Martensitic transformation in micron-scale medium managanese micropillars

Abstract

With the ever-growing demand to develop stronger and more ductile steels for the automotive industry to meet stringent carbon emission standards, extensive resources have been dedicated to finding this ideal steel. One of the most recent and promising candidates is medium manganese transformation-induced plasticity (TRIP) steel in the third-generation steel group. The excellent balance in both production cost and the mechanical properties it can offer attracted the attention of numerous researchers. The unprecedented cost-to-properties factor is generally accredited to the martensitic transformation behavior it can attain. The large metastable austenite volume fraction that retains after thermomechanical processing allows transformation to proceed during plastic deformation. While exploration in the use of medium manganese TRIP steel in the automotive application is proceeds, a lack of resources is dedicated towards its application in microelectromechanical (MEMS) and nanoelectromechanical systems (NEMS). To study its applicability to the micron/nano environment and also to gain a better insight of martensitic transformation itself, a fundamental understanding of the martensitic transformation behavior in the micron-scale needs to be established. The present work studies, for the first time, the martensitic transformations and compression orientation effect on the engineering stress-strain behavior of medium Mn austenitic micropillars. It has been found that large strain instabilities are induced by martensitic transformation and the magnitude largely depends on the grain orientation in which the 3 compression is performed. Micropillars with orientation approximately [1 0 0] show larger strain burst than the counterpart [1 1 0] orientation micropillar. This result can be well attributed to the more significant fraction of transformed martensite found within the microstructure. The difference in volume fraction of transformed martensite can be explained by the formation of intersecting twins. Moreover, the engineering stress level in which the strain instabilities occur can also be attributed to the martensitic transformation that forms during the compression. In micropillars [1 0 0], the large strain burst magnitude can be attributed the stress level in which stacking fault divergence occurs. In [1 1 0] orientation, the large bursts can well be relate to the required stress level to form martensite thermodynamically. Large bursts occur at a lower stress level in micropillars [1 0 0] due to the formation of martensite through the formation of new nuclei from intersecting twins is easier than that to attain thermodynamically.