STRESS DISTRIBUTION NEAR GRAIN BOUNDARY IN ANISOTROPIC BICRYSTALS AND TRICRYSTALS

The rate dependent crystallographic finite element program was implemented in ABAQUS as a UMAT for the analysis of the stress distributions near grain boundary in anisotropic bicrystals and tricrystals, taking the different crystallographic orientations into consideration. The numerical results of bicrystals model with the different crystallographic orientations shows that there is a high stress gradient near the grain boundaries. The characteristics of stress structures are dependent on the crystallographic orientations of the two grains. The existing of triple junctions in the tricrystals may result in the stress concentrations,or may not, depending on the crystallographic orientations of the three grains. The conclusion shows that grain boundary with different crystallographic orientations can have different deformation, damage, and failure behaviors. So it is only on the detail study of the stress distribution can the metal fracture be understood deeply.

Non-uniform Stress Field and Stress Concentration Induced by Grain Boundary and Triple Junction of Tricrystal

The stress characteristics in the anisotropic bicrystal and tricrystal specimens were analyzed using the anisotropic elastic model, orthotropic Hill's model and rate-dependent crystallographic model. The finite element analysis results show that non-uniform stresses are induced by the grain boundary. For bicrystal specimens in different crystallographic orientations, there exist stress concentrations and high stress gradients nearby the boundaries. The activation and slipping of the slip systems are dependent on the crystallographic orientations of the grains and also on the relative crystallographic orientations of the two adjoining grains. For the tricrystal specimens, there is not always any stress concentrations in the triple junction, and the concentration degree depends on the relative crystallographic orientations of the three grains. Different from the bicrystal specimens, there may be or no stress concentration in the vicinity of grain boundaries for the tricrystal specimens, which depends on the relative crystallographic orientations of the three grains. The stress concentration near to the grain boundaries and triple junction can be high enough for the local plastic deformation, damage and voiding or cracking even when the whole specimen is still under the elastic state. It can be further concluded that homogeneous assumption for polycrystalline materials is not suitable to study the detailed meso- or micro-mechanisms for damaging and fracturing.

Crystallographically-Oriented Nanoassembly by ZnS Tricrystals and Subsequent Three-Dimensional Epitaxy

Chemistry gives us the ability to manipulate atoms and molecules into nanometer and micrometer scale building blocks,while the science of crystallography is concerned with the spatial arrangement of atoms,ions,and molecules and thus the morphology and structures of materials.Complex three-dimensional ZnS nanostructures have been fabricated via step-by-step crystallographically-controlled chemical processes.Tricrystals of ZnS whiskers were prepared via a controlled thermal evaporation process,and then the tricrystals were thermally treated in an atmosphere formed by evaporating B N O precursors into N2/NH3 to afford BN-coated arrays of nanobranches.The ZnS nanobranches grew epitaxially on the ternary facets and extended in three[0001]directions forming ordered nanostructures.Meanwhile,the protecting insulating sheath of BN formed on the ZnS nanostructures confined the growth of the nanospines and enhanced their stability.The method may be extended to fabricate other semiconductor nanomaterials with novel structures.

再结晶技术对压电石英晶体质量的影响

压电石英元器件正向高频、小型化、片式化等方向发展,对石英晶体材料的品质提出了更高的要求。该文根据再结晶原理,利用水热法对压电石英晶体进行了一、二和三次结晶,分别测量了三次结晶引起的Q值、包裹体和腐蚀隧道变化,分析和讨论了籽晶、杂质和工艺条件对晶体质量的影响。报道了生长这种高质量压电石英晶体的主要条件。