Strain-induced ε-martensite transformation during nanoindentation of high-nitrogen steel

The strain-induced austenite-to-ε-martensite transformation during nanoindentation of metastable austenite in a high-nitrogen stainless steel was investigated by observation of the microstructural changes underneath the indent. In the nanoindentation load–displacement curve, several small pop-ins were detected at the beginning stage of plastic deformation. The presence of ε-martensite bands underneath the indent and their orientation relationship with the parent austenite strongly suggest that the pop-ins in the load–displacement curve correspond to the transformation from austenite to ε-martensite. A quantitative analysis on the massive movement of Shockley partial dislocations during the transformation was performed to obtain the contraction strain under compressive stress conditions during nanoindentation. It was confirmed that the calculated contraction strain along the indenting direction matches well with the measured length of the pop-ins.