Tilt Grain Boundaries In Bicrystals Research Articles

Crystal Plasticity Modeling of Void Growth on Grain Boundaries in Ni-Based Superalloys

In this work, we explore the effect of misorientation angles of crystal orientations between two grains along the grain boundary (GB) on void growth behavior in polycrystalline Ni-based superalloys by using a crystal plasticity finite element method. Quantitative analysis is conducted to study the coupled roles of the crystal orientation and stress triaxiality in void growth in bicrystals. Based on our simulation results, we find that, as the main loading axis perpendicular to the GB, voids grow more slowly on tilt GBs in bicrystals than those in single and bicrystal samples with twist GBs, while the void growth in single- and bicrystal samples with twist GBs exhibited almost the same rate and increased with the stress triaxiality levels. The interaction between two crystals bonded with the GB activates the effective Schmid factors in each crystal, which results in asymmetric distribution of the equivalent plastic strain around the void and induces distinct irregularly shaped voids during deformation.

HRTEM Study on Σ7 Grain Boundary in Aluminium Bicrystals with and without Ga Doping

A HRTEM investigation was carried out on the Σ7{213} tilt grain boundary in bicrystals of both high purity aluminium and same aluminium doped with 10 ppm gallium. It was determined by image simulation that the bright contrast in the TEM images show the positions of atom columns. By measuring contrast profiles it was found that in the specimen of aluminium-10 ppm gallium, the atoms which are closest to the grain boundary, are anomalously shifted along the ‹121› direction compared to equivalent atom positions of the pure aluminium sample. This anomalous shift makes the disparity in the grain boundary structure of both samples. It is assumed that this anomalous shift is most likely the reason for the higher mobility of the grain boundary of the aluminium-10 ppm gallium sample.

Effect of pressure on grain boundary migration in aluminium bicrystals

The temperature and pressure dependencies of 〈100〉, 〈110〉, and 〈111〉 tilt grain boundaries in bicrystals of pure aluminum were measured, and the corresponding activation enthalpies and activation volumes were determined. While the activation enthalpy strongly depended on orientation, in particular close to low Σ coincidence orientation relationships, the activation volume was found independent of orientation except for 〈110〉 tilt boundaries. The absolute values of the activation volume for 〈100〉 and 〈111〉 tilt boundaries were about the same as for bulk self-diffusion, which agrees with previous measurements on polycrystals. The activation volume for migration of 〈110〉 tilt boundaries substantially exceeded the activation volume for bulk self-diffusion. It is concluded that at least 〈110〉 tilt boundaries migrate by a group mechanism rather than by diffusive jumps of single atoms.

Structure, Growth, and Properties of Twist and Tilt Grain Boundaries in Bicrystals of NaCl

Grain boundaries having from 2° to 45° mismatch across the interface were grown for pure tilt and pure twist misorientation in NaCl by the Czochralski technique. The method of crystal growth and the techniques required for good control of the misorientation and the planarity of the grain boundary are described. The substructure of twist and tilt NaCl bicrystals was examined using both optical and electron-microscopic techniques. The tilt grain boundaries were found to be strong mechanically and not easily cleaved apart. The twist grain boundaries were very weak mechanically, especially at mismatch angles greater than 20°, and exhibited anomalously rapid diffusion or capillary flow effects along the grain boundary. Grain boundaries in the bicrystals were examined by: (1) pulling the crystals apart at the grain boundary and (2) by polishing away one side of the bicrystal to within 0.5 mm of the grain boundary. (The grain boundary surface could then be examined by focusing through the layer of NaCl.) Both of these techniques showed that the twist bicrystals were held together by small regions or spots of continuity at the grain boundary. The spot density ranged from 500/cm2 to 20 000/cm2 for 45° and 10° twist bicrystals, respectively. No substructure was observed for tilt grain boundaries. This may explain not only the extreme mechanical weakness of twist grain boundaries, but may also help to explain brittle fracture in polycrystalline ionically bonded solids generally, including ceramics and other alkali halides, and phenomena, such as the solution embrittlement of AgCl.