Dislocation Microstructure Evolution in Cyclically Twisted Micrometer-Sized Metallic Samples: A Discrete Dislocation Dynamics Analysis

Abstract

Mechanical properties of metals at the micrometer scale are different compared to bulk behavior. At these dimensions, for plastic deformation increasing stresses with decreasing sample sizes are required. This result is explained by the marginal number of activated dislocations which control plasticity. However, for reliable technical devices at small scales, the behavior of dislocations has to be understood in detail. Three‐dimensional discrete dislocation dynamics simulations are an adequate tool to study the dislocation microstructure evolution during the complete deformation process.Dislocation motion in twisted single crystalline aluminum specimens is simulated. Under torsion loading, geometrically necessary dislocations have to be generated to accommodate the strain gradient. Screw dislocations pile‐up in the sample centre where shear stresses are low. Stress gradients hinder the dislocations to move through the sample and escape at the opposite surface. Dislocation density increases linearly as a function of plastic torsion angle which is expected from the theoretical model. These observations are independent of the crystallographic orientation.However, it is found that the dislocation microstructure is non‐reversible if the sample is untwisted. Sessile dislocation reactions lead to an increase in the dislocation density at the untwisted state with increasing loading amplitude and number of cycles. Furthermore, this effect is more pronounced by dislocation cross‐slip. Although the dislocation microstructure changes significantly, less impact on the normalized torsion moment can be found in the first cycles.