Effects of Strain Rate on the Strain Induced Martensite Nucleation and Growth in 301LN Metastable Austenitic Steel

Abstract

The effects of strain rate on the strain induced α’-martensite nucleation and growth were analysed in this work. Tension tests were performed at room temperature at strain rates of 2×10-4 s-1 and 0.5 s -1 using small polished specimens, which fit inside a scanning electron microscope. The specimens were deformed incrementally, and microstructural evolution was tracked carefully at the same location. This approach allows analysing not only the spatial but also the temporal evolution of the α’-martensite. Optical microscopy images and electron backscatter diffraction (EBSD) measurements were taken for each plastic deformation increment. The size and number of α’-martensite particles were evaluated from the EBSD images, whereas the local microlevel plastic strains were obtained with Digital Image Correlation (DIC). According to the results, the number of nucleation sites for α’-martensite does not seem to be affected much by the strain rate. However, there is a notable strain rate effect on how the transformation proceeds in the neighbourhood of freshly formed α’-martensite particles. At low strain rate, repeated nucleation and coalescence leads to notable growth of the α’-martensite particles, whereas at high strain rate, once nucleated α’-martensite particles remain as small isolated islands which do not markedly grow with further plastic strain. This phenomenon can be attributed to local microstructure level heating caused by plastic deformation and the exothermic phase transformation. This reduces the local growth rate of the α’-martensite particles in the vicinity of the above-mentioned islands, thus leading to a lower bulk transformation rate at higher strain rates.