The definition of characteristic times of plastic relaxation by dislocation slip and grain boundary sliding in copper and nickel

• The behaviour of the yield stress of metals is considered from viewpoint of dynamics.
• The integral yield criterion can describe anomalous behaviour of plastic deformation.
• The characteristic time is compared in terms of the integral yield criterion and structural models.
• The behaviour of yield stress versus grain size for nickel in a wide range of strain rates is studied.

The behaviour of the yield strength of metals is considered from viewpoint of dynamics as a function of the limiting stress and the characteristic time of stress relaxation. Experimental curves of the static and dynamic deformation of the coarse-grained copper, microcrystalline and nanocrystalline nickel are analyzed on the basis of the relaxation model of plasticity (the integral criterion) with a constant characteristic relaxation time. The analytical expressions of the relaxation time as a function of variables that characterizes the internal structure of material are obtained from the model of dislocation slip and grain boundary sliding. The strain rate dependences of the yield strength, which are calculated with the help of both the dislocation model and the integral criterion of plasticity, are compared with the experimental data for copper and nickel in a wide range of strain rates (10−3−1011 s−1). It is shown that the proposed analytical expressions give the relaxation time, which order-of-magnitude agrees with the values obtained by fitting the results of the integral criterion. The obtained relations allow us to calculate two separate dependences of the yield strength (one for coarse-grained material, while another for nanomaterial) in Hall–Petch coordinates both for quasi-static and for high-rate deformations.