The formation of martensite (epsilon and alpha') in metastable austenitic Fe-18Cr-(10-11.5)Ni alloys was investigated in situ during cooling. High-energy X-rays were used to study the bulk of the alloys. Both grain-averaged and single grain data was acquired. s played an important role in the formation of a' with an indistinguishable difference in the martensite start temperature. The single-grain data indicated that stacking faults appear as precursors to a An analogy can be made with deformation-induced martensitic transformation, where the generation of nucleation sites would significantly lower the driving force required to overcome the energy barrier in low stacking fault energy Fe-Cr-Ni alloys.

The deformation microstructure of austenitic Fe-18Cr-(10-12)Ni (wt pct) alloys with low stacking fault energies, estimated by first-principles calculations, was investigated after cold rolling. The E >-martensite was found to play a key role in the nucleation of alpha'-martensite, and at low SFE, E > formation is frequent and facilitates nucleation of alpha' at individual shear bands, whereas shear band intersections become the dominant nucleation sites for alpha' when SFE increases and mechanical twinning becomes frequent.

The microstructure of martensite formed athermally or via deformation in Fe-Cr-Ni alloys with different austenite (γ) stability has been investigated using microscopy. Two different types of microstructures, viz. blocky and banded structure, are observed after athermal and deformation-induced martensitic transformation (AMT and DIMT, respectively). The blocky structure form during AMT or DIMT if the stability of γ is low. In both these cases, there is a significant chemical driving force for the transformation from γ to α’-martensite (α’), and if it is not hindered by e.g. planar defects it can grow uninhibited into a blocky morphology without the necessity to nucleate new crystallographic variants to accommodate the transformation strains. On the other hand, the banded structure is due to the formation of ε-martensite (ε) during AMT, or the wider concept shear bands in the case of DIMT. The shear bands, and in particular ε, lower the nucleation barrier for α’ that forms within individual shear bands if the stability of γ is low. Neighbouring α’ units predominantly have a twin-related orientation relationship to accommodate the transformation strains. With increasing γ stability during DIMT, variant selection becomes pronounced with preferred formation of variants favourable oriented with respect to the applied stress/strain field. The formation of α’ at individual shear bands is also rare, since no ε is present and instead α’ forms at the intersection of shear bands for more stable γ. In conclusion, AMT and DIMT for low γ stability lead to similar microstructures, whereas the DIMT microstructure for high γ stability is distinct.

Two transformation-induced plasticity (TRIP) assisted duplex stainless steels, with three different stabilities of the austenite phase, were investigated by synchrotron X-ray diffraction characterization during in situ uniaxial tensile loading. The micromechanics and the deformation-induced martensitic transformation (DIMT) in the bulk of the steels were investigated in situ. Furthermore, scanning electron microscopy supplemented the in situ analysis by providing information about the microstructure of annealed and deformed specimens. The dependence of deformation structure on austenite stability is similar to that of single-phase austenitic steels where shear bands and bcc-martensite (α’) are generally observed, and blocky α’ is only frequent when the austenite stability is low. These microstructural features, i.e. defect structure and deformation-induced martensite, are correlated with the micro- and macro-mechanics of the steels with elastoplastic load transfer from the weaker phases to the stronger α’, in particular this occurs close to the point of maximum rate of α’ formation. A clear strain-hardening effect from α’ is seen in the most unstable austenite leading to a pronounced TRIP effect.

The mechanical behavior of austenite was investigated in a TRIP duplex stainless steel (TDSS). Uniaxial tensile testing was performed and the microstructure was characterized using electron backscatter diffraction. A strong <111> texture develops along the loading direction (LD) due to preferential martensitic transformation and grain rotation. Austenite oriented with <112> and <100> parallel to the LD transform preferentially, since multiple slip planes activate for these orientations. These observations agree with observations in single-phase austenitic alloys with low stacking fault energy. In conclusion, it is indicated that the stability of austenite grains towards martensitic transformation is mainly dictated by crystallographic orientation.

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.