Nanostructured WC/Co composite powder prepared by high energy ball milling

Dislocation model of strain anisotropy is presented. The dislocation theorem of strain broadening is suggested which means that strain broadening can only be caused by dislocation-type lattice defects. Based on this theorem strain anisotropy is modeled and accounted for by assuming that strain broadening is caused by dislocations or dislocation-type lattice defects. The effect of strain anisotropy is summarized in hkl dependent dislocation contrast factors, which can be either averaged over the permutations of hkl indices or are different for each different reflection. The dislocation model of strain anisotropy provides a powerful tool to analyze slip activity, Burgers vector populations, and plasticity on the basis of line profile analysis.

Dislocation model of strain anisotropy

Diffraction line broadening is caused by different defects present in crystalline materials: (1) small coherent domains, (2) dislocations, (3) other types of microstrains, (4) twin boundaries, (5) stacking faults, (6) chemical inhomogeneities, and (7) grain-to-grain second-order internal stresses. Line profile analysis provides qualitative and quantitative information about defect types and densities, respectively. Line profiles can broaden, be asymmetric, and be shifted, and these features can be anisotropic in terms of hkl indices. A few thumb rules help qualitative selection of lattice defect types. If the breadths do not increase globally with hkl, the defects are of size type, i.e., either the domain size is small or twinning or faulting, or both, is present. Whenever the breadths increase globally, the defects produce microstrains. Physically based profile functions can be determined for the different defect types and hkl anisotropy. The qualitative input about defect types based on different experimental observations allows adequate quantitative evaluation of the densities of different defect types by using physically modeled profile functions.

Defect-Related Physical-Profile-Based X-Ray and Neutron Line Profile Analysis

The effects of temperature on the microstrains and crystallite growth in milled alumina have been examined, using a modern method of X-ray diffraction profile analysis. The microstrains may be relieved by annealing at temperatures between 600 and 1000° C, and this relief is accompanied by crystallite size growth. Isotropy was observed with regard to the residual stresses and cleavage properties, in the milled crystals.

The effect of temperature on microstrains and crystallite growth in alumina

This work extends the convolutional multiple whole profile (CMWP) line profile analysis (LPA) procedure to determine the total dislocation density and character of irradiation-induced dislocation loops in commercial polycrystalline Zr specimens. Zr alloys are widely used in the nuclear industry as fuel cladding materials in which irradiation-induced point defects evolve into dislocation loops. LPA has long been established as a powerful tool to determine the density and nature of lattice defects in plastically deformed materials. The CMWP LPA procedure is based on the Krivoglaz–Wilkens theory in which the dislocation structure is characterized by the total dislocation density ρ and the dislocation arrangement parameter M. In commercial Zr alloys irradiation-induced dislocation loops broaden the peak profiles, mainly in the tail regions, and occasionally generate small satellites next to the Bragg peaks. In this work, two challenges in powder diffraction patterns of irradiated Zr alloys are solved: (i) determination of the M values from the long tail regions of peaks has been made unequivocal and (ii) satellites have been fitted separately, using physically well established principles, in order to exclude them from the dislocation determination process. Referring to the theory of heterogeneous dislocation distributions, determination of the total dislocation density from the main peaks free of satellites has been justified. The dislocation loop structure has been characterized by the total dislocation density of loops and the M parameter correlated to the dipole character of dislocation loops. The extended CMWP procedure is applied to determine the total dislocation density, the dipole character of dislocation loops, and the fractions of 〈a〉- and 〈c〉-type loops in proton- or neutron-irradiated polycrystalline Zr alloys used in the nuclear energy industry.

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Characterizing dislocation loops in irradiated polycrystalline Zr alloys by X-ray line profile analysis of powder diffraction patterns with satellites.

X-ray-diffraction measurements have shown that ball-milling of tungsten carbide powder gives rise to considerable line broadening. Analysis of this broadening by integral-breadth and Fourier methods indicates that the main cause of the broadening is lattice microstrains. X-ray studies have been made on tungsten carbide powders, sintered blocks, and tool tips with cobalt binding, to determine how this strain varies at different stages of treatment of the material. It was found that, after sintering, the tungsten carbide was strain-free, but diamond polishing and mechanical work induced strain in the surface layers.

D. Lewis

Papers

Lattice strain in tungsten carbide