Post on 16-Jul-2015
Phase TransitionsBy Saurav Chandra Sarma
CRYSTALLOGRAPHY AND IT’S APPLICATIONS
Outline
Introduction
Classification of Phase Transition
Kinetics of Phase Transition
Martensitic Transformation
BaTiO3 Phase Transition
Glass Transition
Other Examples
Conclusion
Introduction
• A phase transition is the transformation ofa thermodynamic system from one phase or state ofmatter to another one by heat transfer.
• During a phase transition of a given medium certainproperties of the medium change, often discontinuously,as a result of the change of some external condition, suchas temperature, pressure, or others
• For example, a liquid may become gas upon heating tothe boiling point, resulting in an abrupt change in volume.
Classification of Phase Transitions
Classification of
Phase
Transformations
Mechanism
Thermodynamics
Base
d o
n
Ehrenfest, 1933
Buerger, 1951
Order of a phase transformation
B
A
Ehrenfest’s ClassificationFirst order phase transition: Discontinuity in the first
derivative of Gibb’s Free Energy,G.
Second order phase transition: Continuous first derivative but discontinuity in the second derivative of G.
Lambda Transition:
Buerger’s ClassificationReconstructive Transition: Involves a major reorganization
of the crystal structure.
E.g: Graphite Diamond
Displacive Transition:Involves distortion of bond rather than their breaking and the structural changes.
E.g: Martensitic Transformation
Diffusional or Civilian
Military
Transformation involving first coordination
Reconstructive (sluggish) DiamondGraphite
Dilatational (rapid) Rock saltCsCl
Transformation involving second coordination
Reconstructive (sluggish) QuartzCristobalite
Displacive (rapid) LowHigh Quartz
Transformations involving disorder
Substitutional (sluggish) LowHigh LiFeO2
Rotational (rapid) FerroelectricParaelectric NH4H2PO4
Transformations involving bond type (sluggish)
GreyWhite Sn
Buerger’s Classification: full list
Liquid → Solid phase transformation
Solid (GS)
Liquid (GL)
Tm T →
G
→
T
G
Liquid stableSolid stable
T - Undercooling
↑ t
“For sufficient
Undercooling”
On cooling just below Tm solid becomes stable
But solidification does not start
E.g. liquid Ni can be undercooled 250 K below Tm
G → ve
G → +ve
Nucleation
of
phase
Trasformation
→
+
Growth
till
is
exhausted
=
1nd order
nucleation & growth
Kinetics of Phase Transition:
13
Phase TransformationsNucleation
• nuclei (seeds) act as templates on which crystals grow
• for nucleus to form rate of addition of atoms to nucleus must be faster than rate of loss
• once nucleated, growth proceeds until equilibrium is attained
Driving force to nucleate increases as we increase T
– supercooling (eutectic, eutectoid)
– superheating (peritectic)
Small supercooling slow nucleation rate - few nuclei - large crystals
Large supercooling rapid nucleation rate - many nuclei - small crystals
Heterogeneous nucleation
Nucleation occur at the interface between two phases or at the grain boundary.
Homogeneous nucleation
Nucleation occur without any preferential nucleation sites.
Occurs spontaneously and randomly but it requires superheating or supercooling.
An example of supercooling: Pure water freezes at −42°C rather than at its freezing temperature of 0°C. The crystallization into ice may be facilitated by adding some nucleation “seeds”: small ice particles, or simply by shaking
15
r* = critical nucleus: for r < r* nuclei shrink; for r >r* nuclei grow (to reduce energy)
Adapted from Fig.10.2(b), Callister & Rethwisch 8e.
Homogeneous Nucleation & Energy Effects
GT = Total Free Energy
= GS + GV
Surface Free Energy - destabilizes
the nuclei (it takes energy to make
an interface)
24 rGS
= surface tension
Volume (Bulk) Free Energy –
stabilizes the nuclei (releases energy)
GrGV3
3
4
volume unit
energy free volume G
16
Solidification
TH
Tr
f
m
2*
Note: Hf and are weakly dependent on T
r* decreases as T increases
For typical T r* ~ 10 nm
Hf = latent heat of solidification
Tm = melting temperature
= surface free energy
T = Tm - T = supercooling
r* = critical radius
Avirami equation:
Transformations are often observed to follow acharacteristic S-shaped, or sigmoidal.
Initial Slow rate time reqd. for forming asignificant no. of nuclei of the new phase.
Intermediate fast rate nuclei grow in sizeand cross the critical radius
Final slow rate particles already existingbegin to touch each other, forming aboundary where growth stops.
