Tensile deformation characteristics of a Cu−Ni−Si alloy containing trace elements processed by high-pressure torsion with subsequent aging

Experiments were performed to investigate the tensile deformation characteristics of a Cu-2.5 wt% Ni-0.55 wt% Si alloy containing trace elements of Zn, Sn and Mg, processed by high-pressure torsion (HPT) under an applied pressure of 5 GPa for 10 revolutions, and then peak-aged or over-aged at 300 °C. The grain size of the HPT-processed (H) alloy was refined to 70 nm. The peak-aged (PA) alloy exhibited a higher tensile strength σu of over 1 GPa than the H alloy. The over-aged (OA) alloy exhibited a lower value of σu than not only the PA but the H alloy. The PA and the OA alloy revealed unique tensile deformation properties: an extremely small local elongation of 0.1% for the PA alloy, a very small uniform elongation of 0.2% for the OA alloy, and a sudden drop of tensile stress immediately after necking initiation for the OA alloy. The small local and uniform elongation for the PA and OA alloy were explained on the basis of intergranular fracture after and before necking initiation during tension. The stress drop was caused by the rapid propagation of intergranular fracture from the surface of necking part to the interior. Intergranular segregation of Sn promoted by aging at 300 °C resulted in the intergranular fracture of the PA and OA alloy.