Showing papers on "Grain boundary strengthening published in 1997"

Room temperature creep behavior of nanocrystalline nickel produced by an electrodeposition technique

Abstract: Deformation processes of nanocrystalline (6–40 nm) nickel produced by an electrodeposition technique were studied. First, the results of unidirectional tensile tests were discussed with respect to the deviation from the Hall-Petch relationship. It was suggested that such a mechanical behavior exhibited by nanocrystalline materials could be described by a composite model proposed previously. Further experimental work on static and dynamic creep tests under the load control condition showed that nanocrystalline nickel electrodeposits exhibited a significant room temperature creep behavior. It appeared that grain boundary sliding and diffusive matter transport within the intercrystalline region played an important role in terms of deformation mechanisms of nanocrystalline materials. The contributions of dynamic creep to stress-strain behavior and, in turn, to the assessment of the Hall-Petch relationship for nanocrystalline materials are discussed.

The strength of nanocrystalline metals with and without flaws

Abstract: The expectation of large grain-boundary strengthening and extensive grain-boundary sliding has motivated a number of studies of the mechanical properties of nanocrystalline metals. However, poor sample quality frequently has led to results that are far removed from the actual mechanical behavior of the flaw-free material. Several examples, involving elastic moduli, hardness, and yield strength, are given. Recent tests in compression of high density nanocrystalline metals have shown high hardness and yield strength values that are compatible with extrapolation of coarse-grain Hall-Petch data to the nanocrystalline regime.

On the creep behaviour of grain boundary engineered nickel 1

Abstract: Grain boundaries described by low-Σ CSL relationships (ie Σ ≤ 29) have previously been shown to be resistant to grain boundary sliding, cavitation and fracture The present work reports on efforts to reduce creep rates in conventional polycrystalline nickel by increasing the frequency with which these ‘special’ interfaces occur in the microstructure Suitable thermomechanical processing was employed to enhance the frequency of ‘special’ grain boundaries (Σ ≤ 29) in 9999% Ni from 13 to 66%, resulting mostly from the formation of twins (23) and crystallographically-related 29 and 227 boundaries This 53% increase in the fraction of low-2 boundaries produced reductions of 16-fold in the steady-state creep rate and six-fold in the primary creep strain Microstructures having ‘special’ boundary frequencies of less than 50% exhibited significant cavitation almost exclusively along ‘random’ boundaries (ie Σ > 29) at or near triple points No gross cavitation was evident in microstructures containing ‘special’ boundary fractions of 66% Such improvements in creep properties provide considerable promise for the application of a ‘grain boundary engineering’ approach to developing interfacial materials for structural applications

Plastic behavior of nanophase Ni: A molecular dynamics computer simulation

Abstract: We report molecular dynamics computer simulations of low temperature-high load plastic deformation of Ni nanophase samples with several mean grain sizes in the range of 3–5 nm. The samples are polycrystals nucleated from different seeds, with random location and orientation. Among the mechanisms responsible for the deformation, grain boundary sliding and motion, as well as grain rotation are identified. No dislocation activity is detected, in contrast to the behavior of coarse grain metals. Interpreting the results in terms of grain boundary viscosity, a linear dependence of strain rate with the inverse of the grain size is obtained.

Microstructural evaluation of primary crystallization with diffusion-controlled grain growth

Abstract: A model has been developed for evaluating grain size distributions in primary crystallizations where the grain growth is diffusion controlled. The body of the model is grounded in a recently presented mean-field integration of the nucleation and growth kinetic equations, modified conveniently in order to take into account a radius-dependent growth rate, as occurs in diffusion-controlled growth. The classical diffusion theory is considered, and a modification of this is proposed to take into account interference of the diffusion profiles between neighbor grains. The potentiality of the mean-field model to give detailed information on the grain size distribution and transformed volume fraction for transformations driven by nucleation and either interface- or diffusion-controlled growth processes is demonstrated. The model is evaluated for the primary crystallization of an amorphous alloy, giving an excellent agreement with experimental data. Grain size distributions are computed, and their properties are discussed.

