Brittle fracture of nickel by dynamic indentation
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Cracks of up to 40 /xm which are either transgranular cleavage or very low energy "ductile" cracks have been introduced into large-grained fee Ni. The mechanism for introducing this brittle fracture was dynamic indentation. Optical and scanning electron microscopy together with use of selected area channeling patterns were used to confirm that the fracture process is transgranular. The results qualitatively support the hypothesis that dynamic cracks originating in a brittle film can propagate relatively large distances into a ductile face-centered-cubic substrate by a rapid, low energy process.
Except for iridium and rhodium,1 observation of transgranular cleavage in fee materials is always accompanied by one or more of various environmental effects. Specifically, Lichter, Flanagan, Lee, and Zhu2 have investigated the process of transgranular stress corrosion cracking, SSC, in copper-gold alloys and Sieradzki et al? have studied film induced SCC in Cu-Zn and Cu—Al. One standard model developed by Rice and Thomson4 predicts whether a material will respond in a ductile or brittle manner. Their criteria predict that a material will respond in a brittle manner if ^ib/lOy > £o, where /j, is the shear modulus, y is the surface energy necessary to break the bonds at the crack tip, and £0 is the material's dislocation core width divided by the magnitude of the Burgers vector, b . They determined that the low activation energy of nickel places it on the borderline between ductile and brittle materials. Their analysis suggests that Ni, without the aid of environmental effects, but with the aid of thermal fluctuations, could behave as a brittle material. In essence, this implies that Ni, like Fe, could have a ductile-brittle transition temperature. It should be pointed out that even in Fe, which is known to have a transition, substantial dislocation activity precedes cleavage. Recently,5 a modification of Rice-Thomson allows for an emission criterion which is structure dependent, retaining the local Griffith argument while being consistent with far-field plasticity observations. This strongly suggests that in materials like Fe and Ni, specific dislocation arrangements will accompany cleavage. If the modification of the Rice-Thomson model is correct, Ni also may exhibit a ductile-brittle transition under specific conditions of load rate, state of stress, and/or substructural arrangements. Until now, however,
a) Currently
NRC Post-Doctoral Associate, National Institute of Standards and Technology, Gaithersburg, Maryland 20899. J. Mater. Res., Vol. 7, No. 8, Aug 1992 http://journals.cambridge.org
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Ni has not been observed to undergo transgranular cleavage without some environmental effect entering into the picture by lowering y which is derived to be the energy necessary to break the bonds at the crack tip. Recent findings suggest that it is possible to induce brittle cracks into fee nickel without the aid of corrosion effects. Rapid indentation into the polished surface of a large-grained Ni specimen
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