Cyclic deformation behavior of copper bicrystals

Cyclic deformation was applied to three kinds of copper bicrystals with different geometries designated as [<(1)over bar 35>]/[<(1)over bar 35>], [<(1)over bar 35>]/[<(2)over bar 35>] and [<(2)over bar 35>]/[<(2)over bar 35>] as well as a [<(1)over bar 35>] copper single crystal at a plastic shear strain amplitude in the range of 1.66 x 10(-4) to 9.1 x 10(-3) Experimental results showed that the four kinds of crystals all demonstrate a plateau in the cyclic stress-strain (CSS) curves in the range of approximately 1.66 x 10(-4) less than or equal to gamma(p1) less than or equal to 5.0 x 10(-3) except for the [<(1)over bar 35>]/[<(1)over bar 35>] bicrystal, which has a slightly smaller upper plateau value. The initial hardening rate and the cyclic hardening curves for the [<(1)over bar 35>]/[<(2)over bar 35>] bicrystal were found to be similar to those of the [<(1)over bar 35>] single crystal, and the CSS curve shows a plateau stress of about 29 MPa, very close to that of the [<(1)over bar 35>] single crystal. The [<(2)over bar 35>]/[<(2)over bar 35>] bicrystal shows rapid cyclic hardening behavior, and the CSS curve exhibits a high plateau stress of about 36 MPa. For the [<(1)over bar 35>]/[<(2)over bar 35>] non-isoaxial bicrystal, the cyclic hardening rate and the plateau stress are between those of the [<(1)over bar 35>]/[<(1)over bar 35>] and [<(2)over bar 35>]/[<(2)over bar 35>] bicrystals; the plateau stress is approximately the mean of the above, about 32.5 MPa. The above results are in accord with the surface observations. Activation of secondary slip due to grain boundary (GB) constraints and different degrees of dislocation interaction may be responsible for the above results. Slip heterogeneity, slip transfer across the GB and deformation bands were observed on the surfaces of the bicrystals.