In materials science, texture is the distribution of crystallographic
orientations of a polycrystalline sample. A sample in which these
orientations are fully random is said to have no distinct texture. If the
crystallographic orientations are not random, but have some preferred
orientation, then the sample has a weak, moderate or strong texture. The degree
is dependent on the percentage of crystals having the preferred
orientation. Texture is seen in almost all engineered materials, and can have a
great influence on materials properties. Also, geologic rocks show texture due to
their thermo-mechanic history of formation processes.
One extreme case is a complete lack of texture: a solid with perfectly random
crystallite orientation will have isotropic properties at length scales
sufficiently larger than the size of the crystallites. The opposite extreme is a
perfect single crystal, which has anisotropic properties by geometric
necessity. Characterization and representation
Texture can be determined by various methods. Some methods allow a
quantitative analysis of the texture, while others are only qualitative. Among
the quantitative techniques, the most widely used is X-ray diffraction using
texture goniometers, followed by the EBSD method in Scanning Electron
Microscopes. Qualitative analysis can be done by Laue photography, simple X-ray
diffraction or with a polarized microscope. Neutron and synchrotron
high-energy X-ray diffraction are suitable for determining textures of
bulk materials and in situ analysis, whereas laboratory x-ray diffraction
instruments are more appropriate for analyzing textures of thin films.
Texture is often represented using a pole figure, in which a specified
crystallographic axis from each of a representative number of crystallites is
plotted in a stereographic projection, along with directions relevant to the
material's processing history. These directions define the so-called sample
reference frame and are, because the investigation of textures started from
the cold working of metals, usually referred to as the rolling direction RD,
the transverse direction TD and the normal direction ND. For drawn metal
wires the cylindrical fiber axis turned out as the sample direction around which
preferred orientation is typically observed.
Common textures There are several textures that are
commonly found in processed materials. They are named either by the scientist
that discovered them, or by the material they are most found in. These are given
in miller indices for simplification purposes.
Cube component:[100] Brass component:[-112]
Copper component:[11-1] S component:[63-4]
Orientation distribution function The full 3D representation of
crystallographic texture is given by the orientation distribution function which
can be achieved through evaluation of a set of pole figures or diffraction
spectra. Subsequently, all pole figures can be derived from the .
The is defined as the volume fraction of grains with a certain orientation .
The orientation is normally identified using three Euler angles. The Euler
angles then describe the transition from the sample’s reference frame into the
crystallographic reference frame of each individual grain of the polycrystal. One
thus ends up with a large set of different Euler angles, the distribution
of which is described by the . The orientation distribution function, ,
cannot be measured directly by any technique. Traditionally both X-ray
diffraction and EBSD may collect pole figures. Different methodologies exist
to obtain the from the pole figures or data in general. They can be classified
based on how they represent the . Some represent the as a function, sum of
functions or expand it in a series of harmonic functions. Others, known as
discrete methods, divide the space in cells and focus on determining the value
of the in each cell. Origins
In wire and fiber, all crystals tend to have nearly identical orientation in the
axial direction, but nearly random radial orientation. The most familiar
exceptions to this rule are fiberglass, which has no crystal structure, and
carbon fiber, in which the crystalline anisotropy is so great that a
good-quality filament will be a distorted single crystal with
approximately cylindrical symmetry. Single-crystal fibers are also not
uncommon. The making of metal sheet often involves
compression in one direction and, in efficient rolling operations, tension in
another, which can orient crystallites in both axes by a process known as grain
flow. However, cold work destroys much of the crystalline order, and the new
crystallites that arise with annealing usually have a different texture.
Control of texture is extremely important in the making of silicon steel
sheet for transformer cores and of aluminium cans.
Texture in ceramics usually arises because the crystallites in a slurry
have shapes that depend on crystalline orientation, often needle- or
plate-shaped. These particles align themselves as water leaves the slurry,
or as clay is formed. Casting or other fluid-to-solid
transitions produce textured solids when there is enough time and activation
energy for atoms to find places in existing crystals, rather than
condensing as an amorphous solid or starting new crystals of random
orientation. Some facets of a crystal grow more rapidly than others, and the
crystallites for which one of these planes faces in the direction of growth
will usually out-compete crystals in other orientations. In the extreme, only
one crystal will survive after a certain length: this is exploited in the
Czochralski process and in the casting of turbine blades and other
creep-sensitive parts. Texture and materials properties
Material properties such as strength, chemical reactivity, stress corrosion
cracking resistance, weldability, deformation behavior, resistance to
radiation damage, and magnetic susceptibility can be highly dependent
on the material’s texture and related changes in microstructure. In many
materials, properties are texture-specific, and development of
unfavorable textures when the material is fabricated or in use can create
weaknesses that can initiate or exacerbate failures. Parts can fail to
perform due to unfavorable textures in their component materials. Failures can
correlate with the crystalline textures formed during fabrication or use of that
component. Consequently, consideration of textures that are present in and that
could form in engineered components while in use can be a critical when
making decisions about the selection of some materials and methods employed to
manufacture parts with those materials. When parts fail during use or abuse,
understanding the textures that occur within those parts can be crucial to
meaningful interpretation of failure analysis data.
Thin film textures Pronounced textures occur in thin films.
Modern technological devices to a large extent rely on polycrystalline thin
films with thicknesses in the nanometer and micrometer ranges. This holds, for
instance, for all microelectronic and most optoelectronic systems or sensoric
and superconducting layers. Most thin film textures may be categorized as one
of two different types: for so-called fiber textures the orientation of a
certain lattice plane is preferentially parallel to the substrate plane; in
biaxial textures the in-plane orientation of crystallites also tend to
align with respect to the sample. The latter phenomenon is accordingly
observed in nearly epitaxial growth processes, where certain
crystallographic axes of crystals in the layer tend to align along a particular
crystallographic orientation of the substrate.
Tailoring the texture on demand has become an important task in thin film
technology. In the case of oxide compounds intended for transparent
conducting films or surface acoustic wave devices, for instance, the polar
axis should be aligned along the substrate normal. Another example is
given by cables from high-temperature superconductors that are being developed
as oxide multilayer systems deposited on metallic ribbons. The adjustment of the
biaxial texture in YBa2Cu3O7 layers turned out as the decisive prerequisite
for achieving sufficiently large critical currents.
The degree of texture is often subjected to an evolution during thin film growth
and the most pronounced textures are only obtained after the layer has
achieved a certain thickness. Thin film growers thus require information about
the texture profile or the texture gradient in order to optimize the
deposition process. The determination of texture gradients by x-ray scattering,
however, is not straightforward, because different depths of a specimen
contribute to the signal. Techniques that allow for the adequate
deconvolution of diffraction intensity were developed only recently.
References Further reading
Bunge, H.-J. "Mathematische Methoden der Texturanalyse" Akademie-Verlag, Berlin
Bunge, H.-J. "Texture Analysis in Materials Science" Butterworth, London
Kocks, U. F., Tomé, C. N., Wenk, H.-R., Beaudoin, A. J., Mecking, H. "Texture
and Anisotropy – Preferred Orientations in Polycrystals and Their Effect on
Materials Properties" Cambridge University Press ISBN 0-521-79420-X
Birkholz, M., chapter 5 of "Thin Film Analysis by X-ray Scattering" Wiley-VCH,
Weinheim ISBN 3-527-31052-5 External links
aluMatter: Representing Texture MTEX MATLAB toolbox for Texture Analysis
Labotex, ODF/texture analysis software for Microsoft Windows
Crystallographic Texture Combined Analysis