Synergistic effects of Cu and Ni on nanoscale precipitation and mechanical properties of high-strength steels

There is an increasing demand for ultrahigh-strength terrific steels strengthened by nanoprecipitates. Improvement of the precipitation strengthening response requires an understanding of the nanoscale precipitation mechanisms. In this study, the synergistic effects of Cu and Ni on nanoscale precipitation and mechanical properties of ferritic steels were thoroughly investigated, and new steels with ultra-high strength and high ductility have been developed. Our results indicate that Ni effectively increases the number density of Cu-rich nanoprecipitates by more than an order of magnitude, leading to a substantial increase in yield strength. It appears that Ni decreases both the strain energy for nucleation and the interfacial energy between the nucleus and the matrix, thereby decreasing the critical energy for nucleation of Cu-rich nanoprecipitates. Cu and Ni are also found to be beneficial to grain-size refinement, resulting from lowering the austenite-to-ferrite transformation temperature, as determined from thermodynamic calculations. In addition, the strengthening mechanisms of Cu and Ni were quantitatively evaluated in terms of precipitation strengthening, grain refinement strengthening and solid-solution strengthening. The current findings shed light on the composition-microstructure-property relationships in nanoprecipitate-strengthened ferritic steels. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.