The Influence of Impurity Content on Thermal Stability of Low Stacking Fault Energy Silver Processed by Severe Plastic Deformation

The effect of the impurity content on the evolution of the ultrafine-grained (UFG) microstructure in low stacking fault energy Ag and its stability at room and elevated temperatures were investigated. Samples of silver having high (99.995%) and somewhat lower (99.99%) purity levels were processed by equal-channel angular pressing (ECAP) at room temperature (RT) up to 6 passes. Although, the minimum grain size achieved by ECAP was similar to 200 nm for both series, the lattice defect structure was strongly influenced by the impurity content. In the samples processed by 4-16 passes of ECAP a self-annealing occurred during storage RT that was promoted by the higher twin boundary frequency. Both room- and high-temperature thermal stability of 99.99% purity Ag were much better due to the pinning effect of impurities. It was found that a large number of dislocation loops remained in the microstructure even after recrystallization at high temperatures.