Microstructure and mechanical properties of a Cu/NiAl nanoprecipitate strengthened dual-phase steel

Microstructural evolution and its effect on the mechanical properties of Cu/NiAl nanoprecipitate-strengthened dual-phase (DP) steel with various rolling treatments are investigated through a combination of electron back-scattering diffraction (EBSD) and small angle neutron scattering (SANS). The cold-rolled steel after solid solution treatment forms fine and polygonal matrix grains together with more high-angle grain boundaries (HAGBs), austenitic content and annealing twins. The hot-rolled steel maintains the deformed microstructure of the elongated austenite and martensite laths with more low-angle grain boundaries (LAGBs), dislocation density and larger average grain size. However, the Cu/NiAl coprecipitates with similar mean radii and number densities are separated out in the two steels after the same aging treatment, indicating that their precipitation is independent of the matrix microstructure, i.e., the austenitic content, grain size, grain boundaries and dislocation density. The work hardening of the dislocations together with high martensitic content effectively improves the yield strength of the hot-rolled steel, whereas the strong transformation-induced plasticity (TRIP) effect and twins enhance the ductility of the CR-aged steel; thus, the HR-aged steel exhibits higher yield strength and yield ratio, and lower strain hardening rate and elongation-to-failure than the CR-aged steel. The main mechanisms of microstructural evolution and nanoscale precipitation were also addressed and discussed.