Micromechanical Behavior of Transformation-Induced Plasticity-Assisted Annealed Martensitic Steel Using In Situ Neutron Diffraction

Herein, the transformation kinetics of retained austenite (RA) and the deformation mechanisms of constituent phases are quantified for a transformation-induced plasticity (TRIP-assisted) annealed martensitic (TAM) steel. This steel shows a good combination of strength and ductility (product of strength and elongation is 30.3 GPa%). The RA transformation coefficient k (=7.46) is at a medium level, indicating that RA has a suitable mechanical stability compared with other steels. An in situ neutron diffraction technique is used to experimentally study the stress partitioning among the constituent phases of the TAM steel. The results show that it is three times for stress partitioning occurring in TAM steel under uniaxial tensile test. When the engineering stress is just over the elastic limit, the stress from the annealed martensite is transferred to the hard phases and RA. After the specimen reaches the yield strength, the increasing rate of the lattice plane strain of RA declines because the stress from the RA transfers to the harder phase (twin martensite), resulting from the TRIP effect. When the engineering stress reaches 934 MPa, the increasing rate of the lattice plane strain of RA increases again possibly due to the increase in the carbon content of RA.