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 epsilon-martensite, annealing twins and grain boundaries are preferred nucleation sites for athermal alpha'-martensite. Furthermore, when epsilon-martensite forms before alpha'-martensite during quenching it acts as nucleation sites as well as growth obstacles for narrow alpha'-martensite units producing a characteristic microstructure with sharp angular coupling, which is distinct from the packet-type alpha'-martensite that forms when athermal alpha'-martensite is able to grow without any hindrance from prior epsilon-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 alpha'-martensite induced by deformation seem to be mainly at intersections of shear bands and individual shear bands.