14.4: Origins of Texture
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Texture can arise whenever there is a preferred crystallographic orientation within a polycrystalline material.
Directional solidification leads to texture when columnar grains grow in a preferred crystallographic direction in the heat flow direction. Crystals with a fast growth direction parallel to that of the heat flow will dominate the final structure. For more details see: Microstructure and segregation in castings in the Casting TLP
The preferred growth direction of dendrites in cubic metals is <100>. In the animation above it can be seen that close to the chilled mould texture is random, while at great distances from the mould, <100> fibre texture is prominent. This variation in texture occurs because nucleation at the mould wall is a random process with respect to orientation, whereas growth of the grains is strongly dependent on the orientation.
The effect of slip during deformation is described in The stereographic projection TLP and in Slip in single crystals TLP. Slip planes of individual crystals will rotate, so that the direction of slip will rotate towards the tensile axis. If the axis along which force is applied is constrained, so that it does not deviate from its original direction, then the crystal must rotate with respect to the axis. In a polycrystalline sample, an originally random distribution of crystal orientation will become non-random and tend towards an ideal orientation. The ideal orientation depends upon a number of factors including the crystal structure of the material, its temperature, the alloying additions, and the processing route.
The effect of wire drawing, rolling, and annealing on the texture of c.c.p. and b.c.c. metals can be investigated in the flowing animation.
Pronounced texture is very common in thin films. Even when there is no crystallographic matching between the crystal structure of the growing film and the substrate, the grains in the film show a strong preference for a particular plane to be parallel to the substrate surface. For example, films of c.c.p. metals prefer to have a close packed plane parallel to the surface of the substrate.