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Polycrystalline, single crystal and
Non crystalline Materials
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PolycrystallinePolycrystalline material Aggregate of several crystals or
grains
The boundary between the grain is the grain boundary acrosswhich the orientation of the crystal changes.
The point at which three boundaries meet is called the triple
junction.
Triple junction
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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 determination
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Grain size measurement
ASTM grain size number, G Number of grains per unitarea at a particular magnification
ASTM
No.
Grains/mm2 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 Nwhere, N is number of
grains/mm2
G is compared in ASTM
grain size chart to obtain
the grain size.
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Grain size measurement
Calculate the grain size from the micrographs using the line
intercept method
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Grain size Property relationshipHall-Petch Relation
A general relationship between mechanical properties and
grain size is given by the Hall-Petch equation21
kdi
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.
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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-crystal-superalloys/)(www.cmse.ed.ac.uk/AdvMat45/SuperEng.pdf)
Silicon single crystals for semi conductors
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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 increasingthe number of components (alloying elements).
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Bulk Metallic GlassMulti 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
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Properties of BMGs
Mechanical properties High strength, low ductility
Formation of small crystallites in glassy matrix by annealing
can improve ductility.
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
Elastic
strain
Tensile
strength
Youngs
Modulus
Shear
Modulus
Hardness Fracture
toughness
2% 1.9 GPa 96 GPa 34.3 GPa 534 VHN 55 MPa m1/2
Some Properties of Vit1 (R D Conner et al. Scripta mater. 1997;37:13738)
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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
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Nano Materials
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Nanometer = 10-9
mmicrometer = 10-6 m
mm = 10-3 m
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Nano MaterialsNano 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
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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
Nano -Structured Materials (NSM)
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Classification of NSMs according to their chemical
composition and shape
Classification of NSMs
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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)
Processing Routes
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Properties of NSMs
Superior functional properties Photoluminescence,
Electroluminescence, Electronic and magnetic peoperties
Enhanced catalytic activity Very high surface area per unitvolume
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
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Applications
Sunscreens Lotions - Many sunscreens contain nano
particles of zinc oxide or titanium oxide.
Self-cleaning glass:Activ Glass - uses nanoparticles to makethe 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
Scratch resistant coatings addition of aluminum silicate
nanoparticles to scratch-resistant polymer coatings. Scratch-
resistant coatings for cars and eye lenses.
Glossy colors Nano particles of pigments (CoAl2O4).
Shiny, better looking colors for cars
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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.
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Flash memory: Conventional tunneling film must be
sufficiently insulating to store the charge. Limits thickness
reduction and thus miniaturization. Cannot work with defectsin the film.
Nano dot or quantum dot flash memory allows for higher
defect tolerance, lower thickness (and henceminiaturization). 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 - blueQuantum dot display in place of LCDs or OLEDs less
power consumption, better quality pictures.
Applications of NSMs
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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.
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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_crystalhttp://www.appropedia.org/Single_Crystal_Turbine_Blades
Processing Techniques
http://www2.mmae.ucf.edu/~sury/Online_Pub/Nano-Hyperfine.pdf
http://library.iyte.edu.tr/tezler/master/kimyamuh/T000294.pdf
Key words: Polycrystalline; Grain boundary; Amorphous;
Bulk Metallic Glass; Single crystal; Nano Materials
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Quiz1. 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 amorphousmaterial?
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?
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