【正文】 1), deformation (as materials may deform more easily in certain directions). Figure 1 – A solidification microstructure, showing anisotropic grain shapes. Note that, although this material does show texture, this cannot be concluded from the image. All we can say for certain is that the grains have an anisotropic shape, we can say nothing about their crystallographic orientation. Why is Texture Important? Quantification of Microstructure and TextureTexture R Goodall, October 2022 12 The importance of the texture of a material es from the fact that in many crystalline materials, the properties can depend on the direction relative to the crystallographic axis in which they are measured. For example, the elastic modulus of most single crystal metal samples would in fact vary depending on the direction it was tested in. The same is true of such properties as the Poisson’s ratio, magic permeability, electrical conductivity and thermal expansion coefficient, to name a few. These materials are anisotropic (the reason that most materials we encounter do not present such anisotropy is that the random orientation of the grains tends to blend out such variations). However, in a material with a preferred orientation, with a texture, the properties will not blend together equally, and we many find that a macroscopic sample can show a variation in properties. Example – Texture in Aluminium Deep Drawing When rolled, metals can develop a texture, which can influence their behaviour in subsequent processing. In the case of aluminium, rolling can produce a texture that means it deforms more easily along two perpendicular directions in the plane of the sheet than at 45176。 to these directions. The effect of this is seen in the deep drawing operation that constitutes the first stage of the process to produce aluminium cans. As shown in Figure 2, a strong rolling texture results in the aluminium deforming more along the certain directions, resulting in “ears” forming at the top of the drawn cup. This can be a source of significant waste in can production if sheet texture is not controlled, as this excess material is unusable and is cut away. Figure 2 – Deep drawn aluminium cups, showing the effects of, left, weak rolling texture and, right, strong rolling texture on the deformation observed. How is Texture Measured? In modern materials science texture is normally measured by a diffraction of xrays, electrons or neutrons from the crystallographic planes. XRays The most mon method of measuring texture uses xray diffraction and is known as the “Schultz reflection method”. The apparatus used is known as a fourangle diffractometer or a Eulerian cradle, see figure 3. The source of xrays and the detector are oriented so that a particular value of 2θ (the angle between the source and the detector) is specified. This means that only diffraction from a single set of planes, with a particular spacing will be measured. The sample is tilted and rotated systematically, so that all angular orientations are investigated. When the lattice plane specified by the 2θ value is in the right orientation, it will diffract and the detector will record the reflection. For a polycrystalline material, the intensity of detected xrays will increase when there are more grains in that specific orientation, and the intensity for any angle 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. Quantification of Microstructure and TextureTexture R Goodall, October 2022 13 Figure 3 – A schematic diagram of a four angle diffractometer for texture determination. In some materials the bulk and surface textures may be different。 for example after rolling. It is therefore, important to identify which texture is of interest, and which is being measured. Different sources of radiation can lead to different degrees of peration, and hence allow the measurement of either bulk or surface textures. Electrons – EBSD An alternative method of texture determination is that of electron backscattered diffraction, EBSD, which uses a scanning electron microscope. At each single beam position (each spot on the image), the electron beam is rocked about a fixed point on the surface. At particular angles, where the diffraction conditions for particular planes are satisfied, the beam is diffracted, so that there is a change in the intensity of the reflection measured. This leads to the formation of a channelling pattern made up of Kikuchi lines, Figure 4. Figure 4 – Kikuchi lines formed during EBSD of a born doped (001) oriented Si wafer, with lines indicating the reflections identified. Image from DoITPoMS ( The symmetry of the channelling pattern, and vector addition of the different reflections allows the planes generating specific bands to be identified (usually done using mercially available software). This gives the orientation of the material under the beam at that spot, and so it is possible to build up a 2D map of the orientation of grains on the surface of a polycrystalline sample. Quantification of Microstructure and TextureTexture R Goodall, October 2022 14 Neutron Diffraction Neutron diffraction can be used in a similar way to xray diffraction. There is a large reduction in absorption but a much higher angular resolution in neutron diffraction in parison with xray diffraction. This means that neutrons are suitable for situations where a large depth of peration is required, and are relatively rapid, meaning that they can be used to measure insitu texture changes due to environmental factors (. temperature changes and stress). However, beams of su