Notched Tensile Fracture of a Fe-15Mn-0.6C-2Al Twinning Induced Plasticity Steel at Room Temperature

Abstract

The tensile fracture behavior of an Al-bearing TWIP steel was investigated by conducting a series of tensile tests on smooth and notched specimens with different notch geometries, focusing on the effects of evolution of the stress triaxiality and the effective strain during deformation. The flow curve and digital image correlation (DIC) analysis evidenced suppression of dynamic strain aging due to Al addition, and therefore, the effects of local inhomogeneous deformation associated with Portevin-Le Chatelier (PLC) band on fracture could be excluded. The smooth specimen fractured with negligible necking despite the absence of PLC bands. As a result, the effective strain was uniform through the gage section and the stress triaxiality (η) of ~0.33 was nearly unchanged over the entire cross-section up to the maximum load. This led to the fracture surface of the smooth specimen being entirely covered with fine equiaxed dimples. For notched specimens, the fracture strain was drastically reduced with decreasing notch radius, indicating the high notch susceptibility of the steel. The effective strain of the notched specimens was the highest at the edge of the notch root, regardless of the notch radius, so cracks first developed at the surface of the notch root. Although the η at the center of the notched specimens (0.40~0.48 depending on the notch radius) was higher than that of the smooth one, the center of the fracture surface of all notched specimens exhibited dimple features that were very similar to the smooth one, even in size. In contrast, in spite of the same η of ~0.33, fractography at the edge of the notched specimens revealed a fracture mode transition from dimple fracture to void sheet fracture to quasi-cleavage fracture as the notch radius decreased. The present results were rationalized in terms of the local evolution of stress triaxiality and effective strain during deformation, which were analyzed using the finite elemental method and DIC technique. It can be said that the fracture mode of TWIP steel, showing limited necking, was more influenced by the distribution and/or gradient of stress traiaxiality and effective strain rather than their local absolute values - that is, the severer their gradient is, the easier the quasi-cleavage fracture occurs.