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Precipitation hardening

Objective

TheobjectiveofthislaboratoryisforyoutoexplorethekineticsandstrengtheningofAlbyprecipitation

hardening.

Preparation

Readsections10.110.5ofMeyersandChawla.

Equipment

and

samples

First week: Differential scanning calorimeters; ovens for heat treating Al alloys. Second

week: mechanicaltestingfacilitiesinTalbot.

Discs of Al 2024 alloy for DSC studies; bars of Al 2024 for tensile testing and hardness

testing.

Computer,plottingsoftware.

Introduction

PurefccmetalssuchasAlhavelowyieldstrengthsbecausethestressrequiredtomoveadislocationis

small. Onemethodofstrengtheninganalloyisthroughprecipitationreactions. Precipitationhardening

istheprocessofhardeningorstrengtheningofanalloybyprecipitatingfinelydispersedprecipitatesof

thesoluteinasupersaturatedmatrix. Thisprocessinvolvesthefollowingthreebasicsteps:

1. The first step is the solution heat treatment or homogenization. Figure 1 illustrates this

phenomenon. During this step, an alloy of composition X1 is heated to a temperature T1,

betweenthesolvusandsolidustemperaturesandsoakedthereuntilallofthesolutedissolves

intothe phaseandauniformsolidsolutionstructureisproduced.

2. The second step is quenching. Quenching is simply cooling the sample rapidly to a lower

temperature,T3,usuallyroomtemperature,andthecoolingmedium isusuallywateratroom

temperature. Duringquenching,thesoluteisnotimmediatelyabletodiffuseoutofaphaseand

the

alloy

is

said

to

be

supersaturated.

The

rationale

behind

the

quenching

process

is

to

preserve

theuniformsolidsolutionstructureofthealloybelowthehomogenizationtemperature.

3. The finalstep intheprecipitationhardeningprocess isaging. It istheprocessofprecipitating

incoherent precipitates from a supersaturated solid solution. When aging occurs at room

temperature,itiscallednaturalaging. Agingaboveroomtemperatureiscalledartificialaging.

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Figure1.BinaryphasediagramfortwometalsAandBhavingaterminalsolidsolution whichhasa

decreasing

solid

solubility

of

B

in

A

with

decreasing

temperature

[1].

ThephasediagramforAlCuisillustratedinFig.2 BysolutionannealinganAlalloy(2024)containing

4.4Wt%Cu,andthenquenching, i.e.coolingsofastthat littlediffusion ispossible,theCucanremain

dissolved inAl,but it isnot inequilibrium. Itwasmentionedabove that solutehardening isnotan

effective hardening mechanism, and so the quenched alloy must be slowly annealed to precipitate

incoherentAl2Cu (phase)particles. Since thenumberdensityofprecipitatescontrolshardness,not

thesizeofthe individualparticles,thetrick istonucleateasmanyparticlesaspossiblebutkeepthem

fromgrowing. Iftheannealistoolongorattoohighatemperature,someprecipitateswillgrowwhile

others shrinkanddisappear, lowering thenumberofprecipitatesand the strength. Thiscondition is

calledoveraging.

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Figure2.PhasediagramforAlCu[1].

Asitturnsout,Cuinaluminumdonotsimplyprecipitateoutaccordingtothephasediagram,butrather

itformsmetastableprecipitatephase,beforefinallyformingtheequilibriumphase,suchasAl2Cu. For

AlCu,the firstphaseto form isa fullycoherentprecipitatecalledaGP (GuinierPreston)zone.This is

followedbyintermediate"and'phasesbeforethestable (Al2Cu)phaseforms. Asthesereactions

occur, the hardness changes, at first increasing as the precipitates nucleate and grow (or age), and

changing from coherent to incoherent phases. Eventually the strength decreases as the incoherent

precipitatesgrowinsizebutdecreaseinnumber(i.e.,overage),seeFig.3.

Figure3.Microstructuraldevelopmentwithagingtime[1].

The2024alloyisunstableatroomtemperatureaftersolutionheattreatmentandmayreachsubstantial

hardnesslevelswith"naturalaging".

Session

1:

Scanning

calorimetry

to

determine

the

kinetics

of

precipitation

of

Al

2024

alloy

Inthissessionofthe lab,youwillanalyzeprecipitationkinetics ina2024seriesaluminum

alloy. Samples fordifferentialscanningcalorimetryhavebeenpreparedforyouatvarious

annealingtimesandtemperatures. Eachgroupwillworkwithonetemperature,observing

the changes in theDSCdatawith increasingannealing times. You can then lookatdata

acquiredbyothergroupsdata in the class to see theeffect that temperaturehason the

kineticsofthisreaction.

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UseaDSCmeasurementofquenched2024Altomeasurethetotalenthalpyassociatedwith

theprecipitationof2024Al.

UseDSC tomeasure the fractionofCu thathasprecipitatedatasa functionofannealing

time.

Youwillbegiven tensilebarsof2024 toperformyourownagehardeningon in the lab.

Eachstudentwillhaveonesampleandwillpickanagingtime. Threefurnaceswillbesetup

withquenchtanksnexttothem. Afteryoursamplehasbeenagedforthedesiredtime,you

willpullitoutanddropitinthequenchingwater. Itwillthenbemarkedandstoredforuse

intheTalbotMechanicalTestingLabthefollowingweek.

Session

2:

Yield

strength

and

hardness

of

Al

2024

alloy

John Williams and the TA will describe the procedures you will follow at the Talbot

Mechanicaltestinglab.

Instrument

procedures:

DSC

1.

LogontothecomputerstationusingyourNetIDandpassword.

2. UnderStart >AllPrograms >PYRISSoftware,launchPyrisManager.

3. Click

4.

IntheMethodEditor,enterSampleID,OperatorID,andanycomments. Enterthesamplemass.

Under theFileName section,chooseadesired location (Desktopworksbest)andaname for

yourdatafile.

5. UndertheInitialStatetab,changetheInitialTempto25C. Yinitialshouldbe0.

6. IntheProgramtab,heatfrom25to400Catarateof25C/min.

7. Remove the inner and outer lids of the DSC using tweezers. Make sure a reference pan is

present in the furnace. Add the samplepan,making sure it iscenteredon the smallcircular

platform. Replacetheinnerandouterlids.

8. Ifeverythingissetthewayyouwantit,click tostarttherun.

9.

Afterthecurvehasbeengenerated,clickonCalculate >PeakAreaandsettheouterboundson

eithersideofthepeakyouwishtoanalyze.

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10.WaituntiltheDSChascooleddownto25Cbeforechangingsamples.

WhenyouaredoneusingtheDSCfortheday,exitoutofPyrisManagerandtelltheTA. Donotturnoff

theDSC,intercooler,orgasregulator. TransferallofyourASCIIfilesandanyscreenshotsorotherfiles

eitherontoaUSBmemorystickorviaemail,netfiles,etc. TakeyoursampleoutoftheDSCandplaceit

intheprovidedsampletrashbin.

References:

1. W.F.Smith,FoundationsofMaterialsScienceandEngineering,McGrawHill, Inc.,2ndEd,New

York,1993.

2. ThislaboratoryhandoutwasoriginallypreparedforME231. ThehelpofP.Hurath, J.Williams,

T.MackinandB.Coverdillisgratefullyacknowledged.