WEB Crystal plasticity modelling of the martensite/ferrite interface damage initiation in dual-phase steelsFriday (25.09.2020) 13:20 - 13:35 M: Modelling and Simulation 2 Part of:
Martensite/ferrite (M/F) interface damage largely governs the failure of DP steels and is commonly understood to originate from the high phase contrast between martensite and ferrite. This understanding however seems insufficient since it conflicts with a few recent detailed observations, in which considerable M/F interface damage initiation is accompanied by an apparent martensite island plasticity and weak M/F strain partitioning. In fact, martensite has a complex hierarchical structure and demonstrates a strongly orientation-dependent behaviour. Depending on the local stress state, (lath) martensite can indeed be hard to deform but when favourably oriented, the substructure boundary sliding can be triggered at a resolved shear stress comparable to that of ferrite, easily inducing the matersite island plasticity. Moreover, experimental evidence indicates the occurrence of sharp martensite wedges, leading to a jagged M/F interfacial morphology, which could potentially also affect the M/F interface behaviour. By including these substructure and morphology features, that are usually overlooked in the literature, this work re-examines the M/F interface damage initiation mechanism. A systematic crystal plasticity study is performed, which accounts for different loading conditions, phase contrasts, residual strains/stresses due to the preceding martensitic phase transformation. M/F interfaces with various possible morphologies are considered and modelled, for simplicity, as one martensite island embedded in an isotropic elasto-plastic ferrite matrix. Martensite island is constructed by a set of BCC martensite laths along the thickness direction and separated by the inter-lath FCC retained austenite (RA) films enabling the substructure boundary sliding. The results show that the M/F interface damage initiation generally originates from the substructure boundary sliding, which is the favourable plastic deformation mode of martensite islands. This finding thus suggests a new understanding of the physical origin of the M/F interface damage, which is critical for designing more damage-tolerant DP steels.