Shear bands at the fatigue crack tip of nanocrystalline nickel

Abstract

Fatigue crack growth rates in electrodeposited nanocrystalline Ni and its coarse counterpart were experimentally investigated in this paper. The shear bands, called the epsilon plastic zone, were observed on the surface of nanocrystalline Ni samples. The length of the shear bands is close to the estimated size of the plastic zone at the crack tip. Atomic force microscopy measurements indicate that the shear strain in the shear bands and the width of shear bands increase with related crack length.

A Review of Fatigue Behavior in Nanocrystalline Metals

“…The presence of shear lips and dimpled rupture in those studies are also indicative of substantial ductility. Shear banding in the crack tip plastic zone has also been reported in one study of electrodeposited NC Ni [71].…”

“…Other experimental studies using NC Ni have demonstrated the presence of dual shear bands along either side of the crack path, correlating with each loading cycle of the sample [71]. While the length of the shear bands was limited by the monotonic plastic zone size, which was several tens-of-microns under those loading conditions, the spacing between bands correlated with the spacing between striations on the fracture surfaces.…”

“…To date, the majority of NC metals used in experiments are produced by electrodeposition, of which Ni is a prime example [7,8,11,14,20,22,24,30,[58][59][60][61][62][63][64][65][66][67][68][69][70][71][72]. A Watts electrochemical bath [73] is often used in this process, in which current is passed between two electrodes submersed in an aqueous bath of metal salts [ Fig.…”

“…Several of these crack-tip shielding mechanisms are expected to have diminished effect on NC metals. As grain size is refined, the tortuosity of fatigue cracks is reduced [14,22,71] which implies that shielding associated with crack deflection is diminished in NC metals. Also, since NC fracture surfaces have low roughness, corresponding to the fine grain size, roughness-induced crack closure effects may also be minimized in NC alloys.…”

On the length scale of cyclic strain localization in fine-grained copper films

“…Thus, it is suggested that the GB mobility is improved dramatically [15,16] to mediate cyclic plastic deformation. Cyclic plastic strain accumulation will concentrate on GB-involved deformation, such as shear banding [17], GB sliding and grain rotation [18], as a dominant mechanism that would take over the conventional PSB-based fatigue mechanism [7]. Our finding of l min sp clearly indicates that cyclic strain localization through extrusions or slip lines is completely hindered when the length scale of the film is less than l min sp , and the corresponding physical mechanism of fatigue damage has been changed…”