Strain-rate sensitivity of additive-free pulsed-electrodeposited nickel during cyclic loading

Sub-microcrystalline (smc) nickel, produced by pulsed electrodeposition (PED) without additives for grain refinement, was cyclically deformed in the as-prepared state at room temperature at different plastic strain amplitudes 10−4 ≤ ɛpa ≤ 10−3. The plastic strain-rate ε ˙ p = 2 × 10 − 5   s − 1 was repeatedly varied over two orders of magnitude to determine the cyclic strain-rate sensitivity ( ∂ σ a / ∂ ln ε ˙ p ) T of the stress amplitude σa. In the as-prepared state smc PED nickel shows an unusually high value of ( ∂ σ a / ∂ ln ⁡ ε ˙ p ) T , which decreases with increasing ɛpa (and σa). During cycling at constant ɛpa ≥ 2.5 × 10−4, a pronounced softening accompanied by a strong decrease of ( ∂ σ a / ∂ ln ⁡ ε ˙ p ) T is observed. This behaviour is quite different to the behaviour of fine-crystalline (fc) nickel, but seems similar to the reaction of smc nickel produced by equal-channel angular pressing (ECAP). However, whereas in fc and smc ECAP nickel dislocation–dislocation interaction governs the plastic behaviour from the beginning, it is concluded that in smc PED nickel in the as-prepared state, the interaction of dislocations with grain boundaries (GBs) is responsible for the high ( ∂ σ a / ∂ ln ε ˙ p ) T . Its decrease with increasing ɛpa (and σa) is due to enforced long-range dislocation movement leading to enhanced dislocation–dislocation interaction within the grains. GB relaxation processes then, probably, result in the observed decrease of σa and ( ∂ σ a / ∂ ln ⁡ ε ˙ p ) T during cycling at constant ɛpa ≥ 2.5 × 10−4. This also accounts for the considerable reduction of the high internal stresses observed in the as-prepared smc PED nickel.