Effect of orientation relationship on the morphology and strengthening effect of chromium precipitation in Cu-0.67Cr-0.3Zr alloy

High strength and high conductivity Cu-0.67Cr-0.3Zr alloy was prepared by vacuum directional solidification equipment. The face centered cubic (FCC) and body centered cubic (BCC) Cr rich precipitations in Cu matrix were characterized by high resolution transmission electron microscopy. In the as cast and solution treated state, a large amount of coherent FCC Cr precipitates is uniformly distributed in the matrix. These FCC Cr precipitates are covered by close packed {111} crystal planes and presents regular morphology. Due to their small size and cube-cube orientation with the matrix, FCC Cr precipitates follows dislocation shearing mechanism, resulting in limited strengthening effect. After aging treatment, both incoherent ordered nearly spherical and incoherent rodshaped BCC Cr precipitates were formed in the matrix. These incoherent precipitates always follow dislocation bypassing mechanism. Due to its small size and high-volume content, the spherical precipitation plays a major role in improving the strength of the alloy. On the contrary, the influence of rod-shaped precipitates on strength is minimal. According to the minimum free energy theory and local chemical potential equilibrium requirement, the precipitation with Kurdjumov-Sachs (K-S) orientation relationship tends to be rod-shaped extending along the direction of Cu [110] and Cr [111], while the precipitation with Nishiyama-Wassemann (N-W) orientation relationship tends to be nearly spherical.