Strengthening in tension and weakening in torsion in drawn nickel microwires

Abstract

This study deals with the microstructure and texture effects on strength in tension and torsion testing of nickel microwires. Wire drawing was used to produce four different batches of microwires with a diameter of 100 μm with different mechanical treatments. Tension tests showed strain hardening with increasing yield strength upon an increase in drawing strain. However, the torsion tests showed an opposite trend, with a decrease in strength upon increasing wire drawing strain. This unexpected behavior in strengthening during torsion is attributed to the microtextures of drawn wires. Electron backscattered diffraction (EBSD) scans on the cross-sections of microwires revealed a core region texture with strong <111> fiber and a significantly different texture in the peripheral shell region. The fraction of <111> fiber increased with increasing drawing strain. A comparative analysis of Taylor factor M for uniaxial and shear loading of different texture fibers components showed that the effect of grain orientation on the mechanical strength is much higher in shear loading than the uniaxial loading. Crystal plasticity theory has been used to understand the impact of the loading conditions and rigid body rotations on the influence of deformation in grains with different orientations. The rate of hardening during torsion loading was more for microwires with increasing drawing strain, and this is attributed to the generation of more geometrically necessary dislocations in finer grained materials.