Class 20 Class 20 –– Polycrystalline Polycrystalline Materials

Class 20 – Po
Mate

Read

• Chapter 1 in Engler and Randle
• Chapter 2 in Rohrer, pp. 69-80

Prof. M.L. Weaver

olycrystalline
erials

ding

Polycrysta

•Polycrystals composed of many small single cryst
joined at interfaces called grain boundaries.

grain

grain
boundary

•Microstructures have both geometric and crystallo
properties.
•Among geometric are the distribution of grain size
•Among crystallographic are the orientation of the
reference frame (macroscopic surface of the samp
respect to one another (the misorientation).

allography

tals (called grains) with different orientations

Grains tend to be
randomly orientated
Contact atomic force
microscope (AFM) image
of thermally etched Al2O3
polycrystal.

ographic characteristics that influence their
es & aspect ratios.
e crystallites with respect to external sample
ple), and the orientation of the crystallites with

32

Polycrystallogra

•The orientation of the grains with respect to exter
stereographic projection known as a pole figure.

•The grains numerical label is used to designate th
•The center of the circle represents the surface no
•This information can be constructed from XRD po
(electron backscatter diffraction-next slide), or TEM

aphy (continued)

rnal sample surface is typically shown on special
Fig. (001) pole figure
shows the positions of
poles of each Al2O3
polycrystalline grain.

Grains tend to be
randomly orientated

he location of its pole.
ormal [001] ┴ (001).
ole figure (shown above right), SEM/EBSD
M/SAED (selected area electron diffraction).

33

Recall Stereogra

(Case study of how structu

aphic projections

ure determines properties)

Microcrystalline Fe one nanoindent
sample taken with UNT’s
Environmental-SEM (Quanta)
with electron backscatter
diffraction (EBSD) detector

•Hardness and Elastic Modulus
vary from grain to grain which
exhibit different crystallographic
orientation

Figure above. Pole figure for all
Berkovich nanoindentations
(technique to measure hardness
and elastic modulus). Black spots
on the stereographic triangle

represent various indentations. 34

Preferred Orien

•Eq. (3) from last class becomes invalid when you
crystals making up the specimen are not randoml
•Preferred orientation of crystal grains cause large
observed intensities.
•The reason for this is that each peak in the patte
of the particles in the material.
•If the particles are distributed in a truly random fa
orientation should be identical.
•However, if the particles have a shape anisotropy

•Assume particles are hexagonal platelets (below

particles are most likely to lie with their basal plan

•Thus the (0001) diffraction peak will originate from

such as the (1010).

•As a result of this preferred orientation, the (000l

(hki0)-type reflections.

•In general anything that changes the assumed ra

distribution of relative intensities. Figur

•Also, a few large particles, in an otherwise aniso

fine powder pressed sample, can affect the

distribution of intensities (Figure):

•Materials produced by sintering,casting,or

deformation frequently causes some texture.

ntation (Texture)

u have (a) preferred orientation (texture), i.e. the
ly orientated in space.
e disagreements between calculated and

ern is caused by diffraction from a different subset

ashion, then the number of particles in each

y, this might not be true.

w Figure). In a packed powder sample, plate-like
ne, (0001), parallel to the reference plane.
m a greater number of particles than other peaks,

l)-type reflections will be intensified relative to

andom distribution of particles will affect the

re shows texture in a powder diffraction sample. Highly
otropic particles are likely to have similar alignments.

35

Preferred Orien

(contin

•Deformation texturing is due to the tendency of th
during plastic deformation.
•Each grain undergoes slip and rotation in a comp
and by the slip and rotation of adjoining grains, th
•It can occur in metals, ceramics, rocks and in bot
sheets.
•Macroscopic properties of materials are influence

ntation (Texture)

nued)

he grains in a polycrystalline material to rotate
plex way that is determined by the imposed forces
he result is a preferred nonrandom orientation.
th natural and artificial polymeric fibers and
ed by texture due to anisotropy

36

Preferred Orien

(contin

•Pole figures usually give a statistical
distribution of poles from a very large
number of grains.
•Example on right is for Al alloy (6111)
sheet (cubic texture) that has been rolled
and recrystallized causing the <100> axes
to be preferentially aligned along rolling,
normal and transverse directions.

Grains tend to be
preferentially
orientated or
textured along
<100> axes.

Recall the simplified stereo
(001) projection for cubic c

ntation (Texture)

nued)

[001]
[010]
[100]

RD=rolling direction XRD pole
TD=transverse direction figure

maps for
6111 Al
alloy.

3 poles with respect to sample normal.
Pole figures can be prepared for any
set of planes

ographic 37
crystal.

Thin Film Growt

Preferred Orien

•Sputtered (physical vapor deposition) ZnO thin film
•Columnar growth normal to Si substrate with high

2000 X-ray diffraction (XRD):
1800
1600 (002)
1400
Intensity 1200 1 bilayer 200ºC
1000 1 bilayer 250ºC
800 (100) 1 bilayer 350ºC
600 1 layer ZnO 200ºC
400
200 (101) 40

0 35
30 2

(0002) Pole figure
from XRD:

showing strong out of
plane fibrous (0002)
texture (z-axis
perpendicular to the
substrate).

th often results in
ntation (Texture)

m is HCP (Zincite) of Wurtzite crystal structure.
h (0002) texture.

Cross-sectional bright field TEM:

ZnO
Si substrate

38

Preferred Orien

(contin

ntation (Texture)

nued)

Randomly Semi-oriented Highly oriented
oriented PE chains after PE chains after
PE chains
1200% 3600%
deformation deformation

39