Phase reverted transformation-induced nanograined microalloyed steel: Low temperature superplasticity and fracture

The novel concept of phase reverted transformation involving severe cold deformation of martensite at room temperature followed by controlled annealing was adopted to obtain nanograined (NG) microalloyed steel. The NG microstructure was characterized NG (<100 nm) and few ultra-fine (100-300 nm) ferrite grains, together with ~50-80 nm cementite and ~10-20 nm V(C,N) precipitates. The microalloyed steel exhibited superplastic behavior at a low temperature of 500 °C (<0.5Tm) with total elongation exceeding 100%. The fracture surface of the superplastic alloy was characterized by elongated cavities that nucleated and grew parallel to the applied tensile stress. The growth of cavities in the plastically deformed region involved interlinkage of cavities and significant plastic deformation occurred around the cavity, which is envisaged to be the cavitation growth mechanism. Electron microscopy of the deformed region close to the tip of the fracture surface indicated grain boundary migration during plastic deformation, an attribute of grain boundary sliding associated with superplasticity, a significant finding in microalloyed steels.