Deformation Induced Martensitic Transformation In 304 Stainless Steels

304 stainless steel is an austenitic steel widely used for various applications due to a good combination of strength and ductility and relative low cost. It is known to be metastable as the austenite phase can transform into martensite under stress. In this work, a new method (in-situ tensile TEM) and the traditional method (ex-situ tensile tests and TEM, XRD characterization) were used to investigate the mechanisms of deformation-induced martensitic transformation in 304SS samples at different temperatures. The ex-situ tensile tests were conducted under a strain rate of 10-3 s-1 until rupture. After the tensile tests, the fractured area was examined under transmission electron microscopy (TEM) evidencing the phase transformation. Some samples were also interrupted after reaching a strain of 7%, 18%, and 30% with the goal of investigating the intermediate microstructure. Such ex-situ investigation can help evidence the changes incurred by the microstructure but provides limited information on the mechanisms and kinetics of the processes leading to that final microstructure. Thus, in complement to the ex-situ investigation, tensile tests were conducted in-situ in a TEM at 25°C down to cryogenic temperatures (-100°C) using a special straining-stage with the goal of capturing the growth of the martensitic phase as it develops under stress in the material and capture it on video. Through such experiments, it was observed that the austenitic phase (fcc) can transform into both ε-martensite (hcp) and α’-martensite (bcc), and ε-martensite (hcp) can be further transformed into α’-martensite (bcc). Stacking faults (SFs) and mechanical twinning are often formed as an intermediate step during the transformations from γ-austenite to the ε-martensite. Such processes could be observed and recorded in-situ. Through this work, it was thus shown that in-situ tensile TEM, as a small scale tensile technique, is a good technique to investigate the mechanisms of deformation induced phase transformations