Texture Effects in Important Aerospace Materials
This chapter summarizes the effects of processing-induced crystallographic texture on the properties of three important classes of aerospace metallic materials, namely aluminium alloys, titanium alloys and nickel-base superalloys.
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Texture Effects in Important Aerospace Materials R.J.H. Wanhill
Abstract This chapter summarizes the effects of processing-induced crystallographic texture on the properties of three important classes of aerospace metallic materials, namely aluminium alloys, titanium alloys and nickel-base superalloys. Keywords Texture
7.1
Aluminium alloys Titanium alloys Nickel superalloys
Introduction
Three important classes of aerospace metallic materials are (i) aluminium alloys, (ii) titanium alloys, and (iii) nickel-base superalloys. These materials require careful control of texture during processing, in order to obtain optimum properties for design and service use. Most aerospace alloys are used in wrought forms. Notable exceptions are cast nickel-base superalloys for gas turbine blades and vanes, and cast titanium aluminide turbine blades (see Chaps. 9 and 10 in Volume 1 of these Source Books). One of the primary concerns for wrought products is processing-induced crystallographic texture. This can result in anisotropic mechanical properties, thereby affecting the design allowables and a material’s suitability for use. Here another notable example must be mentioned, namely aluminium–lithium (Al–Li) alloys, which are the subject of a recent book [1] and also Chap. 3 in Volume 1 of these Source Books. Texture formation is inevitable for any kind of processing. For example, casting processes lead to solidification textures, and processes such as rolling, forging, extrusion, or drawing cause deformation textures. Also, heat treatments result in annealing textures or transformation textures, depending on whether annealing
R.J.H. Wanhill (&) NLR, Emmeloord, The Netherlands e-mail: [email protected] © Springer Science+Business Media Singapore 2017 N. Eswara Prasad and R.J.H. Wanhill (eds.), Aerospace Materials and Material Technologies, Indian Institute of Metals Series, DOI 10.1007/978-981-10-2143-5_7
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involves recrystallization and grain growth, or crystallographic phase transformations as in many titanium alloys (see Chap. 6 in Volume 1 of these Source Books).
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Texture Definition and Representations
Crystallographic texture is also known as preferred orientation. When most of the crystals in a polycrystalline material have a preference for certain orientations with respect to the sample or product axes, e.g. the rolling direction and tensile axis, the material is said to be textured. Figure 7.1 shows a schematic example of texture-free and textured sheet or plate materials. Pole Figures Textures are most often described by one or a series of pole figures [2, 3], which are determined from specimens/samples by X-ray diffraction methods. A pole figure is a two-dimensional stereographic projection that shows the variation of pole density with pole orientation for a selected set of crystal planes. The poles represent the disposition of plane normals.
For cubic crystal structures, the crystal planes are assigned the Miller indices {hkl}. For hexagonal crystal structures (e.g. the α phas
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