In-situ synchrotron X-ray diffraction study of the effects of grain orientation on the martensitic phase transformations during tensile loading at different strain rates in metastable austenitic stainless steel

In this work, in-situ high-energy X-ray diffraction was used to analyze the effects of strain rate and austenite (gamma) grain orientation on the strain-induced martensitic transformation in metastable austenitic stainless steel 301LN. The diffraction measurements were carried out at strain rates ranging from 10(-3) s(-1) to 1 s(-1) continuously without interrupting the experiment and thus creating nearly adiabatic conditions at the highest studied strain rate. The results indicate that <100>gamma fiber-oriented grains preferentially transform at the strain rate of 10(-3) s(-1) when the true strain is above 0.10, whereas the <111>gamma fiber-oriented grains transform only at later stages of plastic deformation. The phase transformation rate of the <111>gamma and <100>gamma fiber-oriented grains decreases with increase in strain rate. A theoretical model based on stacking fault width as a function of external stress and temperature (stacking fault energy) was used to predict lower-bound estimates for the critical tensile stress needed to start epsilon-martensite and alpha'-martensite phase transformations. The model can predict the experimentally observed phase transformation behavior of the <111>gamma fiber orientations at all strain rates but is unable to predict the decrease of phase transformation rate of <100> fiber-oriented gamma grains with increase in strain rate, which could be related to change in dislocation structure.