A critical assessment of deformation twinning and epsilon martensite formation in austenitic alloys during complex forming operations

The effect of mechanical twinning and ε-martensite formation on the formability of austenitic alloys in complex strain pathways has been examined. One of the alloys studied deformed by deformation twinning and slip (TWIP), and this alloy was examined with two grain sizes. The second alloy showed three concurrent deformation modes (slip, TWIP and ε-martensite TRIP). The TWIP/TRIP alloy showed a markedly reduced formability compared to similar alloys, and this has been attributed to a combination of the early development of strain localizations, and the stress-induced formation of ε-martensite. It is suggested that the TRIP effect may not be desirable in high formability sheet steels if the transformation product is ε-martensite. Microstructural examination revealed that the different strain paths develop different twin volume fractions. Plasticity modelling has shown this to be the result of differences in texture development in the different strain paths. In microstructures where slip, TWIP and TRIP can operate concurrently, the stress to activate twinning is unaffected by the additional deformation mode becoming operative.