R. Birringer

The study of grain size dependence of yield stress of copper for a wide grain size range

Abstract Room temperature yield strength of copper has been measured by means of miniaturized disk-bend test as a function of grain size ranging from about 30 nm to 180 μm. It has been established that grain size dependence of strength does not obey the Hall-Petch relation in the entire range of grain sizes studied. The results obtained suggest that a gradual change of deformation mechanism takes place with decreasing grain size. Nanostructured samples appear to be rather ductile.

Microstructure evolution during rolling of inert-gas condensed palladium

During cold-rolling of nanocrystalline Pd consolidated from clusters we observed a strong increase in stacking fault density, conclusive evidence for lattice dislocation activity. However, the absence of texture and the retention of an equiaxed grain shape even after large deformation suggested grain boundary sliding and grain rotation as concurring processes. The rate of tensile creep at 313 K and at low stress is in agreement with predictions for Coble creep.

Grain-size-dependent Curie transition in nanocrystalline Gd: the influence of interface stress

Abstract The dependence of the Curie transition temperature T C on the average crystallite size of a nanocrystalline material has been studied experimentally in nanostructured Gd prepared by the inert-gas condensation method. Measurements of heat capacity and AC magnetic susceptibility indicate that T C shifts to lower temperatures and the Curie transition takes on an increasingly broadened and rounded appearance with decreasing crystallite size. Isothermal annealing steps resulted in controlled grain growth, thus permitting the functional dependence of the T C shift on crystallite size to be determined. The latter was inconsistent with the finite-size effect observed in Gd films. Rather, the shift in Curie temperature could best be understood as a consequence of the significant internal pressure induced by the interface stress of high-angle grain boundaries in Gd.

Low temperature processing of dense nanocrystalline yttrium-doped cerium oxide ceramics

Nanocrystalline cerium oxide ceramics of high homogeneity and nearly full density were prepared. The starting material was synthesized by the direct homogeneous precipitation method using hexamethylenetetramine (HMT). Two alternative routes for consolidation into green bodies were employed: (i) centrifugal casting of an electrostatically stabilized colloidal solution and (ii) cold isostatic pressing of dried ceria powder. The green bodies were sintered at temperatures between 700 and 1000°C. The sinter activities of both nanocrystalline materials were strongly enhanced if compared to microcrystalline ceria. Green bodies, which were generated by colloidal processing exhibited the highest sinter activities, associated with a unique pore structure. The effect of yttrium doping on the grain size after sintering at high temperatures was also investigated. The combination of yttrium doping and colloidal processing allowed for the synthesis of dense nanocrystalline cerium oxide ceramics by pressureless sintering.

Grain-size-dependent thermopower of polycrystalline cerium oxide

The effect of grain boundaries on the defect chemistry of polycrystalline cerium oxide was investigated by measuring the thermopower of cerium oxide samples with grain sizes in the range of 0.13–11.5 Am. The samples were prepared by sintering pellets from a single batch of nanocrystalline 500 ppm Gd-doped cerium oxide powder at different temperatures. A change in the sign of the Seebeck coefficient as function of the grain size was observed, indicating a transition from predominantly ionic conductivity at large grain size to electronic conductivity at small grain size. The experimental results were analyzed using the space charge model for ionic solids, which has already been successfully employed in the analysis of the grain-size-dependent electrical conductivity of cerium oxide. The agreement between the experimental data and the space charge model, regarding both electrical conductivity and thermopower, suggested that the essential effect of the grain boundaries in cerium oxide is the accumulation/depletion of charge carriers in space charge layers, whereas the effect of microstructure on charge carrier mobilities is negligible. From the analysis of experimental results, a value of DU=0.7 V was obtained for the space charge potential at the grain boundaries in cerium oxide. D 2002 Elsevier Science B.V. All rights reserved.

Interface stress in nanocrystalline materials

Based on a generalization of a capillary equation for solids, we develop a method for measuring the absolute value of grain-boundary stress in polycrystalline samples having a large interface-to-volume ratio. The grain-boundary stress in nanocrystalline Pd is calculated from X-ray diffraction measurements of the average grain size and the residual-strain-free lattice spacings, yielding a value of 1.2 ± 0.1 N/m. The random distribution of crystallite orientations in the sample suggests that this value is characteristic of high-angle grain boundaries in Pd.

The impact of stochastic atomic jumps on the kinetics of curvature-driven grain growth

A model for grain growth allowing stochastic jumps of atoms across boundaries to occur concurrently with curvature-driven boundary migration has been solved numerically. At small grain sizes, the stochastic mechanism controls the overall rate of grain growth, whereas at large sizes, the growth rate is governed by the boundary curvature. The transition between the two regimes occurs smoothly over a range of grain sizes about one order of magnitude in width. Since the upper bound of the transition range is estimated to lie below 20 nm for most polycrystalline materials, the stochastic mechanism is expected to be relevant to the measurement and modeling of grain growth only in nanocrystalline samples.

Tomographic characterization of grain-size correlations in polycrystalline Al-Sn

The inadequacies of current analytical models for grain growth are thought to arise in part from their mean-field nature: they ignore the presence of correlations in the sizes of neighboring grains induced by the process of grain growth itself. Although grain-size correlations have been identified in microstructures generated by computer simulations of grain growth, no comparable evidence has been obtained from real samples - primarily because of the experimental difficulties associated with evaluating this inherently three-dimensional property. Using absorption- contrast x-ray microtomography, we have attempted to characterize the network of grain boundaries in polycrystalline samples of Al doped with up to 3 at.% Sn. In principle, since the tin atoms segregate to the grain boundaries, it should be possible to determine the size and relative position of each grain from a three-dimensional reconstruction of the Sn distribution, from which the desired correlation function could be calculated directly. However, the grain boundaries in Al-Sn are not uniformly decorated with tin, which presents a formidable challenge to quantifying the microstructural properties of such samples. Significant progress toward overcoming this problem has been achieved by applying a constrained phase-field grain-growth algorithm to an approximate microstructure gleaned from the tomographic contrast data.