EFFECT OF GRAIN-SIZE ON AMPLITUDE-DEPENDENT INTERNAL-FRICTION IN POLYCRYSTALLINE COPPER

Internal friction in polycrystalline copper varying grain size was measured as a function of strain amplitude at around 700 Hz by means of the free decay method of flexual vibration. The curves of the amplitude dependence are found to shift to higher amplitude with decreasing grain size. The data are analyzed on the basis of the phenomenological theory of microplasticity and converted into typical mechanical responses such as the microplastic strain expressed as a function of stress. The microplastic flow stress increases with decreasing grain size in a similar way to macroplastic deformation. However, the variation of microplastic flow stress with grain size is much greater than that of macroplastic flow stress. The flow stress at the plastic strain of 1 x 10(-9) cannot be expressed by the Hall-Petch relation, a simple linear function of the inverse square root of grain size, and is almost constant when grain size is larger than 80-mu-m. This means that the constraining force of an individual grain by surrounding ones markedly decreases in coarse-grained specimens under the influence of surfaces. As the plastic strain increases, however, the microplastic flow stress appears to approach the Hall-Petch relation.