Combined X-ray microdiffraction and topography experiment for microstructural analysis of heterogeneous materials

Abstract

A new experimental set-up combining X-ray topography and microbeam diffraction has been designed for the investigation of heterogeneous microstructures with features in the size range of more than 50μm. Built around a four-circle goniometer of 5/1000° angular resolution, this apparatus offers similar facilities to those obtained by TEM but at a different scale. In the Berg-Barrett position, grain and subgrain boundaries can be observed over large surface areas (typically 100 mm2). Based on topography observations, areas of interest of typically 100μm diameter can be precisely selected for microdiffraction measurements. A laser beam directed through the pinhole system of the X-ray microbeam collimating system permits one to directly visualize the irradiated zone. The divergence of the X-ray microbeam is typically 0.034° (full width at half maximum) and permits the measurement of lattice spacing variations (Δa/a) of the order of 10−4. While TEM permits one to examine very localized areas, this new device is appropriate to detect long-distance effects and phase interactions in materials with coarse distributed heterogeneities. In order to demonstrate the versatility of this new device, the orientation distribution, variation of lattice spacing and mosaic structure of dendrites in directionally solidified nickel-base alloys are analysed.