Enhanced mechanical properties and strengthening mechanism of Cu-Ni-Si alloy with trace chromium addition

Cu-3Ni-0.7Si-xCr-0.05Mg (x = 0.1 and 0.4) (wt% nominal composition) alloys were fabricated to investigate the effect of Cr contents on properties evolution and precipitation behavior of Cu-Ni-Si alloy. The results of the property analysis show that trace addition of 0.1 wt% Cr element contributes a better strength (ultimate tensile strength of 823 MPa) after single cold rolling and aging treatment. The precipitation behavior is characterized by transmission electron microscopy. It is found that nanoscale FCC structural Cr precipitates shift into submicron Cr3Si particles with Cr content increasing from 0.1 wt% to 0.4 wt%. Meanwhile, such superior formation of Cr3Si particle decreases the fraction of 8-Ni2Si owing to quick consumption of Si element from Cu matrix. Further quantitative analysis of various strengthening mechanisms reveals that the changed precipitation behavior notably affects their strengthening contribution to Cr contained alloys, resulting in an increase of more than 5.6% in the Orowan-Ashby mechanism. Surprisingly, in the case of the aging state at a temperature of 460 degrees C for a duration of 2 h, it is confirmed that although a much larger strengthening effect origins from nanosized 8-Ni2Si precipitate, the strength variation relies more on the Cr precipitation. That is, the lower strength at higher Cr content is mainly caused by the lower strengthening effect from Cr element related phases. In addition, the effect of the addition of trace Cr element on the operating mechanism on conductivity is also clarified. The results are expected to provide an appropriate content of Cr element and a dependable processing theory guidance for improving the comprehensive properties of the Cu-Ni-Si alloys.