Neutron-diffraction study of stress-induced martensitic transformation in TRIP steel

Neutron diffraction has been used to follow in situ the stress-induced martensitic transformation during uniaxial tensile testing of polycrystalline Fe-25 wt% Ni-0.4 wt% C. This method provides data on the extent of transformation, the development of crystallographic texture and the evolution of lattice strain. Two types of specimen were examined: unswaged, and swaged at 200°C. The swaged material was further aged with greater yield strength, enhancing the transformation. Moreover, it exhibited a stronger texture than the unswaged material. The extent of the transformation is shown to be dependent on grain orientation, with austenite grains having >100<parallel to the tensile axis transforming preferentially. The internal stress state shows that, during plastic flow, the load is transferred from the original austenite to the evolving martensite phase.