Grain Size Altering Yielding Mechanisms in Ultrafine Grained High-Mn Austenitic Steel: Advanced TEM Investigations

Abstract

The underlying mechanism of discontinuous yielding behavior in an ultrafine-grained (UFG) 31Mn-3Al-3Si (wt.%) austenitic TWIP steel was investigated by the use of advanced TEM technique with taking the plastic deformation mechanisms and their correlation with grains size near the macroscopic yield point into account. The discontinuous yielding with noticeable yield drop has been reported in various ultrafine grain materials. However, typical yield drop mechanisms such as the dislocation locking by the Cottrell atmosphere due to the presence of interstitial impurities cannot explain the origin of this phenomenon in the UFG high-Mn austenitic TWIP steel. Here, we experimentally reveal by TEM that the plastic deformation mechanisms in the early stage of deformation, around the macroscopic yield point, shows an obvious grain size dependence, more specifically, the main mechanisms shift from the conventional in-grain slip deformation to deformation by twinning nucleated from grain boundaries as the grain size decreases in the high-Mn steel.Planar dislocation substructure was initially generated from the grain interior in over-1μm grains, whereas stacking faults and deformation twins were directly nucleated at grain boundaries without the presence of dislocations pile-ups in under-1μm grains. This observation indicates that the grain size dependent deformation mechanism transition is also deeply associated with the discontinuous yielding behavior as it could govern the changes in the grain interior dislocation density around the macroscopic yield point.