Grain boundary grooving at the singular surfaces

The unusual topographies of the grain boundary thermal grooves in Ni-rich NiAl were observed after annealing at 1400°C. One of the surfaces forming the grain boundary groove exhibited no curvature measurable in the atomic force microscope, thus indicating its singular character. The theory of grain boundary grooving at singular surfaces was developed in the small-slope approximation and under assumption of negligible diffusivity on these surfaces. The calculated groove shapes are in good agreement with the experimental data and differ considerably from the shapes predicted by the classical Mullins grooving theory for isotropic surfaces.     

High-temperature grain boundary sliding behavior and grain boundary energy in cubic zirconia bicrystals

Misorientation dependence of grain boundary energy and grain boundary sliding at high temperature were examined in cubic zirconia bicrystals with [1 1 0] symmetric tilt boundaries, which were fabricated by diffusion bonding method from two cubic zirconia single crystals. High-resolution transmission electron microscopy observation revealed that the grain boundary in cubic zirconia bicrystals was clean and atomically sharp without any void or grain boundary amorphous layer. Grain boundary energy of the tilt boundaries was estimated from the dihedral angles on thermal grooved surface measured with atomic force microscope techniques. The misorientation dependence of the grain boundary energy in cubic zirconia bicrystals shows similar tendency to that of fcc metal such as aluminum and copper. Grain boundary sliding associated with intragranular dislocation slip in cubic zirconia bicrystals was observed for all specimens. The amount of the grain boundary sliding showed a good correlation with the misorientation factor of each boundary. Grain boundary migration also took place accompanying with the grain boundary sliding. The observed grain boundary sliding and migration can be explained based on a dislocation mechanism for sliding which is based on the movement of lattice dislocations along the grain boundary by a combination of climb and glide.     

Aluminum Σ3 grain boundary sliding enhanced by vacancy diffusion

Grain boundary sliding is an important deformation mechanism for elevated temperature forming processes. Molecular dynamics simulations are used to investigate the effect of vacancies in the grain boundary vicinity on the sliding of Al bi-crystals at 750 K. The threshold stress for grain boundary sliding was computed for a variety of grain boundaries with different structures and energies. These structures included one symmetrical tilt grain boundary and five asymmetrical tilt grain boundaries. Without vacancies, low energy Σ3 grain boundaries exhibited significantly less sliding than other high energy grain boundaries. The addition of vacancies to Σ3 grain boundaries decreased the threshold stress for grain boundary sliding by increasing the grain boundary diffusivity. A higher concentration of vacancies enhanced this effect. The influence of vacancies on grain boundary diffusivity and grain boundary sliding was negligible for high energy grain boundaries, due to the already high atom mobility in these boundaries.     

Atomistic simulation of grain boundary sliding in pure and magnesium doped aluminum bicrystals

Molecular dynamics and statics simulations are used to study grain boundary sliding and energy in bicrystals with symmetric tilt grain boundaries of Al and Mg doped Al. There is an increase in grain boundary energy in Al bicrystals with the presence of Mg depending on the position of Mg atom. Simulations of sliding show a clear dependence of magnitude of sliding on grain boundary energy.     

Grain growth inhibition in thin nanocrystalline Au films by grain boundary diffusion and oxidation of Ti

We studied grain growth in thin nanocrystalline Au films deposited on a sapphire substrate with and without an ultrathin Ti underlayer (adhesion promoter). The samples were annealed at 200 °C for 2 h in air. The reference thin Au film without a Ti underlayer exhibited significant grain growth during annealing, whereas no changes in microstructure of the Au layer were observed in the Au/Ti bilayers. This stabilization of the microstructure of the Au layer was attributed to thermal grain boundary grooves on the Au surface filled with Ti oxide. The grooves exhibited an elongated morphology characterized by a low apparent dihedral angle value, atypical for thermal grain boundary grooves in pure metals. We demonstrated that grooves with this morphology are very efficient in pinning grain boundary motion. We also developed a quantitative model of grain boundary grooving coupled with grain boundary interdiffusion in thin bilayer films. The model predicted the formation of long, narrow grooves at the grain boundaries, which are very efficient in suppressing grain growth in nanocrystalline thin films.     

Creep and stress relaxation in free-standing thin metal films controlled by coupled surface and grain boundary diffusion

An analytical solution is presented that interprets the effects of grain size, surface and grain boundary diffusivities, surface and grain boundary free energies, as well as grain boundary grooving on the creep rate in free-standing polycrystalline thin metal films. The Coble creep in the film plane is also taken into account; this has a significant effect on the creep rate of the film. The effects of diffusion ratio and free energy ratio between surface and grain boundary on film agglomeration are illustrated as well. A closed-form expression for stress relaxation in films under constrained strain conditions is derived from this solution. An exponential decay in stress is found as a function of the film microstructure. Results predicted by the solution are shown to be in agreement with the experiments.     

