Deformation in small dimensions studied by thin wires in torsion

Abstract

This thesis comprises studies of deformation of thin wires in torsion. Generally, experiments in small-scale plasticity usually focus on small-sized samples. However, studying thin wires with lengths up to a meter in torsion has the advantage of giving extremely high strain resolution and reversal of the loading direction. These experiments allow the transition from elastic to plastic deformation to be studied in forward and reversed loading and subsequently to high strains. In this way the work explored the very early stages of plastic deformation. This is important since structural failure is usually a consequence of exceeding the elastic limit. Micro-strain plastic deformation, dislocation creep, Bauschinger effects and the thermal activated recovery were easily observed. The onset of irreversible deformation was also observed and associated with a few dislocations in the largest grains throughout the wire. Easy plastic deformation on reversal of the loading direction was observed following this initial plastic deformation but not before. Strain hardening behaviour was also studied. Comparing to the traditional torque-torsion method, much higher sensitivity was achieved. Data was fitted with the Ramsberg-Osgood equation to reveal the facts underlying strain hardening and flow stress. Size dependence of the plastic deformation was studied in wires with different diameters and grain sizes. The onset of plastic deformation and subsequent hardening could be related to the combined length-scale of the wire diameter and grain size. The results in this thesis are important to understanding plasticity in confined volumes and at very low strains. The control of strength by the length-scale of the materials provides a new technique for controlling strength and fatigue resistance in metallic materials