Characterization of bulk ultrafine grained and nanocrystalline materials

Thermal stability in bulk ultra fine grained (UFG) 5083 Al that exhibited initial grain size of 305 nm, and that was processed by gas atomization followed by cryomilling, consolidation and extrusion, and in bulk nanocrystalline (nc) Ni, initial grain size of 15 and 20 nm, prepared by electrodeposition was investigated. In both the materials, two grain growth regimes were identified: a low temperature region and a high temperature region. In the low temperature regime, relatively low activation energy was found: 25 +/- 5 kJ/mol for UFG 5083 Al and 11 +/- 3 kJ/mol for nc-Ni. It is suggested that this low activation energy represents the energy for the reordering of thermodynamically non-equilibrium grain boundaries in the UFG and nc-materials. In the high temperature regime the value of activation energy for UFG 5083 Al (124 +/- 5 kJ/mol) lies in between that for grain boundary diffusion and lattice diffusion of polycrystalline Al. For nc-Ni an approximate activation energy of 105 +/- 3 kJ/mol, which is close to the activation energy for grain boundary diffusion in polycrystalline Ni, was measured. The value of the grain growth exponent, n, for both the materials (deduced from the grain growth data) were higher than the value of 2 predicted from elementary grain growth theories. The discrepancy was attributed to the operation of strong pinning forces on boundaries during the annealing treatment. An examination of the microstructure suggests that the origin of the pinning forces is most likely related to the presence of impurities and dispersion-particles on the grain boundaries.; Creep and ductility behavior of UFG 5083 Al were also studied in the temperature range of 523 K-648 K in the present investigation. The curve of ductility as a function of strain rate shows the presence of a maximum that shifts to higher strain rates with increasing temperature. An analysis of the experimental data indicates that the true stress exponent is about 2, and that the ductility characteristics of 5083 Al are a reflection of its creep behavior as a superplastic alloy and not as a solid-solution alloy. In addition, the observation of optimum elongations of more than 300% at strain rates higher than 0.1 s-1 is indicative of the occurrence of high strain rate (HSR) superplasticity. Substructural evidence for the occurrence of HSR superplasticity in UFG 5083 Al includes the retention of equiaxed grains after deformation, the observation of features associated with the occurrence of boundary sliding, and the formation of cavity stringers. Grain size stability during the superplastic deformation of the alloy is attributed to the presence of dispersion particles that are introduced during gas spraying and cryomilling. These particles also serve as obstacles for dislocation motion, a process that may account for the threshold stress estimated from the creep data of the alloy.