The Effect of Texture on the Hall-Petch Relationship in Severely Drawn Ni-Co Microwires

The influence of texture on strength was investigated by altering the amount of cobalt in Ni-Co alloys. The purpose of incorporating Co was to reduce the stacking fault energy (SFE) of the alloy. A decrease in the SFE reduces the ease of cross-slip and enhances planar slip, leading to changes in texture during cold working. Three Ni-Co alloys (33, 50, and 60 wt pctCo) were wire drawn at room temperature in a specially designed setup to wire diameters of 500, 200, and 100 μm, corresponding to a drawing strain of 2.67, 4.05, and 5.88. EBSD analysis revealed a refinement in grain size with drawing strain as well as a transition from elongated to equiaxed grains in Ni-50 and Ni-60 Co alloys. Line profile analysis and bulk texture analysis showed that increasing drawing strain leads to a higher defect density, an increased <100> fiber fraction and a decreased <111> fiber fraction, with the trends increasing with an increase in Co content. Tensile tests on the drawn wires show that Ni-60 Co alloy has higher strength with superior ductility for all wire diameters. The HP coefficients for the annealed conditions, calculated by annealing Ni-Co wires of 100 μm diameter, was found to be independent of SFE. The HP coefficients in drawn conditions were found to be higher than in the annealed conditions. The activation volume measured from the multiple stress relaxations in the drawn wires ranged from 20 to 5.5 b3, suggesting that dislocation interactions remained the underlying operating deformation mechanism.