Dispersion Strengthening Dispersion Strengthening by Heat Treatmentby Heat Treatment

Chapter 11a – 4Chapter 11a – 4thth Edition Edition

Chapter 12a- 5Chapter 12a- 5thth Edition Edition

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

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

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

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

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

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

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

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

Nucleation of Carbon Dioxide Nucleation of Carbon Dioxide bubbles bubbles

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

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

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

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

Avrami PlotAvrami PlotF

ract

ion

Con

vert

ed

Time (sec)

Conversion is 50% Complete

is the time required for

50% conversion| Incubation Time |

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)

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

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

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)

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

Effect of Temperature on Phase Effect of Temperature on Phase TransformationTransformation

Growth Rate

Nucleation Rate

Overall Transformation

RateTem

pera

ture

Rate

Equilibrium transformation temperature

Effect of Temperature on Phase Effect of Temperature on Phase TransformationTransformation

Tem

pera

ture

Time

Time for 50% Transformation

Minimum Time required for Transformation

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

Aluminum Copper Phase DiagramAluminum Copper Phase Diagram

Intermetallic Compound

Partial Al – Cu Phase DiagramPartial Al – Cu Phase Diagram

Al % Cu

Many Aluminum Alloys are strengthened with copper

How Does the Solid Form?How Does the Solid Form?

Liquid

L +

This is what we would like to happen

Al % Cu

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

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

#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

#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

#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

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

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

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

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

Aluminum Copper Phase DiagramAluminum Copper Phase Diagram

Intermetallic Compound

Does the Aluminum Copper system meet the requirements for Age Hardening (Precipitation Hardening)?

Aluminum - Copper AgingAluminum - Copper Aging

#1 Solution Treatment

#2 Quench

ss

#3 Aging

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

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

What about other compositions in the What about other compositions in the Titanium-Aluminum system?Titanium-Aluminum system?

Ti