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Advanced Physical Metallurgy Advanced Physical Metallurgy

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Advanced Physical Metallurgy Advanced Physical Metallurgy

Amorphous Materials Amorphous Materials

Eun Eun Soo Soo Park Park

2009 spring

04. 01. 2009

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Contents for previous class Contents for previous class

• • Measuring Glass Transition Temperature Measuring Glass Transition Temperature (1)

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• • Measuring Glass Transition Temperature Measuring Glass Transition Temperature

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• • Measuring Glass Transition Temperature Measuring Glass Transition Temperature

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• • Measuring Glass Transition Temperature Measuring Glass Transition Temperature

The Dynamic Mechanical Analysis is a high precision technique for measuring the viscoelastic properties of materials. It consists in applying a sinusoidal deformation to a specimen of material and measuring the resulting force transmitted through the specimen.

Specimen

Applied displacement

Amplitude

Resulting force

Time

Loss angle δ

F D

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Contents for previous class Contents for previous class

• • Glass: Glass: all types of bonding all types of bonding

• • Glass transition: Glass transition: Volumetric measurement Volumetric measurement

depends on cooling rate depends on thermal history

T T

gg

glass transition temp. glass transition temp.

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Contents for previous class Contents for previous class

• • Thermodynamics for Thermodynamics for glass transition glass transition

~ not thermodynamic nature

~ close to second order phase transition

V, H. S changes continuously.

V, H. S changes continuously.

α α

TT

C C

PP

K K

TT

changes discontinuously. changes discontinuously.

at T at T

gg

G changes continuously. G changes continuously.

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• • Order (degree) of transition Order (degree) of transition

C

D

P E

Continuous phase transitions

:

occur when the minimum in the thermodynamic potential evolves smoothly into two equal minima.

An example is seen in the model of phase separation, along the co-existence line.

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• • Order (degree) of transition Order (degree) of transition

C

D

P E

• CD, DE, DP: Equilibrium of 2 phases

latent heat

Volume change 1st order transition

• T and P beyond point p

: vapor and liquid are indistinguishable.

No boiling pt. no latent heat Higher order transition

Single phase: only property changes.

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~ Brass is a 50:50, Cu:Zn alloy with a b.c.c structure.

~ At low temperatures, T<460K, the Zn and Cu atoms for an ordered

structure (eg. Cu atoms in the body-centre sites in top diagram)

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Contents for previous class Contents for previous class

• • derivative of thermodynamic properties derivative of thermodynamic properties

~ dramatically decrease Cp near Tg during cooling

~ dramatically increase Cp near Tg during heating

discontinuous

discontinuous (C

(Cpp, α, αTT, K, KTT))

Experimental observation during heating

because the monitoring is very difficult during cooling

Glass transition: heat endothermic

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Fragile network glass : Vogel-Fulcher relation

Strong network glass : Arrhenius behavior

]

0

exp[

RT E

a

η

η =

] exp[

0

0

T T

B

= η − η

< Quantification of Fragility >

Fragility Fragility

g n

g g T T

T n T

g d T T

T d

T T

d

T m d

=

=

=

= ( / )

) ( log )

/ (

) ( log

,

,

τ η

< Classification of glass >

Fragility ~ ability of the liquid to withstand changes in medium range order with temp.

~ extensively use to figure out liquid dynamics and glass properties corresponding to “frozen” liquid state

Slope of the logarithm of viscosity, η (or structural relaxation time, τ ) at Tg

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T increase

Supercooled liquid

1012~1014 poise

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Strong liquid vs. Fragile liquid

Fragility Fragility

• Strong glass forming liquid

covalent bond of SiO2

small difference of Cp between SCL and glass at Tg

(small difference of structure)

SCL: relatively low entropy

• fragile glass forming liquid

non-directional bonding

(Van der waals bonding)

large difference of C at T

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Endo.

Exo. heating

Glass SCL crystal Liquid

Glass Supercooled

liquid

liquid cooling

Tm Tg

Tx

Tp

Ts TL

Glass transition

: Endo.

Crystallization

: Exo.

melting

: Endo.

C+L

< Thermal map >

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Use of Gibbs energy curves to construct a binary phase diagram of the eutectic type.

High temperature

T1 T2 T3

T4 T5

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