Texture and Microstructure

• Microstructure contains far more than qualitative descriptions (images) of cross-sections of materials.

• Most properties are anisotropic •it is important to quantitatively characterize the microstructure including orientation information (texture). In latin, textor means weaver

In materials science, texture  way in which a material is woven.

Polycrystalline material is constituted from a large number of small crystallites (limited volume of material in which periodicity of crystal lattice is present). Each of these crystallites has a specific orientation of the crystal lattice.

A randomly texture sheet

A strongly textured sheet

The cube texture (001)  ND (Sheet Normal Direction) [100]  RD (Sheet Rolling Direction) Crystallographic texture is the orientation distribution of crystallites in a polycrystalline material

Texture :Metallurgists and Materials Scientists

Fabric :Geologists and Mineralogists

Preferred Orientation :Everybody Why textures?

Texture influences the following properties:

•Elastic modulus •Yield strength •Tensile ductility and strength •Formability •Fatigue strength •Fracture toughness •Stress corrosion cracking •Electrical and Magnetic properties

Major fields of application

A. Conventional • Aluminium industry • Steel industry • LC steels • Electrical steels

• Titanium alloys • Zirconium base nuclear grade alloys

B. Modern • High Tc superconductors • Thin films for semiconducting and magnetic devices • Bulk magnetic materials • Structural Ceramics • Polymers Beverage Cans

Aluminium beverage cans Stages in the drawing and ironing of a beverage can

Anisotropy in the mechanical properties can cause 'earing' in the can body. The ears must be cut off, leading to wastage. Photo courtesy of VAW aluminium AG Materials whose properties may be influenced by texture

Engineering Materials: Metals and Alloys Ceramics Polymers Composites

Natural Materials: Geological Minerals Ores Rocks Salts Ice Biological Bones Shells Teeth Wood Texture Formation processes

Texture changes due to:

(i) Crystallisation / solidification (from a non-crystalline / liquid state)

(ii) Plastic deformation (by glide or slip and twinning)

(iii) Annealing (from the same phase)

(iv) Phase transformation (from a different phase) Factors deciding texture  Crystal structure

 Material - Solid solution impurities or alloying additions, Insoluble impurities or second phases  Prior heat treatment - Initial texture Initial grain size Deformation variables Deformation system (slip, twinning) Strain rate Overall deformation (amount, mode of deformation etc.) Deformation temperature  Annealing Temperature and time Heating and cooling rates Furnace atmosphere Texture formation in thin films

Depends on:

 Structural anisotropy of the substrate

 Geometry of the substrate

 Solidification / Deposition conditions

 Temperature gradient

 Mechanical stresses

 Electrical field

 Magnetic field Some general information regarding texture  elongated or flattened grains do not imply a certain texture, or even the presence of texture at all

presence of equiaxed grains does not imply a random orientation.

texture is a statistical concept. If 30-40% of the grains have common orientation, material can be said strongly textured. textures may be of simple type or they may be complex Effect of texture on diffraction patterns How to Describe Textures ?

For uniaxial deformation or other processes, miller indices of directions [uvw] aligned along the specimen axis,

For biaxial deformation, like rolling, a combination of miller indices of sheet plane and the miller indices of the directions parallel to the longitudinal axes, say, (hkl) [uvw]

<100> fibre texture in wire {110}<112> sheet texture Stereographic projection

 mapping of crystallographic planes and directions in a convenient and straightforward manner.

 two dimension drawing of three dimensional data.

 planes are plotted as great circle lines, and directions are plotted as points.

Sometimes, planes are also indicated by the normal to them, that is, by a point.

What do we learn further :  geometrical correspondence between crystallographic planes and directions with their stereographic projections.

 important crystallographic directions that lie in a particular plane of a crystal. What is a pole?

How to Measure Textures ?

 X-ray diffraction

 Neutron diffraction

 Synchrotron radiation

 Electron diffraction Pole Figures

• Pole figures are the most common source of texture information; cheapest, easiest to perform.

• Pole figure = variation in diffracted intensity with respect to direction in the specimen.

• Representation = map in projection of diffracted intensity.

• Map of crystal directions w.r.t. sample reference frame. Principle of pole figure measurement Texture Goniometer goniometer for texture analysis:

4-circle spectrometer

4 angles: ,, ,  pole figure angles ,, 

Reflection geometry Transmission geometry Thick samples (X-ray) Thin samples (X-rays, neutrons)

Types of texture:

Bulk / macro-texture

Micro-texture

Meso-texture

Mathematical descriptors of orientation

Orientation matrix

Ideal orientation

Euler angles

Angle/axis of rotation

Rodrigues vector

Oorientation through angle / axis