Characterization of fracture in medium Mn steel

In the present study, the damage mechanisms operating during the tensile deformation of intercritically annealed Fe-0.3%C-6.0%Mn-3%Al-1.5%Si medium Mn steel were investigated. The steel was annealed at different temperatures to obtain a range of strain hardening properties in uniaxial tension by activating the twinning-induced plasticity and transformation-induced plasticity effects. The initial microstructure consisted of coarse delta-ferrite grains and an ultra-fine grained (UFG) constituent containing ferrite (alpha) and austenite (gamma). However, the volume fraction of martensite (alpha') increased significantly by phase transformation from austenite as the material deformed plastically. The internal damage and the fracture appearance after monotonic standard uniaxial tension tests and in-situ interrupted tensile experiments were characterized at macro-and micro-scale. The fracture features were analyzed as a function of the intercritical annealing temperature, which is the most important processing parameter for medium Mn steel. Three mechanisms contributed to the damage that developed in these materials. First, nucleation and growth of voids occurred at non-metallic inclusions. Second, debonding of the alpha-alpha' and alpha'-alpha' interfaces due to a local loss of interfacial strength was observed in the UFG constituent. While the voids initiated at non-metallic inclusions were in the order of several microns, the size of those initiated at the alpha-alpha' and alpha'-alpha' interfaces in the UFG constituent was limited by the initial grain size, with little or no growth. Finally, in addition to void damage, longitudinal cleavage-like cracks formed along the delta-ferrite layers, and parallel to the sheet plane, were observed in the fractured specimens. These longitudinal cleavage-like cracks were the consequence, but not the cause, of a fracture process triggered by plastic flow localization during uniaxial tension testing.