Cyclic deformation behavior of a copper tricrystal and bicrystal

Cyclic deformation was applied to a copper tricrystal (TC) as well as a bicrystal (BC) at constant axial plastic strain amplitude in the range of 1×10−4≤εpl≤4×10−3. The initial cyclic hardening of TC was found to be similar to that of BC at low strain amplitudes. The cyclic hardening of TC is higher than that of BC at higher εpl and the distinction becomes pronounced with εpl increasing. The cyclic stress–strain (CSS) curve of TC is obviously higher than that of BC. No matter for TC and BC specimens, σs increases slowly with increasing εpl and the difference between them is almost independent of εpl when εpl<1.5×10−3, σs of TC and BC as well as the difference between them increase obviously after εpl>1.5×10−3. Both curves show almost no plateau. The above mechanical behaviors are in accord with surface morphologies and simple analysis. Two effects of the grain boundary triple junction (TJ) on slip have been found: one is retardation to the primary slip at lower strain amplitudes, the other is the activation of multiple slip systems due to the internal stresses caused by the strain incompatibilities. Deformation bands appeared near grain boundary for BC at higher strain amplitudes while these did not occur for TC at the same strain amplitude. Microcracks nucleated near TJ for εpl greater than 0.7×10−4.