Texture and Properties - I
Which of the properties are affected by texture?
 Physical, Chemical and Mechanical properties depend on
crystal orientation.
 Since the basic building block of a polycrystalline
material, a grain or crystal, displays anisotropic response
to most of the above mentioned properties, their
aggregate (the polycrystal) also exhibits anisotropy.
 The extent of anisotropy depends on crystallographic
texture, which essentially tells how many crystals are
constructively adding to anisotropy and how many are
destructively adding to reduce the anisotropy.
What all properties do depend on texture ?
 Elastic properties – Young s modulus and Poisson s ratio
 Plastic properties
Strength
Ductility
Fracture toughness
Fatigue
Applies to cubic materials as
well as materials with lower
crystal symmetry
 Crevice corrosion and stress corrosion cracking
 Magnetic properties
 Magnetic permeability
 Energy of magnetisation
Thermal expansion
Electrical conductivity
Hexagonal materials
Elastic properties
In the figure below, the variation of elastic modulus with angle from the
rolling direction is plotted for two predominant texture components. It is
clear that the variation is be quite significant for some type of texture and is
not so pronounced for the other type of texture.
Courtesy: R.K. Ray
For differently textured aluminium alloys (texture being
represented by the ODF plots indicated alongside), elastic
modulus has been plotted as a function of angle from the
rolling direction:
It can be seen that the elastic modulus
variation shows an opposite trend. This
clearly shows the dependence of elastic
modulus on texture.
(Courtesy: O. Engler)
Key points in texture dependence
of mechanical property
 Effect of texture on the stress necessary for yielding
and plastic flow:
Texture strengthening
 Influence of orientation on strength:
Anisotropy of flow
 Fracture behaviour of materials:
may /may not be related to anisotropic plasticity
depending on circumstances
Variation of yield
strength with angle
from the rolling
direction for differently
textured material
(important texture
components indicated
below)
(Courtesy: R.K. Ray)
Texture and Formability
 Forming operations involve a combined stress system thus
plastic behaviour of a material has to be accessed in terms of
Yield locus.
 The form of plane strain yield surface is strongly dependent
on texture:
Texture
• For an isotropic material,
strengthening
yield locus is an ellipse
Isotropy
•
For anisotropic materials,
yield locus is deviated from ellipse
This leads to strengthening of material in one
direction, which is known as “Texture
strengthening”.
The concept of texture strengthening is used in
tailoring the deep drawing operation.
Texture can have two types of effect on deep drawability
 Uneven flow due to variation in plastic properties in different
directions
 Peaks and troughs around the periphery of the drawn cup - earing
desirable cup showing earing
Figure courtesy: J. Hirsch, RWTH, Aachen
 Both these effects need to be controlled for cup drawing.
Out of the two types of effects,
Uneven flow due to variation in plastic properties in different directions,
that leads to peaks and troughs around the periphery of the drawn cup
(earing) is known as Planar
anisotropy.
Figure courtesy:
J. Hirsch, RWTH,
Aachen
Further, to obtain extensive deformation in drawing operation, metal flow
in the flange region should occur easily without build up of high stress.
Unless this is acheived there would be fracture in the vicnity of the punch.
This is due to Normal
anisotropy.
Suitable texture can introduce a differential strengthening between the inplane (flange) and the through-thickness (punch) modes of plastic deformation.
Depending on competition between plastic flow processes in different areas of
the metal undergoing deformation, there are regions of easy plastic flow
Three important zones
Requires:
Easy plastic flow
High normal anisotropy
Strength against failure
Figure adapted from:
M. Hatherley and W.B. Hutchinson, Introduction to Texture,
Institute of Materials, London
Best condition
- achieved by high concentration of {111} planes lying parallel to sheet plane
High concentration of {100} planes parallel to sheet plane – most unfavourable
- Strengthens zone A relative to zones B and C
The sheet formability of a material is accessed in terms of Lankford
parameter (r), which is the ratio of the width (plastic) strain w to
the through-thickness strain t . Sometimes, it is also denoted as the
R value.
•R varies from 0 (no widening) to infinity (no thinning) and equals 1
for isotropic materials.
•Sometimes the contraction ratio
measured in terms of
the strain rates along the sample width and length, i.e.
and varies from 0 to 1.
•To characterize the overall resistance to thinning of a sheet, an
average of the R values at three angles is determined, which is
indicator of normal anisotropy
•To measure the amplitude of the planar anisotropy (related to the
ear heights) the difference value is used
• For 90 ears, R  0, and for 45 ears, R  0.
• For a fully isotropic sheet,
= 1 and R = 0.
• value greater than 1 means that during elongation the sheet
contracts more than it thins - indicates good formability.
R value strongly depends on texture of material.
Variation of Lankford parameter (r) with angle from the
rolling direction for differently textured materials
For good deep drawability, the value of R should
be as high as possible, and this can be achieved with
suitable texture. In steels used for making car
bodies, tailoring a suitable texture is a must. For
maximumR and minimum R, out of the above
three examples, {111}<110> is the most suitable one.
Figure courtesy: R. K. Ray
Questions
1. For fully isotropic sheet material
(a) normal anisotropy is zero
(b) normal anisotropy is 1
(c) planar anisotropy is zero
(d) planar anisotropy is 1
2. The condition for good deep drawability of a sheet material is
(a) normal anisotropy should be high
(b) planar anisotropy should be high
(c) normal anisotropy should be low
(d) planar anisotropy should be low
3. Optimum condition for deep drawing of aluminium correspond to minimization of
ears and differential strengthening of the sheet. This is achieved by
(a) high concentration of {110} planes parallel to sheet plane
(b) high concentration of {100} planes parallel to sheet plane
(c) high concentration of {111} planes parallel to sheet plane
(d) high concentration of {123} planes parallel to sheet plane
4. Explain Anisotropy of properties in materials.
5. Why does properties depends on the texture of materials?
6. What is detrimental texture component of steel flowability?