Change of deformation mechanism through nano-structuring of pearlite: An in-situ study

Abstract

The deformation mechanisms and fracture of nano-structured pearlitic steels with inter-lamellar spacing between 60 and 160 nm have been studied in-situ using a tensile stage inside a scanning electron microscope. Fragmentation of cementite lamellae for finer pearlite was observed post ultimate tensile strength (UTS) in regions where the lamellae were aligned along the tensile axis. For coarse pearlite, cementite fragmentation initiated prior to UTS which became more severe post UTS. For coarse pearlite, the fragmented regions were distributed randomly and fragmentation was also observed for lamellae inclined to the tensile axis, thus suggesting a shear cracking dominated fracture. With refinement of lamellae spacing, fragmentation of cementite lamellae occurred simultaneously traversing the entire width of a pearlitic colony because of the strain energy release suggesting a dominating fiber stress transfer failure mechanism. For coarser pearlite, cracks in multiple colonies got connected after reaching the UTS.