(1)

1. (1) where b is the Burgers vector, d is the grain size, Dgbo is the frequency factor for grain boundary diffusion, R is the gas constant, Qgb is the activation energy for grain boundary diffusion, M is a stress concentration factor, is the applied shear stress, T is the absolute temperature, k is Boltzmann’s constant. The term 2Mb3 in Equation (1) represents the activation volume v. Fig. 2 Creep rate, , is plotted as a function of shear stress, t, on a double logarithmic scale for 40 and 100 nm nc-Ni at 393 K. The figure also shows the prediction of the proposed model. Correlations Between the Model of Dislocation-Accommodated Boundary Sliding and Experimental Data for Nanocrystalline Ni – (I) Farghalli A. Mohamed and Manish Chauhan, University of California, DMR 0304629 Introduction. Consideration of guiding information on deformation in nanocrystalline (nc) materials and utilization of fundamental knowledge on deformation has led to the development of a new deformation process that is based on the concept of dislocation-accommodated boundary sliding. In developing the model, it has been 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 rate-controlling equation can be represented by: Purpose.The purposeof this study is to examine whether the model of Dislocation Accommodated Boundary Sliding that was recently put forward can explain the deformation characteristics of nc- materials such as nc-Ni, which has served as a model material. Correlation with experimental results.[+]The deformation rates as a function of stress for nc-Ni agree reasonably well with the predictions of the above model (Fig.2). Also, the model as represented by Equation (1) predicts that Qadecreases with increasing applied shear stress. This prediction agrees with the experimental finding on nc-Ni that the apparent activation energies are 141 kJ/mol, 134 kJ/mol, and 126 kJ/mol at 141MPa, 165 MPa, and 188 MPa, respectively. Furthermore, the model predicts that the deformation rate depends on 1/(d3). This prediction agrees with the experimental data as shown for 100 nm in Fig.2. . Significance. This research is significant since it shows that a recently developed model based on dislocation accommodated boundary sliding can explain the creep characteristics of nc-Ni that include the variation of the stress exponent and the apparent activation energy with stress, and grain size sensitivity. 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. [+] F.A. Mohamed and M. Chauhan, Metallurgical and Materials Transactions A, (2006), 37a (12): 3555-3567