A study of quench hardening in copper

Quenching studies of copper, under clean conditions, have revealed a two stage hardening mechanism. The first stage corresponds to the formation of small vacancy clusters and occurs around room temperature, while the second stage is centered around 250°C. Measurements of the dislocation loop density in quenched, aged specimens in the second stage of hardening, show that Frank vacancy loops are the major obstacle to dislocation motion. The temperature dependence of the critical resolved shear stress, in quenched specimens, was also measured throughout the aging process and compared with various expressions for the interaction of mobile dislocations with different obstacles. These results indicate quite clearly that Seeger's theory of obstacle shearing is favored over Fleisher's theory of asymmetrical distortion in this case. An experimental value of the cutting process is about 5 eV. In agreement with this mechanism, electron microscope observations show that Frank dislocation loops, within a slip plane, are removed during the first few percent of plastic deformation. Finally, the quench hardening is only removed at very high temperatures, in the same temperature range where the Frank loops are removed.