An investigation was made of the rate sensitivity of flow stress and the extent of strengthening in polycrystalline Cu containing various volume fractions of nano-sized twins, but having the same average grain size. The specimens were produced by pulsed electrodeposition, in which the concentration of twins was varied systematically by varying the processing parameters. Depth-sensing instrumented indentation experiments performed at loading rates spanning 3 orders of magnitude on specimens with a higher density of twins (twin lamellae width of about 20nm) revealed an increase (up to seven-fold) in the rate-sensitivity of hardness as compared with essentially twin-free pure Cu of the same grain size. A reduction in twin density at a given grain size (with twin lamellae width of about 90nm) also resulted in a noticeable reduction in rate-sensitivity and hardness. The presence of a high density of nano-scale twins was also seen to impart a significant hardness which was comparable to that achieved in nano-grained Cu. Post-indentation analyses of indented Cu with nano-scale twins in the transmission electron microscope revealed deformation-induced displacement of coherent twin boundaries, formation of steps and jogs along coherent twin boundaries, and blockage of dislocations at coherent twin boundaries. These processes appeared to influence significantly the evolution of thermal activation volume for plastic flow which was some 3 orders of magnitude smaller than that known for microcrystalline Cu. Transmission electron microscopy also revealed coherent twin boundaries with a high density of dislocation debris and suggested that displaced coherent twin boundaries could serve as barriers to dislocation motion and could also provide sources for dislocation nucleation; especially near to stress concentrations, very much like grain boundaries.

Nano-Sized Twins Induce High Rate Sensitivity of Flow Stress in Pure Copper. L.Lu, R.Schwaiger, Z.W.Shan, M.Dao, K.Lu, S.Suresh: Acta Materialia, 2005, 53[7], 2169-79