1. Fig. 2 Correlations Between the Model of Dislocation-Accommodated Boundary Sliding and Experimental Data for Nanocrystalline Ni – (II) Farghalli A. Mohamed, 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 purpose of this study is to examine whether the model of Dislocation Accommodated Boundary Sliding as represented by Eq. (1) can explain the deformation characteristics of nc- materials such as nc-Ni. Correlation with experimental results. [+] Two types of reported results are considered herein.. (b) Ductility. The results of investigations on nc-Ni at room temperature have shown that elongations to failure (ductility) are much lower than those reported for micrograined alloys; ductility is in the range 2%-6%. The present model offers a possible explanation. The stress exponent for creep, n, as predicted by the model exhibits high and variable values; n > 5. Accordingly, it is expected that ductility in nc-Ni would be much lower than those characterizing micrograined superplastic alloys, for which n = 2, since ductility depends on 1/n (n = 1/m), as illustrated, for example, by the following expression: Fig. 1 Comparison between the prediction of model and available data on strain rate sensitivity; Fig.2 Comparison between the prediction of the model for the percentage elongation to failure, ef % (ductility) and experimental data at room temperature for 20 nm nc-Ni. ef % = [exp(C/(n –1)) – 1] x 100 (2) where the value of C = (n-(1/n)) ln (400/n); n = 1/m. The results of Fig.2 are combined with Equation (2) for the purpose of plotting ef %, against tensile strain rate at room temperature. This plot is shown in Fig. 2. Included in the figure are the experimental earlier data on ductility. It is clear that the agreement is good in terms of the trend. Significance. This research is significant since it shows that a recently developed model based on dislocation accommodated boundary sliding can explain experimental trends noted for nc-Ni at room temperature: (a) the occurrence of very small changes in strains sensitivity with strain rate or stress, and (b) the very low values of ductility. (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, 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. Fig. 1 [+] F.A. Mohamed and M. Chauhan, Metallurgical and Materials Transactions A, (2006), 37a (12): 3555-3567. (a) Strain rate sensitivity as a function of stress. Very recently, extensive measurements relating strain rate sensitivity, m (1/n to applied tensile stress,, in nc-Ni of nc-Ni at room temperature were reported. In Fig. 1, the reported values of m are plotted as function of applied tensile stress,. Also, included in the figure is the prediction from Equation (1) with 2M b 3 = v = 20 b 3. An inspection of the figure shows that the prediction agrees well with experimental measurements.