Effect of temperature on impact properties and microstructural evolution of twinning induced plasticity steel

The Charpy V notch impact toughness of twinning induced plasticity steels was investigated at 20, -50, -80, -110 and -196 degrees C. The fracture morphologies were analysed by scanning electron microscopy. The microstructural characteristic near the fractures is investigated by optical microscopy, X-ray diffraction and transmission electron microscopy. The impacted twinning induced plasticity steels exhibit better impact toughness due to the simultaneous generation of deformation twins and martensite, which hinders crack propagation and does not display ductile to brittle transition. The ruptures exhibit cleavage characteristic, and traces of deformation twins can be observed in the cleavage facets, so the cracks should propagate through the deformation twin, martensite and grain. The martensite transformation occurs because of the intensive high rate force with rapidly changing direction. The nucleation and growth of deformation twins are accomplished by the pile-up of dislocations at the positions of stress concentration, and the stacking fault grows through the moving and vanishing dislocation.