Internal friction during ultrasonic deformation of alpha-brass

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Abstract
A high‐intensity ultrasonic apparatus was utilized to study the variation with strain amplitude ε of internal friction Q−1 and modulus defect ΔS/S in α‐brass, as influenced by composition, grain size, and prestress. Q−1(ε) manifests three regions: I, where Q−1 is independent of ε; II, where Q−1 rises and then levels off; III, where Q−1 rises rapidly. Regions I and II are superficially similar to the Granato‐Lücke regions, but different mechanisms seem to be acting; region II has not been recognized before in brass. The strain at the onset of region III, εu, is found to be proportional to the flow stress and to increase with decreasing grain size. Above εu, ΔS/S varies much faster than Q−1, both quantities increasing with increasing grain size, increasing with a small prestress, but decreasing when the prestress exceeds the yield stress. The slope m′≡d logQ−1P/d logε decreases with increasing ε, reaching values in the range 0–1 for ε≳2εu. Between εu and 2εu, m′ is sensitive to structure, increasing with zinc content and prestress, decreasing with grain size. It is concluded that the effects are due to the operation of the few Frank‐Read sources most easily turned. Simple models based on this idea produce some agreement with experiment. It appears that when ε≳2εu, the loops thrown off by the source are only viscosity limited, as suggested by Frost and Ashby, reaching the grain boundaries in times of the order of 10−8 sec and piling up there in the manner described by Rosenfield and Kanninen.