The disappearance of serrated flow in two copper alloys

Abstract

The disappearance of serrated flow was investigated for a commercial Cu2Be and for Cu3AgZr alloy (where the compositions are in approximate weight per cent.) In both of these alloys the serrated flow disappears at higher temperatures and slower strain rates by a progressive delay in the onset of serrated flow. The activation energy for the disappearance was found to be 1.16 eV for the Cu2Be alloy and 1.68 eV for the Cu3Ag0.38Zr alloy. The disappearance of serrated flow in both alloys is suggested to be due to the presence of solute clusters in the matrix which act as a second-phase sink which depletes the dislocation lines of their atmospheres during the dislocation arrest time at the clusters. In the temperature-strain rate regime where the disappearance occurs, serrations appear on the flow curve only after the matrix solute cluster sinks approach saturation (i.e. the clusters are approaching their stable equilibrium size) or when the atmosphere species is arriving at a rate greater than the cluster growth rate. Serrated flow then progresses with the stress decrement increasing progressively until failure of the specimen. The second-phase matrix sinks responsible for the disappearance in Cu2Be are suggested to be beryllium-rich solute clusters. The sinks responsible for the disappearance in the Cu3Ag0.38Zr alloy are suggested to be silver-rich clusters. In the higher temperature regime of the Cu3Ag0.38Zr alloy, serrated flow also disappears off the outer end of the flow curve. In this regime, after saturation of the silver cluster sinks, serrated flow initiates and persists for a small strain and then disappears. This disappearance near the end of the flow curve in the higher temperature regime is suggested to occur by a precipitation reaction between the retained silver atmosphere on the dislocations and substitutional zirconium atoms. In this high temperature regime, increasing temperature or strain and decreasing strain rate allows the zirconium atoms to diffuse through the copper matrix to the dislocation lines containing the silver atmospheres. A precipitation reaction then occurs between the silver and zirconium, resulting in an Ag nZr intermetallic directly on the dislocation lines, which thus depletes the dislocation line of its atmosphere.