Enhanced fracture properties by heterogeneous grain structures and dual nanoprecipitates

Abstract

Excellent synergy of yield strength and fracture toughness has been achieved in a lightweight steel by deploying heterogeneous structures and dual nanoprecipitates. Two microstructures with similar yield strength (about 1.1 GPa) have been fabricated, one is the fully recrystallized heterogeneous grain structure with higher volume fraction of nanoprecipitates (HGS1), and the other one is the heterogeneous lamella structure consisting of both un-recrystallized and recrystallized areas with lower volume fraction of nanoprecipitates (HLS). The HGS1 shows higher uniform elongation and higher fracture toughness compared to the HLS. The HGS1 displays larger size of plastic zone and higher hardening capacity around the crack tip compared to the HLS. The plastic deformation around the crack tip is accommodated by the planar dislocation slips and the formation of parallel slip bands on two {111} planes for both samples. The average interspacing of the slip bands for the HGS1 sample is found to be smaller than that for the HLS sample, indicating a stronger hardening around the crack tip for the HGS1 sample. The higher fracture toughness of the HGS1 sample can be attributed to the stronger hardening around the crack tip by the smaller spacing of planar slip bands and the stronger precipitation hardening.