Polycrystalline, single crystal and Non crystalline Materials

Polycrystalline, single crystal and
Non crystalline Materials
Polycrystalline
Polycrystalline material – Aggregate of several crystals or
grains
The boundary between the grain is the grain boundary across
which the orientation of the crystal changes.
The point at which three boundaries meet is called the triple
junction.
Triple junction
Grain size determination
Line intercept method
This is one of the most commonly used methods. Number of
grains intersecting a given length of a random line is counted.
Grain size D = Length of the line/no of grains intersected
Grain size measurement
ASTM grain size number, G – Number of grains per unit
area at a particular magnification
ASTM Grains/mm2
No.
Grains/mm3
Avg. grain
size, mm
-1
3.9
6.1
0.51
0
7.8
17.3
0.36
1
15.5
49.0
0.25
2
31.0
138
0.18
3
62.0
391
0.125
4
124
1105
0.09
5
248
3126
0.065
6
496
8842
0.045
7
992
25010
0.032
8
1980
70700
0.022
9
3970
200000
0.016
10
7940
566000
0.011
G = -2.9542 + 1.4427 ln N
where, N is number of
grains/mm2
G is compared in ASTM
grain size chart to obtain
the grain size.
Grain size measurement
Calculate the grain size from the micrographs using the line
intercept method
Grain size – Property relationship
Hall-Petch Relation
/
i
o
  
d
k
A general relationship between mechanical properties and
grain size is given by the Hall-Petch equation
1 2
o is the yield strength, d is the grain size and i and k are
material dependent constants.
Finer grain size means more grain boundaries or higher grain
boundary area per unit volume. Deformation in metals takes
place by dislocation motion and grain boundaries act as
obstacles to dislocation motion. Hence, presence of more
grain boundaries (finer grain size) will increase the resistance
to deformation and enhance the strength.
Single Crystals
Single crystal – only one grain or crystal and hence, no
grain boundaries.
Useful for applications where grain boundaries are harmful.
For example, high temperature deformation or creep
resistance (as creep takes place by grain boundary sliding) single crystal turbine blades
(http://blog.makezine.com/2012/01/16/single-crystalsuperalloys/)(www.cmse.ed.ac.uk/AdvMat45/SuperEng.pdf)
Silicon single crystals for semi conductors
Non crystalline or Amorphous materials
Amorphous – random arrangement of atoms: Silicate glass,
Polymers
Metallic amorphous materials
Bulk Metallic Glass (BMG)
A metallic system can be made amorphous by decreasing the
chance of crystallization :– Allow less time for crystallization
during solidification – Rapid solidification processing (RSP)
Increase confusion for a particular crystal form by increasing
the number of components (alloying elements).
Bulk Metallic Glass
Multi component – elements with different chemical nature
and atomic size and number together reduce the chance of
crystallization
Zr41.2Ti13.8Cu12.5Ni10Be22.5 [=(Zr3Ti)55(Be9Cu5Ni4)45], commonly
referred to as Vitreloy 1 (Vit1)
Processing routes:
RSP - Melt spinning
Ball milling
Solid-state amorphisation reaction
Nuclear irradiation
Properties of BMGs
Mechanical properties – High strength, low ductility
Formation of small crystallites in glassy matrix by annealing
can improve ductility.
Some Properties of Vit1 (R D Conner et al. Scripta mater. 1997;37:1373–8)
Elastic
strain
Tensile
strength
Young’s
Modulus
Shear
Modulus
Hardness
Fracture
toughness
2%
1.9 GPa
96 GPa
34.3 GPa
534 VHN
55 MPa m1/2
Magnetic properties – excellent soft magnetic properties due
to absence of crystalline magnetic anisotropy FINEMET –
Fe-Si-B-Cu-Nb
Excellent corrosion resistance
Good acoustic properties
Applications of BMGs
Golf heads – BMG golf heads can transfer 99% energy
compared to 60% for steel heads and 70% for Ti heads
Thin yet strong electronic casing – MP3 player, mobile
phones, digital cameras, PDA
Surgical instruments, Prosthetic implants (Biomaterials)
Sensors for electronic article surveillance, cores for high
frequency transformers
Self-sharpening tank armor penetrator
Nano Materials
Nanometer = 10-9 m
micrometer = 10-6 m
mm = 10-3 m
Nano Materials
Nano Structured Material (NSM) – At least one component
of the microstructure e.g. grains, particles or dispersoids, is
nano meter in size.