Factors controlling the fatigue strength of nitrided titanium

Abstract: — The factors affecting the fatigue strength of nitrided titanium were clarified. The fatigue strength depended strongly on the fracture strength of the compound layer formed on the surface by nitriding. We found a Hall-Petch relationship between the fatigue strength of nitrided titanium and the grain size. The findings indicated that the reduction in the fatigue strength by nitriding results from both the formation of the compound layer possessing low fracture strength and grain growth occurring from ordinary nitriding. Furthermore, low-temperature nitriding (620°C, 24 h) was proposed to suppress grain growth. This treatment method improved not only the wear resistance and the corrosion resistance but also the fatigue strength of titanium.

Improvement of high-temperature creep resistance in fine-grained Al2O3 by Zr4+ segregation in grain boundaries

Abstract: The high-temperature creep resistance in Al2 O3 is improved greatly by ZrO doping. Zirconium ions are found to be segregated in Al O grain boundaries. The activation energies for creep in high-purity Al2 O3 and ZrO-doped Al O are - estimated to be 430 and 650 kJ mol1, respectively. The grain boundary diffusivity of Al ions is expected to be reduced by the segregation of Zr4+ in the grain boundaries.

Low and high temperature hardness of WC-6 wt%Co alloys

Abstract: This paper reports hardness measurements on WC-6 wt%Co of three different grain sizes in the temperature range from −196 to 900 °C. Coarser grades have been found to soften with increasing temperature at a higher rate than finer grades. It has been confirmed that hardness decreases with increasing grain size over the whole range of temperatures and it has been shown that the decrease in hardness with increasing grain size follows a Hall-Petch-type relationship at all the temperatures tested.

Grain boundary mobility during recrystallization of copper

Abstract: Average grain boundary migration rates during recrystallization of cold-deformed copper were estimated from stereological measurements. In the same material, the instantaneous driving forces for boundary migration during recrystallization were calculated from calorimetric measurements of the release of the stored energy of cold work. The migration rate dependence on driving force was analyzed in the context of grain boundary migration rate theory, and within experimental error, a linear dependence was observed. The average mobility of grain boundaries migrating during recrystallization of cold-worked copper at 121°C was calculated to be 6.31×10−10 (m4 s−1 MJ−1). This result was found to be consistent with single boundary, curvature-driven grain boundary mobilities measured in copper at higher temperatures. It was also demonstrated that the average grain boundary mobility was reasonably within the expectation (order of magnitude uncertainty) of the Turnbull single process model of boundary migration with a process akin to grain boundary self-diffusivity as the rate-controlling atomic mechanism.

Stability of the grain configurations of thin films—A model for agglomeration

Abstract: We have calculated the energy of three distinct grain configurations, namely, completely connected, partially connected, and unconnected configurations, evolving during a spheroidization of polycrystalline thin film by extending a geometrical model due to Miller et al. to the case of spheroidization at both the surface and film-substrate interface. “Stability” diagram defining a stable region of each grain configuration has been established in terms of the ratio of grain size to film thickness versus equilibrium wetting or dihedral angles at various interface energy conditions. The occurrence of spheroidization at the film-substrate interface significantly enlarges the stable region of unconnected grain configuration thereby greatly facilitating the occurrence of agglomeration. Complete separation of grain boundary is increasingly difficult with a reduction of equilibrium wetting angle. The condition for the occurrence of agglomeration differs depending on the equilibrium wetting or dihedral angles. The agglomeration occurs, at low equilibrium angles, via partially connected configuration containing stable holes centered at grain boundary vertices, whereas it occurs directly via completely connected configuration at large equilibrium angles except for the case having small surface and/or film-substrate interface energy. The initiation condition of agglomeration is defined by the equilibrium boundary condition between the partially connected and unconnected configurations for the former case, whereas it can, for the latter case, largely deviate from the equilibrium boundary condition between the completely connected and unconnected configurations because of the presence of a finite energy barrier to overcome to reach the unconnected grain configuration.