5 - Polycrystalline

8/10/2019 5 - Polycrystalline

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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?

5 - Polycrystalline

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