Deformation Strength of Nanocrystalline Thin Films

The data of deformation strength and microstructure of thin films of nanocrystalline Pd recently provided by Colla et al. have been analysed. It is shown that the properties of the films with cylindrical grains of30 nm diameter extending over a significant portion of the film thickness（≈90 nm） are quantitatively comparable to what is known from nanocrystalline bulk material. This is explained in terms of boundarymediated processes governing emission, storage, and recovery of dislocations.

Correct Interpretation of Creep Rates: A Case Study of Cu

Traditionally the deformation resistance in creep is characterized by the minimum creep rate εmin and its sensitivity to stress （stress exponent n） and temperature （activation energy Q）. Various values of constant n have been reported in the literature and interpreted in terms of specific mechanisms. The present case study of coarse-grained Cu at 573 K yields a stress exponent n = 9 for εmin. in tension and a relatively low activation energy. The evolution of the deformation resistance with strain at constant tensile creep load and comparison with creep in compression without fracture indicates that the tensile εmin. result from transition from uniform deformation to strain localization during fracture. This is confirmed by the results of creep in compression where fracture is suppressed. Both the tensile εmin, and the compressive creep rate at strains around 0.3 can be described using existing equations for quasi-stationary deformation containing the subgrain boundary misorientation θ as structure parameter. While in the latter case constant θ leads to monotonic increase of n with stress, the tensile nine-power-law results from variable θ, and has no simple meaning. The result of this case study means that uncritical interpretation of minimum tensile creep rates as stationary ones bears a high risk of systematic errors in the determination of creep parameters and identification of creep mechanisms.

Effects of Grain Refinement by ECAP on the Deformation Resistance of Al Interpreted in Terms of Boundary-Mediated Processes

Results of a large set of tensile and compressive creep tests on pure Al were reanalyzed for the influence of low-and high-angle grain boundaries on the deformation resistance at the temperature T = 473 K = 0.51 Tmwhere Tm is the melting point.Thermomechanical treatment by equal channel angular pressing followed by heating to T led to strong increase of areal fraction of high-angle boundaries in a structure of subgrains of ≈10^-6m in size,accompanied by significant reduction of subgrain strengthening and of the stress sensitivity of the deformation rate.（Sub）grain strengthening by low-angle boundaries is most effective;the strengthening effect virtually disappears during creep as the boundary spacings coarsen toward their stress-dependent,quasi-stationary size wqs.The same type of coarsening is found for（sub）grain structures with large fraction of high-angle boundaries;in the quasi-stationary state they lead to softening at low and strengthening at high stresses,and a significant increase in tensile fracture strain to values up to 0.8.The results are analogous to former results for Cu and are explained in the same way by the influence of boundaries on storage and recovery of crystal defects and the homogenization of glide.