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The mechanism of Harper-Dorn creep

1989; University of Toronto Press; Volume: 37; Issue: 8 Linguagem: Inglês

10.1016/0001-6160(89)90147-8

1878-0768

F. R. N. Nabarro,

Study of Mite Species

Abstract Existing theories of Harper-Dorn creep rely on the assumption that, under conditions where Harper-Dorn creep is observed, the dislocation density rapidly achieves a characteristic value which is independent of the method of preparation of the sample, of the applied stress, and of the extent of creep strain, but they do not attempt to justify this assumption. We perform a dimensional analysis in which it is assumed that the ratio of strain rate ϵ to applied stress σ, which is experimentally observed to be characteristic of the material and independent of the history of the material and of the applied stress, can depend only on the Burgers vector, the shear modulus μ, and the drag coefficients B c for dislocation climb and B g for dislocation glide. If we also accept the experimental observation that ϵ/σ has the activation energy of self-diffusion, the resulting relation indicates a value of ϵ/σ which is 10 11 times too large. The only other large number which can enter the theory is the ratio of μ to the Peierls stress σ p , and this indicates that the Peierls stress must play a significant role in the mechanism. We obtain a numerically reasonable theory by assuming that the equilibrium dislocation density is that at which the stress exerted by a dislocation on its neighbour is equal to the Peierls stress, while the motion of the resulting dislocation array under the applied stress is controlled by climb. The internal stresses in the Harper-Dorn regime are less than the applied stress.