The parameter ‘n’ depends on shape of β-phase particles (the Dimension):
Spherical→ n=3 (3D) Disk-shaped→ n=2 (2D) Rod-shaped→ n=1 (1D)
Rate of Phase Transformation
Avrami equation => y = 1- exp (-kt n)
• k & n are transformation specific parameters
transformation complete
log t
Fra
ctio
n tra
nsfo
rme
d, y
Fixed T
fraction
transformed
time
0.5
By convention rate = 1 / t0.5
Adapted from
Fig. 10.10,
Callister &
Rethwisch 8e.
maximum rate reached – now amount unconverted decreases so rate slows
t0.5
rate increases as surface area increases
& nuclei grow
Temperature Dependence of Transformation Rate
• For the recrystallization of Cu, since
rate = 1/t0.5
rate increases with increasing temperature
• Rate often so slow that attainment of equilibrium
state not possible!
Adapted from Fig.
10.11, Callister &
Rethwisch 8e.
(Fig. 10.11 adapted
from B.F. Decker and
D. Harker,
"Recrystallization in
Rolled Copper", Trans
AIME, 188, 1950, p.
888.)
135C 119C 113C 102C 88C 43C
1 10 102 104
The martensitic transformation occurs without composition change
The transformation occurs by shear without need for diffusion
The atomic movements required are only a fraction of the interatomic
spacing
The shear changes the shape of the transforming region
→ results in considerable amount of shear energy
→ plate-like shape of Martensite
The amount of martensite formed is a function of the temperature to
which the sample is quenched and not of time
Hardness of martensite is a function of the carbon content
→ but high hardness steel is very brittle as martensite is brittle
1) Martensitic Transformation:
Example??
Martensite
FCCAustenite
FCCAustenite
Alternate choice of Cell
Tetragonal
Martensite
Austenite to Martensite → 4.3 % volume increase
Possible positions of
Carbon atoms
Only a fraction of
the sites occupied
20% contraction of c-axis
12% expansion of a-axis
In Pure Fe after
the Matensitic transformation
c = a
C along the c-axis
obstructs the contraction
C
BCT
C
FCCQuench
% 8.0
)( '
% 8.0
)(
What happens actually???
Martensite Austenite
2) BaTiO3 Phase transition
>120oC
Click me
Cubic Structure(Paraelectric)
Tetragonal Strucure(Ferroelectric)
Experimental Techniques:• DSC• EXAFS (Extended X-ray absorption fine structure)• XANES (X-ray absorption near-edge structure)• PDF (Pair Distribution Function)- To undertand local structure distortion.
Change in hysteresis loop pattern
Source: Ferroelectricity, domain structure and phase transition of Barium Titanate, Reviews of Modern Physics, 22,3 (1950)
Source: Ferroelectricity, domain structure and phase transition of Barium Titanate, Reviews of Modern Physics, 22,3 (1950)
In the high-symmetry cubicphase, no reflections are split.In the tetragonal phase, (222)remains a single peak whereasthe (400) reflection is dividedinto (400/ 040) and (004) peakswith an intensity ratio of 2:1
Source: J. AM. CHEM. SOC.,130, 22, (2008)
Tetragonal system: 10 Raman active
modes but 18 observed due to LO-TO splitting.
Cubic system: Should be Raman inactive
but 2 modes observed due to displace Ti
position
Raman Spectra:
Glass forming liquids are those that are able to “by-pass” the melting point, Tm
Liquid may have a high viscosity that makes it difficult for atoms of the liquid to diffuse (rearrange) into the crystalline structure
Liquid maybe cooled so fast that it does not have enough time to crystallize
Temperature
Mola
r V
olu
me
liquidglass
2) Glass transition:
Examples of Poor Glass Formers:
Why is water, H2O, found to be a very “weak” glass former
Requires cooling the liquid faster than 1,000,000 oC/min
300 to 150K in 9 milliseconds
H2O
No bonding between
molecules and molecules
can easily flow by each
other
Examples of “Good” Glass Formers:
Why is silica, SiO2, found to be a very “strong” glass former?
Can be cooled at
10-10C/min and still by-pass Tm without crystallizing
2,000 oC to 1,000 oC in 20 million years!!
SiO2
Each Si is tetrahedrally bonded to O,
each O is bonded to two Si. Si and O
atoms cannot move unless other
neighboring atoms also move
Typical DSC thermogram
Determination of Glass Transition temperature by dilatometry
Other Examples:
Tetragonal Orthorhombic
Conclusion
• Experimental techniques used to understand the phase transitiondepends on the type of phase transition.
• Depending on the type of transition, it shows various type ofcomplexity.
• Understanding of phase diagram is must to deal with phasetransition.