The cyclic deformation behaviour of nickel polycrystals and monocrystals has been investigated as functions of applied electrochemical potential. Experiments were conducted in air and in 0.5_N H_2SO_4 at conditions of active dissolution and under conditions of electrochemical passivity. It has been shown that, for polycrystals, active dissolution of the metal during cyclic deformation results in a shift of crack initiation and propagation from transcrystalline to intercrystalline paths with a marked reduction in fatigue resistance. Cyclic deformation in the passive regime, on the other hand, results in preferential attack of persistent slip bands (PSBs). It has been shown that active dissolution of the monocrystalline surfaces results in enhanced plasticity with an enhancement of PSB formation. When the metal is in the passive condition, preferential attack of the PSbs occurs, and current transients reflect the stress cycling, increasing with increasing hardening and stabilising at saturation. When the metal is passive, the passive film is not ruptured until near stress saturation, and the passive current density is unchanged. Upon the formation of persistent slip bands however, intense strain localisation is observed, the passive film is locally ruptured and selective attack of the persistent slip bands occurs. This interaction results in further strain localisation and enhanced passive film rupture. This apparent synergism between strain localisation and film reformation/rupture explains the dramatic decrease in fatigue resistance for many passive alloys.
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