An atomic simulation of the influence of hydrogen on the fracture behavior of nickel
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H. L. Heinisch Battelle Pacific Northwest Laboratories, Richland, Washington 99352 (Received 16 January 1992; accepted 20 April 1992)
A model exploring the effect of the presence of a single hydrogen interstitial on the crack tip configuration of nickel is described. The model is based on a EAM-type potential developed by Daw, Baskes, Bisson, and Wolfer for describing the Ni-Ni, Ni-H, and H—H interactions, and involves the crack tip region of a semi-infinite crack in an infinite solid. Several types of interactions are observed to occur. In a model oriented such that dislocation emission is difficult, hydrogen is observed to increase the crack tip opening displacement (CTOD), exert a force on the crack tip due to interaction between the dilatancy of the defect and the hydrostatic component of the field of the crack, and increase the local tensile stresses. However, the largest contribution to extending the crack derives from the energy released when a hydrogen interstitial escapes to the crack surface. A hydrogen interstitial is also observed to assist dislocation emission in models with an easy emission orientation.
I. INTRODUCTION When a crack or similar defect is present, hydrogen, as a dissolved interstitial, degrades fracture toughness and promotes subcritical crack growth in many metals. Hydrogen has also been shown to cause a significant reduction in the ductility of an intermetallic, FeAl,1 and it may be responsible for the low ductility of others. Hirth2 has reviewed a number of mechanisms that have been proposed to account for degradation, noting that more than one mechanism may be involved in any particular metal/H system. Some of these explanations propose events occurring near a crack tip on an atomic level which include hydrogen enhancing interatomic separation, crack tip plasticity, and grain boundary separation. These mechanisms may, in general, be classified as types of Rice—Thomson arguments3 in which hydrogen atoms influence the balance between dislocation emission and crack extension at a crack tip. Therefore, hydrogen must affect the evolution of the crack tip configuration with increasing K in a way that reduces the applied K needed to initiate crack growth. The processes leading to hydrogen embrittlement are not easily investigated experimentally, because the region of greatest interest, the crack tip, is rather inaccessible and the processes occur on an atomic scale. Consequently, theoretical techniques to characterize the interatomic forces and energies in such problems have been attractive. One technique involves ab initio calculations to determine the electronic mechanisms underlying the influence of hydrogen on the atoms of the host 2080 http://journals.cambridge.org
J. Mater. Res., Vol. 7, No. 8, Aug 1992 Downloaded: 14 Mar 2015
(cf. Briant and Messmer4 and Fu and Painter 5 ). The advantage of ab initio calculations is that they provide relatively accurate and reliable solutions for the type of atomic configurations they consider. They suffer a disadvantage in that they rely on the
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