A New Deformation Model for the Creep Behavior of Nanocrystalline Materials in Terms of Dislocation-Accommodated Boundary Sliding DMR Farghalli A. Mohamed, University of California, Irvine, CA 92697, USA Introduction. Nanocrystalline (nc) materials that are characterized by a grain size in the range nm offer interesting new possibilities related to many structural applications. In order to explore some of these possibilities, an understanding of the origin and nature of deformation processes in nc-materials is essential. Purpose. The purpose of this study is to develop a model that can explain the deformation characteristics of nc- materials. Development. In developing the model, it is suggested that plasticity in nc- materials is the result of grain boundary sliding accommodated by the generation and motion of dislocations under local stresses, which are higher than applied stresses due to the development of stress concentrations. The model was quantitatively developed using the following sequence of events: (a) as a result of sliding of a group of grains, the shear stress becomes concentrated at triple point, that obstructs motion of this group; (b) this local high stress that is higher that applied can then generate dislocations in the blocking grain (or initiate voids); and (c) the generated dislocations move one by one to the opposite boundary where they climb to their annihilation sites (see Fig. 1). where b is the Burgers vector, d is the grain size, D gbo is the frequency factor for grain boundary diffusion, R is the gas constant, Q gb is the activation energy for grain boundary diffusion, M is a stress concentration factor, t, is the applied shear stress, T is the absolute temperature, k is Boltzmann’s constant. The term 2Mb 3 in Equation (1) represents the activation volume v. According to the above model, strain during the creep of ED nc-Ni is produced by boundary sliding while the creep rate is governed by the time for the climb of a dislocation along the boundary until annihilation occurs. Rate Equation. By expressing the sliding rate,,in terms of the time, t c, for a dislocation to climb through a distance h along the boundary (h was assumed to be equal d/3), the following rate-controlling equation was derived: Significance. The research is significant since a new deformation model for nc-materials that is based on dislocation accommodation boundary has been developed for the first time. The model provides several predictions regarding the stress exponent, the apparent activation energy, the grain size sensitivity, and the activation volume. (1) Predictions. The model as represented by Equation (1) predicts the following: (a) The stress exponent is variable and increases with increasing applied stress. (b) The grain size sensitivity, s, is 3. (c) The apparent activation energy is variable and decreases with applied stress (d) The activation volume, v, is equal to term 2Mb 3. Consideration of available data on superplastic ultra fine-grained ceramics (grain size in the range of 200 nm- 400 nm) along with theoretical calculations for the stress required move a boundary dislocation into the interior of the grain indicates that the value of M are in the range Accordingly, the model predicts that is the range of 10 b b 3. Fig.1 Schematic diagram for the proposed model, showing that as a result of boundary sliding, dislocations are generated at a triple junction and then traverse the grain to the opposite grain boundary where they climb and are annihilated. Fig. 2 Creep rate,, is plotted as a function of shear stress,, on a double logarithmic scale for M = 20, 15 and 10. Grain sizes shown in the figures refer to those determined during steady-state creep.