Measurement of Texture
In the past, optical methods and etching have been used to determine grain orientation, but recently texture is almost exclusively measured by diffraction techniques. Diffraction of x-rays, electrons, and neutrons will all be discussed in this TLP.
The most common method of measuring texture uses x-ray diffraction and is known as the “Schultz reflection method”. The apparatus used is known as a four-angle diffractometer or a Eulerian cradle.
The source of x-rays and the detector are oriented so that a particular value of 2θ is specified. This allows for a single Bragg reflection to be measured. The stage of the cradle is tilted and rotated systematically, so that all angular orientations of the sample are investigated.
The animation below shows how the Eulerian cradle is used for measurement of texture using the reflection method.
When the specified lattice plane of a crystallite fulfils the Bragg condition, the detector will record the reflection. For a polycrystalline material, the intensity of detected x-rays will increase when there are more crystallites in a specific orientation. The intensity for a given orientation is proportional to the volume fraction of crystallites with that orientation. Areas of high and low intensity suggest a preferred orientation, while constant intensity at all angles would occur in a random polycrystalline aggregate.
X-ray diffraction may be carried out so that the x-rays are reflected from the surface of the sample, or they may penetrate via transmission. Transmission is only suitable for thin films or wires because of the high absorption of the x-rays by many materials.
In some materials the bulk and surface textures may be different, e.g. in some rolled textures. Therefore, it is important to identify which texture is of interest. Different sources of radiation can lead to different degrees of penetration, and hence allow the measurement of either bulk or surface textures.