Effect of Hydrogen on Dislocation Emission from a Crack Tip in Nickel
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EFFECT OF HYDROGEN ON DISLOCATION EMISSIJON FROM A CRACK T'IP IN NICKEL
RICHARD G. HOAGLAND, Dept. of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164-2920
ABSTRACT A method for determining the intrinsic resistance to dislocation emission and glide in atomic models is presented and applied to EAM models of nickel containing a single-ended crack tip. The method is an adaptation of the Peierls approach in which the work done on the glide plane is computed as a function of dislocation position and its derivative is the intrinsic resistance to dislocation motion. The results indicate that there are two parts to the resistance to dislocation glide from the crack tip: a distinct barrier to injection of the dislocation from the crack tip and a periodic resistance associated with the lattice. When a hydrogen interstitial is placed on the slip plane near the crack tip the barrier height is reduced but its width is increased. INTRODUCTION Recent atomic simulation calculations involving hydrogen interstitials in nickel by Daw, et al [1], indicate that crack extension becomes easier, i.e., occurs at a lower applied stress, in a situation where a hydrogen interstitial is near the tip of a slot in a nickel lattice parallel to 1100) planes. Their calculations were based on a many-body, EAM-type potential that accommodates Ni-Ni, Ni-H, and H-H interactions in a way that approximately reproduces the elastic constants and stacking fault energy of nickel, and the heat of solution of hydrogen in nickel. There is a considerable advantage of this particular many-body potential over a twobody potential, as is at least partly evident in the elastic constants, which for pair potentials, are constrained to satisfy the Cauchy relations. Hoagland and Heinisch [2], using the same potential developed by Daw, et al., but applied to a model with a single-ended crack tip in Ni, examined the effect of the position of a hydrogen interstitial on the system energy, the crack tip opening displacement, the stress field, and the crack tip driving force. The variation in each of these parameters with the interstitial position provided some quantitative indication of the effect of hydrogen in assisting crack extension. Their results also contained some qualitative evidence that hydrogen may, in certain circumstances assist dislocation emission. In particular, they observed that partial dislocations were emitted simultaneously from the crack tip and moved away from it symmetrically when hydrogen was absent. However, when a hydrogen interstitial was placed near the slip plane on one side of the crack plane, the dislocation on that side was emitted much sooner. The purpose of the present paper is to describe a method that determines the resistance to dislocation emission from a crack tip in an atomic simulation in a more quantitative way than presented in Ref. 2. In particular, we determine the work done on the slip plane as the dislocations gradually emerge and glide away from the crack tip. The analysis involves an anal
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