Effect of high-pressure torsion on high cycle fatigue of commercially pure Cu: Some insights from formation of surface micro-cracks

In most technical applications, fatigue is related to highly localized load distributions. While high-pressure torsion (HPT) materials cannot prevent crack initiation, they promise to improve the resistance to crack nucleation and propagation. Herein, commercially pure Cu was processed through HPT at a pressure of 6 GPa up to 50 turns. Small “cantilever” type samples were fabricated from annealed and HPT samples. The cantilever samples were subjected to fully-reversed cyclic bending. The maximum stress amplitude was chosen to reach the high cycle fatigue regime, and the experiments were stopped when the resonant frequency decayed by 20%. Compared to the annealed samples, the HPT samples showed higher lifetimes. The grain size in the HPT samples remained stable during fatigue, and dislocation substructures, a stacking of parallel dislocations, could be observed in all samples. In HPT samples, the area fraction of surface micro-cracks increased with the local stress amplitude. This can be attributed to the inhibition of crack nucleation at low stresses due to the high strength of HPT samples and the crack arrest at the boundaries of their ultra-fine grains. The obtained insights into the microstructure-fatigue response relationship are vital for understanding the initial stages of fatigue failure in ultra-fine-grained materials and their technological adoption for applications in extreme environments.