Lean duplex TRIP steel : Role of ferrite in the texture development, plastic anisotropy, martensitic transformation kinetics, and stress partitioning

Constitutive micromechanical behavior of transformation induced plasticity (TRIP) stainless steel (SS) alloys was investigated using high-energy synchrotron x-ray diffraction (S-XRD) and in-situ neutron diffraction techniques. First, four different steel alloys were designed and produced: (1) a metastable austenitic TRIP SS, (2) a stable austenitic SS, which is a stable counterpart of the TRIP SS, (3) a lean duplex TRIP SS with ferrite and metastable austenite phases, and (4) a lean duplex stable SS, which is a stable counterpart of duplex TRIP SS. Then, effects of chemical composition, microstructure, and texture on the plastic anisotropy, martensitic transformation kinetics, and residual stress concentration during a tensile deformation were investigated. The results show that the plastic anisotropy, governed by the initial microstructure and texture, has insignificant effect on macroscopic tensile properties and martensitic phase transformation kinetics despite different R-values observed along different loading directions. On the other hand, the interplay between stress partitioning among constituent phases and martensitic phase transformation plays a critical role in the micromechanics of plastic deformation, and, consequently, determines the distributions of in-situ martensite fraction and residual stresses. The phase stress partitioning in the TRIP alloy clearly shows that a large tensile residual stress of 1.8 GPa can concentrate on the martensite phase with 30% plastic strain. In contrast, the introduction of the tensile load-sharing ferrite phase in the cost-effective lean duplex TRIP alloy significantly reduces the tensile residual stress concentration in the martensite phase, which could improve the formability of high-strength TRIP steels.