Evaluation of Microplastic Flow Stress in Copper Alloys from Amplitude-Dependent Internal Friction

Stress-strain relations are evaluated from amplitude-dependent internal friction in polycrystalline solid solution copper alloys. The flow stress in microplastic strain range much below the yield point is examined for eight kinds of specimens, prepared by alloying commercial-grade pure copper with 0.3 at% solute atoms (aluminum, silicon, nickel, gallium, germanium, indium, tin and gold). Internal friction is measured at room temperature under atmospheric pressure using the free-decay method of flexural resonant vibration with both free ends around 600 Hz. The flow stress of Cu–Sn alloys required to cause the plastic strain of 1×10−9 is about 440 times larger than that of pure Cu, while that of Cu–Ni alloys only 6 times larger. Thus the flow stress evaluated from amplitude-dependent internal friction reflects sensitively the change of solute elements. The flow stress is examined in terms of the misfit parameters between solute and solvent atoms proposed by Fleischer, and it is shown that the flow stress in microplastic strain range is controlled by the elastic interactions between screw dislocations and solute atoms in solid solution copper alloys.