Nano particles possess very high surface area /unit volume
which gives rise to unique physical and chemical properties.
In a nano-grained material the grain boundary width is
comparable with the grain size.
Nano particles
Nano Ni-ZrO2 composite Nano grains in Al
Nano -Structured Materials (NSM)
A two-dimensional representation of a nanostructured
material. Red circles indicate atoms in the grain while
open circles indicate atoms in the grain boundary region
Classification of NSMs
Classification of NSMs according to their chemical
composition and shape
Processing Routes
 Inert gas condensation
 Vapour phase condensation
 Wet chemical method
 High energy mechanical alloying
 Plasma processing
 Combustion synthesis
 Super critical liquid
 Chemical vapor deposition (CVD)
 Physical vapor deposition (PVD)
Properties of NSMs
Superior functional properties – Photoluminescence,
Electroluminescence, Electronic and magnetic peoperties
Enhanced catalytic activity – Very high surface area per unit
volume
Structural properties – Very high strength – Hall-petch
equation 0 = i + kd-1/2
Ultrafine grain size - Superplasticity
High Hardness – Nanocrystalline WC-Co composites
High stiffness - CNTs
Applications
Sunscreens Lotions - Many sunscreens contain nano
particles of zinc oxide or titanium oxide.
Self-cleaning glass: Activ Glass - uses nanoparticles to make
the glass photocatalytic and hydrophilic.
•Clothing: coating fabrics with a thin layer of zinc oxide
nanoparticles, better protection from UV radiation.
•Stain resistant clothes - have nanoparticles in the form of little
hairs or whiskers that help repel water and other materials
Glossy colors – Nano particles of pigments (CoAl2O4).
Shiny, better looking colors for cars
Scratch resistant coatings – addition of aluminum silicate
nanoparticles to scratch-resistant polymer coatings. Scratchresistant coatings for cars and eye lenses.
NSMs in Electronics – Better and Smaller
The advent of the nano technology has immensely helped
in miniaturization which is an essential part in many
electronic gadgets.
The nano technology is already in use in many electronic
gadgets like mobile phones, I-pads, cameras, palm tops and
so on.
Quantum dots - nano-scale semiconductor crystals.
Dramatic improvement in digital storage (Flash memory)
Less power consumption, better resolution (in visuals),
smaller size.
Applications of NSMs
Flash memory: Conventional – tunneling film must be
sufficiently insulating to store the charge. Limits thickness
reduction and thus miniaturization. Cannot work with defects
in the film.
Nano dot or quantum dot flash memory allows for higher
defect tolerance, lower thickness (and hence
miniaturization). Less power consumption
Quantum dots display - By altering the size of the nano
particles, the color they emit can be changed.
6 nm – red, 2 nm - blue
Quantum dot display in place of LCDs or OLEDs – less
power consumption, better quality pictures.
Applications of NSMs contd….
IBM has developed Microscopic LED: A thin indium-nitride
nanowire that emits infrared light when a current is applied
(http://www.technologyreview.in/communications/19129/)
It is believed that the nanowire LEDs could eventually be
used for telecommunications and for faster communications
between devices on microchips.
References
http://www.iue.tuwien.ac.at/phd/holzer/node39.html
http://science.howstuffworks.com/nanotechnology3.htm
http://203.208.166.84/mjrahman/Class%20Note_Jellur.pdf
http://en.wikipedia.org/wiki/Single_crystal
http://www.appropedia.org/Single_Crystal_Turbine_Blades
Processing Techniques
http://www2.mmae.ucf.edu/~sury/Online_Pub/NanoHyperfine.pdf
http://library.iyte.edu.tr/tezler/master/kimyamuh/T000294.pdf
Key words: Polycrystalline; Grain boundary; Amorphous;
Bulk Metallic Glass; Single crystal; Nano Materials
Quiz
1. What aspect of the crystals changes across the grain
boundaries?
2. How is the gain size measured in a polycrystalline material?
3. What is effect of grain boundaries on strength of metals?
4. Are grain boundaries desirable for high temperature
structural application? Give reasons for your answer.
5. What is meant by amorphous material?
6. How can a metallic system be made into an amorphous
material?
7. Why bulk metallic glasses (BMG) are generally
multicomponent?
8. What is nanostructured material? What imparts unique
properties to these materials?
9. Classify nanostructured materials.
10. What are the processing routes of nano materials?