A Microstructural Investigation of Athermal and Deformation-induced Martensite in Fe-Cr-Ni Alloys

Abstract

The microstructure of athermal and deformation-induced martensite in four high-purity Fe-Cr-Ni alloys has been investigated. The investigation is conducted using light optical microscopy, electron backscatter diffraction and electron channeling contrast imaging. It is found that ɛ-martensite, annealing twins and grain boundaries are preferred nucleation sites for athermal α’- martensite. Furthermore, when ɛ-martensite forms before α’-martensite during quenching it acts as nucleation sites as well as growth obstacles for narrow α’-martensite units producing a characteristic microstructure with sharp angular coupling, which is distinct from the packet-type α’-martensite that forms when athermal α’-martensite is able to grow without any hindrance from prior ɛ-martensite formation. It is also found that the transformation strain imposed by the martensitic transformation can be relieved by either autocatalytic martensitic transformation or generation of planar defects, i.e. shear bands. Finally the nucleation sites of α’-martensite induced by deformation seem to be mainly at intersections of shear bands and individual shear bands.