The effects of size, temperature and strain-rate on the mechanical properties of face-centered cubic metals

Drawn and annealed copper wires of diameters ranging from 50 μ to 900 μ were tested in tension and the results examined for evidence of size-effects. No size-effect on yield-stress or work-hardening rate has been definitely established. The results were discussed in terms of the fraction of the number of grains in the specimen which have a free surface. The ultimate tensile strength and ductility decrease with decreasing diameter for diameters below 200 μ. An explanation has been put forward in terms of void-formation during deformation. Polycrystals and single crystals of copper were tested at room temperature with the strain-rate cycled between 10⁻¹ min⁻¹ and 10⁻³ min⁻¹. Polycrystalline copper obeys the Cottrell-Stokes law but shows a variation in the ratio of flow-stresses with varying grain diameter and with a varying value of the fraction of grains in the specimen which show a free surface. Copper specimens were also tested with the temperature cycled between 78°K and 293°K. Copper polycrystals do not obey the Cottrell-Stokes law, the deviation depending on the grain size. These results are discussed in terms of stacking-fault energy and several possible explanations are considered. Aluminum polycrystals were tested with the temperature cycled between 78°K and 293°K. Aluminum obeys the Cottrell-Stokes law for temperature variation. A work-softening effect accompanies the yield-drop found at 293°K after prior deformation at 78°K. This was discussed in terms of cross-slip and dislocation climb mechanisms.Applied Science, Faculty ofMining Engineering, Keevil Institute ofGraduat