Fatigue behavior and retained austenite transformation of Al-containing TRIP steels

In material selection for the design of advanced lightweight automotive steel components, fatigue performance is of particular significance. High strength TRIP steels offer very good cyclic behavior, especially under cyclic plastic strains, which is assisted by the Transformation Induced Plasticity effect (TRIP). In the present study the TRIP effect has been quantified under both elastic (HCF regime) and plastic (LCF regime) cyclic strains for two Al-containing TRIP steels with similar chemical composition and different initial retained austenite (RA) content. The results illustrate that transformation behavior differs for the two materials under elastic and plastic cyclic straining and fatigue behavior is in both cases linked to the amount of RA transformation. The latter is discussed in the paper considering relevant RA microstructural aspects like content and particle size. In the investigation the fatigue crack initiation resistance of the materials has been experimentally evaluated, resulting in different damage tolerance ability for the two steels. A numerical simulation is developed to determine the local strains at the notch tip under monotonic loading conditions and is used with the LCF material characteristics to discuss the differences obtained in crack initiation resistance.