Cavitation and grain boundary sliding during creep of Mg-Y-Nd-Zn-Mn alloy

Creep of squeeze-cast Mg-3Y-2Nd-1Zn-1Mn alloy was investigated at the constant load in the stress range of 30-80 MPa. Tensile creep tests were performed at 300℃up to the final fracture.Several tests at 50 MPa were interrupted after reaching the steady state creep;and another set of creep tests was interrupted after the onset of ternary creep.Fraction of cavitated dendritic boundaries was evaluated using optical microscopy.Measurement of grain boundary sliding by observation of the offset of marker lines ...     

A scanning force microscopy study of grain boundary energy in copper subjected to equal channel angular pressing

We employed a scanning force microscopy technique to determine the ratio of grain boundary and surface energies in copper using the thermal grooving method. Samples of ultrafine grain copper obtained by four passes of equal channel angular pressing were heat treated in a reducing atmosphere at 400 °C for 15 min and at 800 °C for 2 h. The average dihedral angles of the grain boundary grooves after the former and the latter heat treatments were 152.4 ± 6.3° and 164.2 ± 4.3°, respectively, which can be translated into the difference by a factor of 1.8 in average grain boundary energies. This difference implies that the grain boundaries in ultrafine grain copper produced by equal channel angular pressing are in a state of high non-equilibrium that cannot be fully relaxed after a short annealing at 400 °C, but that undergoes significant relaxation after annealing at 800 °C.     

A predictive model for microstructure evolution in metallic multilayers with immiscible constituents

Focussing on the thermal stability of layered structures, we developed a predictive model to study the microstructure evolution of metallic multilayers with different morphologies including aligned and classical staggered grain geometries. We found that the zig-zag microstructure experimentally observed in multilayers forms when grains in each upper layer have a relative shift less than half the in-plane grain size to the lower layer. During this formation process, the non-equilibrium triple junctions move, driven by the imbalance of tensions of interphase and grain boundaries, corresponding to an extension to classical grooving theory. Numerical simulations show that a finite mobility of the triple junction can effectively impede the development of grooves, suggesting that the classical t^1/4 dependence of groove depth with the assumption of an infinite triple junction mobility might be questionable at low temperatures to predict the time to pinch-off. Further, a map for the stability of layered structure in Cu/Nb system is developed in terms of the aspect ratio of grain dimensions and the ratio of the distance between two nearest triple junctions to the in-plane grain size. A criterion for this stability is also proposed for multilayers with similar grain boundary energies based on simplified geometrical consideration. Both the map and the simple criterion are in good agreement with the experiments for Cu/Nb multilayers.     

Modeling of Microstructure Evolution in Metallic Multilayers with Immiscible Constituents

Thermal stabilities of Cu/Nb, Cu/Ag, and Cu/Mo multilayers are studied by a recently developed model for microstructure evolution in multilayers with immiscible constituents, which actually is an extension to the classic grooving theory. The experimentally evidenced zig–zag microstructure is found to form through grooving when grains are staggered in a “stair-like” fashion. Furthermore, stability maps for these systems are developed in terms of the aspect ratio of grain dimensions and the ratio of the distance between two nearest triple junctions to the in-plane grain size. A comparison of stability among the three systems shows that the ratio of the grain boundary energy to the interphase boundary energy is more important than the ratio of the two grain boundary energies in controlling the stability. A simple criterion is also proposed for a quick estimation of the stability. Both maps from the model and from the simple criterion are in good agreement with the experiments for multilayers.     

Plastic deformation of nanocrystalline aluminum at high temperatures and strain rate

The deformation of nanocrystalline aluminum was studied using molecular dynamics simulation at homologous temperatures up to 0.97. The microstructures and stress–strain response were examined in a polycrystalline and bicrystal configuration. The activation energies for dislocation-based deformation as well as grain boundary sliding and migration were quantified by fitting simulation data to temperature using an Arrhenius relation. The activation energy for the flow stress response suggests that deformation is largely accommodated by sliding and migration of grain boundaries. This is in agreement with simulated microstructures, indicating a negligible degree of dislocation interaction within each grain, and microstructural observations from high strain rate processes are also consistent with this result. A steady-state grain size is maintained in the recrystallized structure following yielding due to boundary migration and grain rotation mechanisms, rather than by diffusion-based dislocation climb.     

