dispersion strengthening by heat treatment chapter 11a – 4 th edition chapter 12a- 5 th edition
TRANSCRIPT
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Dispersion Strengthening Dispersion Strengthening by Heat Treatmentby Heat Treatment
Chapter 11a – 4Chapter 11a – 4thth Edition Edition
Chapter 12a- 5Chapter 12a- 5thth Edition Edition
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In the last two chapters…In the last two chapters…
We looked at some We looked at some fairly simple 2 fairly simple 2 component phase component phase diagrams in some diagrams in some detaildetail
Copper and Nickel Copper and Nickel benefit from solid benefit from solid solution strengthening solution strengthening to improve propertiesto improve properties
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Of course some were Of course some were more complicated more complicated than othersthan othersThe lead-tin phase The lead-tin phase diagram diagram demonstrates a demonstrates a eutectic composition, eutectic composition, which makes which makes dispersion dispersion strengthening strengthening possiblepossible
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We also introduced more complicated two We also introduced more complicated two component phase diagramscomponent phase diagrams
The copper-zinc The copper-zinc phase diagram phase diagram includes a number of includes a number of 3 phase 3 phase transformation points transformation points
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We We did notdid not examine in detail how the examine in detail how the phase change from one solid phase to phase change from one solid phase to another or others occursanother or others occurs
We just accepted that it doesWe just accepted that it does
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PrecipitationPrecipitation
What we want to do What we want to do now is examine the now is examine the precipitation process as precipitation process as we go from one solid to we go from one solid to two solids, much as we two solids, much as we examined the examined the solidification process solidification process from a liquid to a solidfrom a liquid to a solid
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Solid State ReactionsSolid State Reactions
The change from one solid to another has The change from one solid to another has a lot in common with the solidification a lot in common with the solidification process (from liquid to solid)process (from liquid to solid)
It does not happen instantlyIt does not happen instantly Need time for nucleationNeed time for nucleation Need time for growthNeed time for growth
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RecallRecall
For solidificationFor solidification G = 4/3 G = 4/3 r r33 GGvv +4 +4 r r22 Volume free energy + surface energyVolume free energy + surface energy
For one solid phase changing to anotherFor one solid phase changing to another G = 4/3 G = 4/3 r r33 GGvv +4 +4 r r22 + 4/3 + 4/3 r r33 Volume energy + surface energy + strain Volume energy + surface energy + strain
energyenergy Because the new solid does not take up the Because the new solid does not take up the
same volume as the old solidsame volume as the old solid
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In solids it is easier to start a grain growing In solids it is easier to start a grain growing than from liquidsthan from liquids
There is already an existing crystal, which There is already an existing crystal, which functions a lot like a seed crystal in functions a lot like a seed crystal in heterogeneous nucleation from a liquidheterogeneous nucleation from a liquid
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Nucleation of Carbon Dioxide Nucleation of Carbon Dioxide bubbles bubbles
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Let’s look at the two factors Let’s look at the two factors controlling the growth of new controlling the growth of new
crystalscrystals
NucleationNucleation
GrowthGrowth
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NucleationNucleation
Nucleation usually occurs at grain Nucleation usually occurs at grain boundaries – because that’s where there’s boundaries – because that’s where there’s room!room!
Unlike solidification, it isn’t too hard to get Unlike solidification, it isn’t too hard to get a nucleus goinga nucleus going
The nucleation rate increases as the The nucleation rate increases as the temperature goes down – just like in temperature goes down – just like in solidification from a meltsolidification from a melt
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GrowthGrowth
The nucleus grows as material diffuses The nucleus grows as material diffuses through the surrounding solid material to through the surrounding solid material to the sitethe site
Diffusion is a function of temperatureDiffusion is a function of temperature
If you cool the material off immediately, it If you cool the material off immediately, it is hard for diffusion to occuris hard for diffusion to occur
Supersaturated non-equilibrium structures Supersaturated non-equilibrium structures can occurcan occur
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KineticsKinetics
The combination of Nucleation and growth The combination of Nucleation and growth determine how fast the transformation will occur.determine how fast the transformation will occur.
At a constant temperature the transformation is At a constant temperature the transformation is described by the Avrami relationshipdescribed by the Avrami relationship
f=1-exp(-ctf=1-exp(-ctnn)) f is the fraction convertedf is the fraction converted t is timet is time c and n are constants for a given temperaturec and n are constants for a given temperature
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Avrami PlotAvrami PlotF
ract
ion
Con
vert
ed
Time (sec)
Conversion is 50% Complete
is the time required for
50% conversion| Incubation Time |
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Growth RateGrowth Rate
Often expressed as 1/Often expressed as 1/minminhe growth rate is a function of he growth rate is a function of temperaturetemperatureOften, the higher the temperature, the Often, the higher the temperature, the faster the solid transformsfaster the solid transformsWhy?Why?Diffusion dominates in many systems Diffusion dominates in many systems because diffusion is hard (slow) and because diffusion is hard (slow) and nucleation is easy (fast)nucleation is easy (fast)
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Growth Dominated Phase Change is Growth Dominated Phase Change is common for many metals, especially during common for many metals, especially during
recrystallization after cold workrecrystallization after cold work
For these metals nucleation occurs readilyFor these metals nucleation occurs readily
The only factor that changes with The only factor that changes with temperature then is the growth rate – temperature then is the growth rate – which is diffusion controlledwhich is diffusion controlled
For these metals, the solid to solid phase For these metals, the solid to solid phase change always occurs faster at higher change always occurs faster at higher temperaturestemperatures
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Effect of Temperature on Copper Effect of Temperature on Copper Recrystallization after Cold WorkRecrystallization after Cold Work
Fra
ctio
n T
rans
form
ed
Time
135 C
120 C80 C
is the time required for 50% conversion
Higher temperature equals faster conversion
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Growth rate follows an Growth rate follows an Arrhenius Relationship Arrhenius Relationship
Growth rate = 1/
Growth rate is proportional to overall transformation rate
Growth rate = A exp(-Q/RT)
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What happens if nucleation controls What happens if nucleation controls the phase change?the phase change?
