PNNL

Abstract

The deformation mechanisms and associated microstructure changes during tensile loading of an annealed twinning-induced plasticity (TWIP) steel with the chemical composition of Fe-20Mn-3Si-3Al-0.045C (wt.%) were systematically investigated using in situ time-of-flight (TOF) neutron diffraction in combination with postmortem transmission electron microscopy (TEM). The initial microstructure of the investigated alloy consists of equiaxed austenitic grains with the initial a'-phase of ~7% in volume. In addition to dislocation slip, twinning and two kinds of martensitic transformations from the austenite to a'- and epsilon martensites were observed as the main deformation modes during the tensile deformation. In situ neutron diffraction provides a powerful tool to establish the deformation mode map for elucidating the role of different deformation modes in different strain regions. The critical stress is 520 MPa for the martensitic transformation from the austenite to a'-martensite, whereas a higher stress (>600 MPa) is required for actuating the deformation twin and/or the martensitic transformation from -martensite. Both epsilon- and a'-martensites act as the hard phases whereas mechanical twinning contributes to both strength and ductility of the studied steel. TEM observations confirmed that the twinning process was facilitated by the parent grains orientated with or parallel to the loading direction. The nucleation and growth of twins are attributed to the pole and self-generation formation mechanisms, as well as the stair-rod cross-slip mechanism.