Effect of multiphase microstructure on fatigue crack propagation behavior in TRIP-assisted steels

The effect of various multiphase microstructures on the fatigue properties of intercritical annealing, quenching and partitioning (I&Q&P) steel, transformation-induced plasticity (TRIP)-assisted annealed martensite (TAM) steel and TRIP steel was investigated by S-N curve, fatigue crack growth (FCG) rate and in-situ scanning electron microscopy tests. The fatigue limits of I&Q&P, TAM and TRIP steels, determined by a derivation of the Stromeyer relationship, are 670 MPa, 770 MPa and 795 MPa, respectively. The region II of FCG curve was fitted by Paris and exponential models, and the result indicates that I&Q&P steel has the highest FCG rate, followed by TAM steel and TRIP steel. In I&Q&P steel, fatigue crack mainly propagates along the interfaces between ferrite and martensite or ferrite and martensite and austenite islands, resulting in the presence of many secondary cracks; in TAM steel, the fatigue crack propagation (FCP) can be retarded when the crack passes through the mixed microstructure containing annealed martensitic laths and film-like retained austenite at an angle (not parallel); in TRIP steel, the crack branching and interlocking can hinder the FCP effectively, benefitting from bainite densely distributing in ferrite matrix as strengthening phase and the two phases having almost identical grain size. Therefore, the multiphase microstructure of TRIP steel has the best effect on retarding FCP among the three steels. The findings are essential for the optimization of multiphase microstructure in designing a steel with excellent fatigue properties.