The objective of this study was to investigate the effect of the surface area of grain boundaries on the stress relaxation behavior over a wide range of grain sizes. Stress relaxation tests were performed using single crystal (SC), coarse grained (CG), and ultra-fine grained (UFG) samples. Additionally, pure copper was used to eliminate the effects of the solid solution and precipitates. The initial stress relaxation behavior was investigated using the stress relaxation rate at 0.2 s after the beginning of strain holding. Furthermore, the internal stress was investigated as the end of the stress relaxation behavior. The stress relaxation rate increased with the surface area of the grain boundaries per unit volume (SV). Although the activation volume of the UFG sample was smaller than that of the CG samples, the stress relaxation rate was higher. This suggested that the grain boundary sliding contributed to the stress reduction of the UFG sample. Therefore, the stress relaxation rate increased with SV even if grain boundary sliding occurred. The internal stress increased with SV, except for the SC sample. Furthermore, in the CG range, the internal stress could be approximated by the Hall-Petch (H-P) relation with a coefficient almost equal to that of the flow stress. This behavior of internal stress inherently explains the dislocation pile-up model assumed in the H-P relation. The internal stress of the UFG sample decreased below the expected value based on the H-P relation. It is suggested that the depletion of dislocation sources prevented the internal stress from increasing.
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