Corrosion fatigue of ultra-fine grain copper fabricated by severe plastic deformation

Corrosion fatigue is investigated of ultra-fine grain (UFG) copper fabricated by severe plastic deformation through equal-channel angular pressing (ECAP). The corrosion fatigue behavior in 1M NaNO2 aqueous solution is compared with results of testing in air under low-cyclic regimes with constant plastic strain amplitudes Deltaepsilon(pl)/2 ranged from 2x10(-3) to 5x10(-2). The significant enhancement of fatigue life in the UFG state is achieved in comparison with polycrystalline copper of ordinary grain size. It is shown that a higher overall resistance against corrosion fatigue can be obtained after grain reduction via ECAP. The detailed structural and morphological investigations have been performed on different stages of cycling. While the crack nucleation occurs transgranularly in polycrystals with relatively large grain size, the intergranular fracture is observed in UFG specimens. It is shown that cyclic softening, which occurs both in air and in aggressive environment, is associated with abnormal grain growth. Formation of a dislocation structure characteristic for persistent slip bands (PSBs) is evident in the grains which have been coarsened during fatigue. The PSB - grain boundary interaction is supposed to facilitate strongly the intergranular fatigue fracture, which is particularly pronounced in corrosion fatigue where the grain boundaries are preferentially attacked by aggressive spices. The findings of the present investigations are discussed in context of former fatigue studies and results of general corrosion and stress-corrosion tests. The possible scenario of intergranular corrosion fatigue damage in UFG copper is proposed.