In Situ Straining TEM Experiments for the Characterization of Deformation Induced Phase Transformations

The spatial resolution of transmission electron microscopy (TEM) makes it an invaluable characterization technique to track the real-time response of microstructure under tensile stress, when the tensile experiment is done <i>in-situ</i> in the microscope. For that matter, this work will show how in-situ straining experiments done in the TEM can shed light on the deformation mechanisms in materials and the subsequent phase transformations, through the example of the deformation induced martensitic transformation in metastable austenitic stainless steel 304, an alloy with relatively low Stacking Fault Energy (SFE) which is known to transform from austenite to martensite under deformation.<br/>The small scale tensile straining experiments were conducted in-situ in a TEM at 298K (25 °C) down to cryogenic temperatures using a special straining-stage with the goal of capturing the nucleation and growth of the martensitic phase upon deformation. The in-situ observations at room temperature evidenced how the austenitic phase γ (fcc) could transform into both ε-martensite (hcp) and α’-martensite (bcc). Regardless of the temperature, stacking faults (SFs) were observed to form as an intermediate step during the transformation from γ-austenite to the hcp ε-martensite. The formation of such SFs mostly originating from Grain Boundaries (GBs) and the subsequent formation of ε-martensite (hcp) through their overlapping/bundling were captured in-situ. ε-martensite shear bands were also observed to serve as nucleation sites for α’-martensite which grew into the neighboring γ-austenite matrix as evidenced in the evolution of the diffraction pattern. Additionally, α’-martensite was also observed to nucleate at austenite GBs.<br/>At cryogenic temperatures down to 173K (−100 °C) , the formation and growth of SFs was also observed in situ and the following transformation paths were evidenced in-situ: (i) accumulation and superimposition of SFs resulting in formation of ε-martensite (γ →SFs →ε), (ii) γ transforming into α’ directly (with interface migration upon pulling the sample) (γ→ α’) and (iii) the case where after inducing some stress in the sample by pulling the γ→ α’ transformation occurred upon further cooling (with no further pulling), indicating again how stress and temperature are both effective on the transformation.<br/>Overall, the presentation will showcase how in-situ straining in a TEM, as a small scale tensile testing technique, is proved to be an effective and powerful technique to visualize and investigate the mechanisms of deformation induced transformations in materials.