High cycle fatigue behaviour of copper deformed by high pressure torsion

Influence of purity and process parameters on the fatigue behaviour of ultrafine grained (UFG) copper produced by high pressure torsion (HPT) was studied. A special experimental set up in combination with an ultrasonic resonance fatigue testing system was used to obtain S–N curves from miniaturized specimens in the range of 106–109 loading cycles. Fatigue life curves were compared with conventional grain sized (CG) copper and UFG copper processed by equal channel angular pressing (ECAP). Due to the extreme fine microstructure of the HPT specimens and their enhanced tensile strength a considerable increase of fatigue life was found for all HPT specimens up to about 107 loading cycles. In excess of 107 cycles fatigue resistance of high purity HPT Cu decreased considerably whereas commercially pure HPT Cu showed excellent performance up to 109 cycles. Microstructural examination of the specimens confirmed that this behaviour was mainly related to the stability of the structure during the cyclic loading. The high purity HPT Cu showed a remarkable grain coarsening in the region of very high cycle fatigue leading to a strong reduction of the fatigue strength. The superior fatigue resistance of the commercial purity HPT copper was related to the higher stability of the microstructure due to the presence of impurities which prevent cyclic induced recrystallization and grain coarsening.