Temperature Dependent Deformation Mechanisms of a High Nitrogen-Manganese Austenitic Stainless Steel

The influence of temperature on the deformation behaviour of a Fe-16.5Cr-8Mn-3Ni-2Si-1Cu-0.25N (wt%) austenitic stainless steel alloy was investigated using transmission electron microscopy and X-ray diffraction measurements. Recrystallized samples were deformed under tension at -75 degrees C, 20 degrees C, and 200 degrees C and the microstructures were characterized after 5% strain and after testing to failure. Deformation to failure at -75 degrees C resulted in extensive transformation induced plasticity (TRIP) with over 90% alpha'-martensite. The sample deformed to 5% strain at -75 degrees C shows that the austenite transformed first to epsilon-martensite which served to nucleate the alpha'-mariensite. Transformation induced martensite prohibits localized necking providing total elongation to failure of over 70%. At room temperature, in addition to some TRIP behaviour, the majority of the deformation is accommodated by dislocation slip in the austenite. Some deformation induced twinning (TWIP) was also observed, although mechanical twinning provides only a small contribution to the total deformation at room temperature. Finally, dislocation slip is the dominant deformation mechanism at 200 degrees C with a corresponding decrease in total elongation to failure. These changes in deformation behaviour are related to the temperature dependence on the relative stability of austenite and martensite as well as the changes in stacking fault energy (SFE) as a function of temperature.