Internal Stress of Plate Martensite Depending on Aspect Ratio via fcc-hcp Martensitic Transformation in Metastable Austenitic Stainless Steels

Internal stress generated by athermal martensitic transformation from fcc (face-centered cubic) to hcp (hexagonal close-packed) was investigated in 18Cr–8Ni metastable austenitic stainless steel. Lattice parameter ratio of hcp-martensite, c/a, formed by subzero treatment was evaluated accurately by means of electron back scatter diffraction technique. The evaluation revealed that hcp-martensite surrounded by fcc-austenite matrix has obviously higher c/a compared with the reference value in the case when hcp-martensite exists in a single body, and the c/a is continuously increased with increasing the aspect ratio of the plate-shaped hcp-martensite. However, additional formation of martensite with bcc (body centered cubic) reduced the c/a of hcp-martensite suddenly. From the point of view of micromechanics, it can be concluded that internal stress is dynamically increased by the thickening of plate martensite on fcc-hcp martensitic transformation, and the subsequent hcp-bcc martensitic transformation is stimulated to accommodate the internal stress.