Strength and microstructure evolution in nickel during large strain wire drawing

This study deals with the evolution of strength and microstructure in pure nickel during wire drawing from an initial diameter of 1.74 mm to 30 μm, corresponding to a total true strain of 8.1. Electron backscattering diffraction (EBSD) and X-ray diffraction (XRD) have been used for microstructural characterization. In the later stages of deformation, the fraction of low angle boundaries decreased as did the misorientations within a grain or cell. The dislocation density stabilized at ∼2 × 1015 m−2, and the activation volume decreased from ∼100 b3 at a strain of 1 to ∼ 15 b3 at strains ≳4, where b is the magnitude of the Burgers vector. Cross-sectional EBSD of the wires revealed that a core region had <111> fiber texture, whereas a peripheral shell region had a complex fiber texture due to redundant shear strain. Viscoplastic self-consistent (VPSC) simulations were in good agreement with the crystallographic texture evolution in the wires during drawing. The wire drawing data is compared with other deformation techniques, to provide a broad framework for analyzing microstructure-strength-ductility relationships. At large strains of >2, with an essentially constant dislocation density, the strength can be related solely to Hall-Petch strengthening by a refinement of the transverse grain size.