation at Elevated, Ambient, and Subzero Temperatures

Abstract

The deformation-induced martensite variant selection in a supermartensitic stainless steel (SMSS) has been examined in the temperature range from −60 °C to 150 °C, using in-situ tensile testing in combination with electron backscatter diffraction (EBSD) analyses in the scanning electron microscope (SEM). In the as-received (i.e., intercritically annealed) condition, the base material contains about 40 vol pct of retained austenite. At each testing temperature, this austenite transforms back to martensite during plastic deformation at a rate which is controlled by the accumulated plastic strain in the material. On the other hand, the applied strain rate and crystallographic orientations of the prior austenite grains do not affect the overall transformation rate. Moreover, the subsequent Schmid factor analysis reveals that the martensite variant selection is independent of the local slip activity within the austenite. Therefore, no new martensite variants, besides those already present in the parent steel, develop during the phase transformation. At the same time, their individual intensities remain approximately constant within each prior austenite grain. This means that the deformation-induced martensite variants nucleate from the same sites as those that are operative in the intercritically-annealed base material. Thus, the observed variant selection is another example of the inherent reversible nature of the martensite transformation.