Molecular dynamics simulations of grain boundary sliding: The effect of stress and boundary misorientation

Molecular dynamics simulations were used to study the effect of applied force and grain boundary misorientation on grain boundary sliding in aluminum at 750 K. Two grains were oriented with their 〈1 1 0〉 axes parallel to their boundary plane and one grain was rotated around its 〈1 1 0〉 axis to various misorientation angles. For any given misorientation, increasing the applied force leads to three sliding behaviors: no sliding, constant velocity sliding and a parabolic sliding over time. The last behavior is associated with disordering of atoms along the grain boundary. For the second sliding behavior, the constant sliding velocity varied linearly with the applied stress. A linear fit of this relationship did not intersect the stress axis at the origin, implying that a threshold stress for sliding exists. This threshold stress was found to decrease with increasing grain boundary energy. The ramifications of this finding for modeling grain boundary sliding in polycrystals are discussed.     

Stress induced grain boundary motion

We investigated the motion of planar symmetrical and asymmetrical tilt boundaries in high-purity aluminium with <112>- and <111>-tilt axes under the influence of an external mechanical stress field. It was found that the motion of low-angle grain boundaries as well as high-angle grain boundaries can be induced by the imposed external stress. The observed activation enthalpies allow conclusions on the migration mechanism of the grain boundary motion. The motion of planar low- and high-angle grain boundaries under the influence of a mechanical stress field can be attributed to the movement of the grain boundary dislocations which comprise the structure of the boundary. A sharp transition between low-angle grain boundaries and high-angle grain boundaries was observed at 13.6°, which was apparent from a step of the activation enthalpy for the grain boundary motion. For the investigated boundaries the transition angle was independent of tilt axis, impurity content and tilt boundary plane.     

Comparing calculated and measured grain boundary energies in nickel

Recent experimental and computational studies have produced two large grain boundary energy data sets for Ni. Using these results, we perform the first large-scale comparison between measured and computed grain boundary energies. While the overall correlation between experimental and computed energies is minimal, there is excellent agreement for the data in which we have the most confidence, particularly the experimentally prevalent Σ3 and Σ9 boundary types. Other CSL boundaries are infrequently observed in the experimental system and show little correlation with computed boundary energies. Because they do not depend on observation frequency, computed grain boundary energies are more reliable than the experimental energies for low population boundary types. Conversely, experiments can characterize high population boundaries that are not included in the computational study. Together the experimental and computational data provide a comprehensive catalog of grain boundary energies in Ni that can be used with confidence by microstructural scientists.     

Whisker and hillock growth via coupled localized Coble creep,grain boundary sliding,and shear induced grain boundary migration

A model incorporating the effects of grain geometry and size and grain boundary properties on the growth of whiskers and hillocks has been developed based on coupling between localized Coble creep and grain boundary sliding. For both whiskers and hillocks accretion of atoms by Coble creep on grain boundary planes normal to the growth direction is limited by grain boundary sliding on planes parallel to the direction of whisker growth. If the accretion-induced shear stresses are not coupled to grain boundary migration a whisker forms when sliding occurs. In the case of hillocks an additional coupling between grain boundary sliding and shear-induced grain boundary migration leads to the observed lateral growth. By incorporating grain size and geometry, a structure-dependent grain boundary sliding coefficient and measured film stresses the local conditions for whisker growth, including the growth rates, can be calculated. As described here, other commonly observed whisker and hillock morphologies and geometries are consistent with this model, as are the effects of a surface oxide film and thermal cycling.     

Crossover from grain boundary sliding to rotational deformation in nanocrystalline materials

A theoretical model is suggested which describes cooperative action of grain boundary (GB) sliding and rotational deformation in mechanically loaded nanocrystalline materials. Focuses are placed on the crossover from GB sliding to rotational deformation occurring at triple junctions of GBs. In the framework of the model, gliding GB dislocations at triple junctions of GBs split into dislocations that climb along the adjacent boundaries. The splitting processes repeatedly occurring at triple junctions give rise to climb of GB dislocation walls that carry rotational deformation accompanied by crystal lattice rotation in grains of nanocrystalline materials. The role of GB sliding, rotational deformation and conventional dislocation slip in high-strain-rate superplastic flow in nanocrystalline materials is discussed.     

Deformation mechanism at impact test of Al-11% Si alloy processed by equal-channel angular pressing with rotary die

Al-11%Si(mass fraction)alloy was transformed into a ductile material by equal-channel angular pressing(ECAP)with a rotary die.Two mechanisms at impact test,slip deformation by dislocation motion and grain boundary sliding,were discussed.The ultrafine grains with modified grain boundaries and the high content of fine particles(<1μm)were necessary for attaining high absorbed energy.The results contradict the condition of slip deformation by dislocation motion and coincide with that of grain boundary sliding.Many fine zigzag lines like a mosaic were observed on the side surface of the tested specimens.These observed lines may show grain boundaries appeared by the sliding of grains.