Analysis and design of a three-phase TRIP steel microstructure for enhanced fracture resistance

The goal of this paper is to predict how the properties of the constituent phases and microstructure of a transformation induced plasticity steel influence its fracture resistance. The steel selected for study was a three-phase quenched and partitioned (QP) sheet steel comprised of 50% ferrite, 42% martensite and 8% retained austenite (RA) with ∼ 980 MPa tensile strength. Experiments show that ductile fracture in the steel involves nucleation, growth, and coalescence of micron-scale voids. Accordingly, the failure process is modeled at the microstructure scale by idealizing the individual phases of the steel using elastic-viscoplastic constitutive relations that account for the loss of strength resulting from cavitation, as well as the effects of transformation of metastable RA to martensite. The flow behavior of the phases and the transformation kinetics of RA are calculated by homogenizing the microscale calibrated crystal plasticity constitutive relations from a previous study (Srivastava in J Mech Phys Solids 78:46–69, 2015) while the damage parameters are determined by void cell model calculations. The microstructure-scale simulations are used to compute the fracture and instability loci for the steel, which are used to calibrate the GISSMO (Generalized Incremental Stress State Dependent Damage Model) (Andrade in Int J Fract 200:127–150, 2016). The microstructure-informed GISSMO model for QP980 is found to predict fracture strains within 18% of experimental measurements of ligament-type test specimens. Finally, a series of virtual steel microstructures are analyzed to determine the influence of the phase volume fractions on the fracture resistance of the steel. Two candidate microstructures are identified that exhibit increased engineering fracture strains (> 57 %) without significantly compromising (within 6%) the tensile strength when compared to the baseline QP980.