Influence of inter-lamellar spacing on the strain rate sensitivity and deformation mechanisms in nano-pearlite

Abstract

This study investigates the influence of inter-lamellar spacing on the strain rate sensitivity and deformation mechanisms in nano-pearlite. Two blocks of pearlitic steels with inter-lamellar spacing of 67 ± 7 nm and 110 ± 7 nm were obtained by austenitization at 1000 °C followed by isothermal holding at 575 °C and 700 °C respectively. The blocks, referred to as NP575 and NP700, were subjected to ambient temperature tensile testing at strain rates ranging from 2 × 10⁻¹ s⁻¹ to 3 × 10⁻⁶ s⁻¹. The finer pearlitic block NP575 didn't show any significant sensitivity to the applied strain rate at all values of strains, primarily due to the predominance of fiber loading deformation mechanism in finer pearlite. However, NP700 is highly sensitive to the applied strain rate at lower strains (ε $<0.018)\text{,}$ while the strain rate sensitivity of NP700 diminished significantly at higher strains (ε $>0.018)$. Electron Backscattered Diffraction (EBSD) analysis revealed significant differences between the samples regarding the distribution and activity of dislocations. NP700 showed larger areas with high kernel average misorientation (KAM) values at the lowest strain rate but smaller areas at the highest strain rate of testing. However, at the highest as well as the lowest strain rate, NP575 showed a similar area fraction of regions with high KAM values and the appearance of KAM distribution was similar to NP700 at the highest strain rate, suggesting that dislocation activity considerably reduced at the highest strain rate for NP700. Post-deformation TEM observations for NP700 specimen tested at the lowest strain rate showed the shearing of cementite lamellae as a precursor to failure. However, failure in both the lowest and highest strain rate tested specimens of NP575 as well as the highest strain rate tested specimen of NP700 occurred due to the breaking of cementite lamellae through the fiber loading mechanism. These findings elucidate the differences in the deformation mechanisms and micro-mechanical behavior of nano-pearlite enabled by changes in strain rate and inter-lamellar spacing.