Abstract
A local elongation of 8% for nanoscale precipitated steel was observed via tensile testing, which is higher than that of 5% for bainitic steel. To determine the factor underlying this difference, void nucleation, growth, and coalescence mechanisms in the nanoscale precipitated steel and the bainitic steel were examined using electron backscattering diffraction and subsequent observation by synchrotron radiation X-ray laminography during tensile testing. Synchrotron radiation X-ray laminography analysis of void growth and coalescence revealed that the critical strain and the critical void volume fraction for fracture in the bainitic steel were smaller than those for the nanoscale precipitated steel. Secondary-ion mass spectrometry analyses revealed that C atoms were segregated at grain boundaries in the bainitic steel. Void nucleation sites in the nanoscale precipitated steel were nanoscale precipitates inside the grain and at grain boundaries and coarse precipitates; however, in the bainitic steel, void nucleation sites were entirely at grain boundaries. Nanoindentation hardness measurements showed a larger plastic strain gradient between the grain boundary and matrix in the bainitic steel than in the nanoscale precipitated steel. From these results, the high local elongation exhibited by the nanoscale precipitated steel was concluded to be due to the reduced plastic strain gradient with a uniform hardness distribution between the grain boundary and the grain interior.