Nucleation happens more readily at Nucleation happens more readily at LOWER temperaturesLOWER temperatures
That means the overall transformation That means the overall transformation goes UP as the temperature goes downgoes UP as the temperature goes down
However… eventually as the temperature However… eventually as the temperature gets colder and colder diffusion slows to gets colder and colder diffusion slows to the point where it plays a factorthe point where it plays a factor
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Effect of Temperature on Phase Effect of Temperature on Phase TransformationTransformation
Growth Rate
Nucleation Rate
Overall Transformation
RateTem
pera
ture
Rate
Equilibrium transformation temperature
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Effect of Temperature on Phase Effect of Temperature on Phase TransformationTransformation
Tem
pera
ture
Time
Time for 50% Transformation
Minimum Time required for Transformation
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C-curveC-curve
Typical of many metals, ceramics, glasses Typical of many metals, ceramics, glasses and polymersand polymers
Ex. Iron changes phase this wayEx. Iron changes phase this way
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Aluminum Copper Phase DiagramAluminum Copper Phase Diagram
Intermetallic Compound
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Partial Al – Cu Phase DiagramPartial Al – Cu Phase Diagram
Al % Cu
Many Aluminum Alloys are strengthened with copper
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How Does the Solid Form?How Does the Solid Form?
Liquid
L +
This is what we would like to happen
Al % Cu
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How Does the Solid Form?How Does the Solid Form?
Liquid
L +
This is what typically happens
We want to avoid this structure, which is
caused by slow cooling
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Age hardening or Precipitation Age hardening or Precipitation HardeningHardening
A treatment used on non optimum alloy A treatment used on non optimum alloy structuresstructures
Produces a uniform dispersion ofProduces a uniform dispersion of FineFine HardHard Coherent PrecipitateCoherent Precipitate
In a softer, more ductile matrixIn a softer, more ductile matrix
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#1 Solution Treatment#1 Solution Treatment
Reheat the alloy up to a temperature where only one Reheat the alloy up to a temperature where only one solid phase exists (above the solvus)solid phase exists (above the solvus)
This dissolves the second solid phase (This dissolves the second solid phase ( for example) for example) into the primary phase (which may take significant time)into the primary phase (which may take significant time)
Don’t exceed the eutectic temperatureDon’t exceed the eutectic temperature
L
L
Al Cu
Tem
pera
ture
Tem
pera
ture
Time
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#2 Quench#2 Quench
Rapidly cool to room temperature or belowRapidly cool to room temperature or below
This results in a supersaturated – This results in a supersaturated – nonequilibrium structurenonequilibrium structure
The second phase does not form, because The second phase does not form, because diffusion is so slow!!diffusion is so slow!!
L
L
Al Cu
Tem
pera
ture
Tem
pera
ture
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#3 Aging#3 Aging
Reheat to a temperature below the solvusReheat to a temperature below the solvus
Diffusion is still slow, so the atoms can only Diffusion is still slow, so the atoms can only diffuse a short distancediffuse a short distance
Results in a fine precipitateResults in a fine precipitate
There is an optimum aging timeThere is an optimum aging time
L
L
Al Cu
Tem
pera
ture
Tem
pera
ture
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Coherent vs. Non-Coherent Coherent vs. Non-Coherent PrecipitatePrecipitate
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Non-Coherent Coherent
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Coherent PrecipitatesCoherent Precipitates
Form FirstForm First
Eventually grow until they snap out of Eventually grow until they snap out of solutionsolution
Produce more hardeningProduce more hardening
If you over age – the strength goes down If you over age – the strength goes down becausebecause Precipitate goes from coherent to Precipitate goes from coherent to
noncoherentnoncoherent
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AgingAging
See the animations on the CDSee the animations on the CD
Artificial aging – elevated temperaturesArtificial aging – elevated temperatures
Natural aging – room temperatureNatural aging – room temperature
Not suitable for use at high temperatureNot suitable for use at high temperature Why?Why?
Problems with weldingProblems with welding
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Requirements for Age Requirements for Age HardeningHardening
Must have a phase diagram that exhibits a Must have a phase diagram that exhibits a change from a single solid phase to two change from a single solid phase to two solid phases (solid phases (Matrix should be soft and ductileMatrix should be soft and ductile
Precipitate should be hard and strongPrecipitate should be hard and strong
Must be quenchableMust be quenchable
Must have a coherent precipitateMust have a coherent precipitate
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Aluminum Copper Phase DiagramAluminum Copper Phase Diagram
Intermetallic Compound
Does the Aluminum Copper system meet the requirements for Age Hardening (Precipitation Hardening)?
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Aluminum - Copper AgingAluminum - Copper Aging
#1 Solution Treatment
#2 Quench
ss
#3 Aging
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What About the Lead Tin System?What About the Lead Tin System?
There is no There is no intermetallic intermetallic phase, so it is phase, so it is probably not a probably not a good candidategood candidate
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What about compositions between 22 wt% and 36 What about compositions between 22 wt% and 36 wt% Al in the Titanium-Aluminum system?wt% Al in the Titanium-Aluminum system?
Ti
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What about other compositions in the What about other compositions in the Titanium-Aluminum system?Titanium-Aluminum system?
Ti