Precipitation strengthening of Cu/NiAl co-precipitates in a martensite-austenite dual-phase steel

Abstract Precipitation strengthening of Cu/NiAl co-precipitates and its influence on mechanical properties in the martensite-austenite dual-phase steels under aging treatment are studied through a combination of atom probe tomography (APT), small angle neutron scattering (SANS) and theoretical calculation of strengthening mechanisms. The APT and SANS results show that the co-precipitates of NiAl and Cu nanoparticles are only separated out in the martensite phase upon aging treatment, and then the co-precipitates with high coarsening coefficient (4.17 nm3‧h−1) rapidly grow up. In the peak aging state, the co-precipitates with mean radius of 2.1 nm and number density of 1.1 × 1024 m−3 in the martensite, make a strength-ductility synergy in the dual phase steel that the yield strength increases from 474 ± 10 MPa to 964 ± 21 MPa and the uniform elongation simultaneously increases from 20% to 25%. Beyond peak aging, the mean radius of the co-precipitates increases to ~3.6 nm and its number density decreases to 1.4 × 1023 m−3, and the contribution of precipitation strengthening slightly decreases but the uniform elongation can be maintained. The precipitation strengthening in the peak aging is mainly contributed by shearing mechanism, while Orowan looping becomes dominant in beyond peak aging. Uniformly, both of shearing and Orowan looping mechanisms partly contribute to the strength and the ratios are related to the martensitic phase content, owing to a composite effect of precipitation strengthening in the martensite and weakening effect in the austenite. The effect of microstructure and nanoprecipitates on the ductility is also addressed and discussed.