Precipitation in original Duralumin A-U4G versus modern 2017A alloy

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Brunet, Magali and Malard, Benoît

and Ratel-Ramond, Nicolas and

Deshayes, Christophe and Joulié, Sébastien and Warot-Fonrose, Bénédicte and

Sciau, Philippe and Douin, Joël and De Geuser, Frédéric and Deschamps,

Alexis Precipitation in original Duralumin A-U4G versus modern 2017A

alloy. (2019) Materialia, 8. 100429. ISSN 2589-1529

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Precipitation in original Duralumin A-U4G versus modern 2017 A alloy

Magali Brunet

a

_,

_*

_{, Benoit Malard}

b

_{, Nicolas Ratel-Ramond}

a

_{, Christophe Deshayes}

a

_,

Sébastien Jouli

e

, Bénédicte Warot-Fonrose

a

_{, Philippe Sciau}

a

_{, Joël Douin}

a

_{, Frédéric De Geuser}

c

_,

Alexis Descham ps

c

• CEMES, CNRS, UniYemti de Toulouse, 29 rue Jeanne MIJJ'Vi& BP 94347 31055 Toulouse Cedex 4, Fronce • CIRIMA T, TNP Toulouse, CNRS, 4 aHé.e Emile Manso, 31030 Toulouse, France

• UniYemté Grenoble Alpes, CNRS, Grenoble TNP, SJMoP, F-38000 Grenoble, France

ARTICLE INFO ABSTRACT

Keyword.s: Duralumin Al-Co-Mg-Si alloy Predpitation SAXS STEM-HMDF

Precipitation in Duralumin, a historie quatemary alloy of the type: Al-Cu-Mg-Si, was never fully studied nor observed bycurrentelectron microscopytechniques. This article presents the full characterization and comparison of two alloys: a Duralumin (A-U4G) from the 19SOs collected on a vintage aircraft and its modern equivalent: a 2017 A alloy. The as-received and peak-aging states were analysed with DSC, SAXS and TEM advanced techniques. It is shown that old Duralumin and modern 2017A present a similar nanoprecipitation in the as-received state and behave similarly upon artificial aging. As opposed to what has been reported in the past, three types of precipitates participating in hardening were found upon aging: 8'-Al2Cu, Q'(Q)-AICuMgSi and Q-Al2Cu.

1. Introduction

Duralumin, with a composition close to 4 Cu 0.7 Mg 0.6 Mn 0.6 Si wt°/4, is the oldest age hardening aluminium alloy. Discovered in 1906 by Alfred Wilm, a German metallurgist [1], it triggered the develop ment of aeronautics at the beginning of the XXth century. At the sci entific and technical research centre in Neubabelsberg, Wilm experi mented many treatments on Al Cu Mn alloys with small amounts of magnesium (0.5 wt%). He found out that by quenching from tempera tures below its melting point (about 450'C) and by letting it age natu rally for a few days, the new alloy exhibited enhanced mechanical prop erties (strength and hardness). Thanks to its low density and strength, Duralumin soon became the prime choice for airplanes construction, well illustrated by the airplane Breguet 14 whose production reached 12,000 during World War I [2]. From then on, aircraft production, pre viously a craftsmen business, transformed into a real industry. Following on this discovery and for reaching even higher performance, metallur gists in the 1920s tuned aluminium alloys via the addition of microal loying elements. This was done empirically, as no explanation could yet be provided for the age hardening phenomenon. Merica et al [3] were the first team in 1929 to explain that the hardening during aging of du ralumin was due to fine and highly dispersed Al2Cu precipitates. But the concept of dislocation, essential to a complete understanding of the structural hardening mechanism, was introduced by Orowan in 1934 [4]. The discovery of coherent copper rich zones in the Al Cu 4% al loy was achieved independently by Guinier [5] and Preston [6] with X ray experiments in 1938. lt took even longer to be able to observe

• Corresponding author.

E-mail address: magali.brunet@cemes.fr (M. Brunet).

directly the nanoprecipitates responsible for the hardening: first obser vations were made on oxide replicas in France in 1952 by Saulnier and

Syre [7].

Original Duralumin was thus improved experimentally by increas ing the magnesium content (up to 1.5wt%), leading to a new alloy, called Duralumin FR in France in the early 1930s [8]. Duralumin and Duralumin FR became A U4G and A U4Gl respectively in 1943, to fol low a new designation defined by AFNOR1 _{[9]. From the 1930s to the} 1950s, because of its lack of strength and ductility, A U 4G was replaced gradually in aeronautics construction by A U4Gl. ln 1954, international standardization of aluminium alloys took place [10]: the numerical des ignation with 4 numbers was adopted. A U4G was designated as 2017 A and A U4Gl as the well known 2024.

Original Duralumin has not received a major focus in the scientific literature related to the precipitation at nanoscale. ln fact, published works on the precipitation occurring in Duralumin and even on the modern 2017 A alloys are scarce. Original studies on the precipitation of industrial Duralumin were published in the 1950s. ln 1950, Lambot studied the precipitation of an industrial Duralumin by X ray abnor mal scattering [11]. ln accordance with results found by Guinier and Preston in Al Cu alloys, after quenching and room temperature aging, he detected scattering due to copper clusters or Guinier Preston Zones (GPZ). Then, upon aging at moderate temperatures, a first form of pre cipitates was evidenced, with a structure of three aluminium rich planes

1 _{Created in 1926, AFNOR is the French association for standardization.}

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Table 1

Elementalcompositionofalloys(inwt%)obtainedbyICP-OES.

Al Cu Mg Mn Si Fe Ti Zn Cr

2017A Base 4.32 ± 0.08 0.68 ± 0.01 0.611 ± 0.002 0.618 ± 0.006 0.34 ± 0.01 0.043 ± 0.001 0.20 ± 0.04 0.029 ± 0.003

A-U4G Base 4.18 ± 0.04 0.710 ± 0.005 0.67 ± 0.01 0.61 ± 0.01 0.285 ± 0.008 0.010 ± 0.001 0.082 ± 0.008 0.007 ± 0.0001

in between two copper richplanes. This phase evolved tothe well

distinguished𝜃’ Al2Cuphasewhenagingathighertemperatures.Other scatteringpatterns,superimposedtotheonesoriginatingfromAl2Cu precipitates,weredetected.TheywereattributedtotheAl4CuMg5Si4 phase.Thisphaseisexpectedatequilibriumstate,inalloysofMg/Si ratioslowerthan1.73[12].Twoyearslater,SaulnierandSyre[7]iden tiﬁedinasimilarindustrialDuralumin,thankstoelectronmicroscopy (usingoxidereplicas),thepresenceoftheso calledphaseX(renamed 𝜃’’later)afteragingat260°Cevolvingintothe𝜃’ Al2Cuphaseat290°C,

conﬁrmingtheﬁndingsofLambot.Anotherphasewasevidencedwith

thermaldilatometry.However,itwasattributedtoMg₂Sineedles.In thiswork,theQ Al4CuMg5Si4phasecouldnotbeevidenced.Similarly,

YanoandKodain 1968[13], observedby transmissionelectronmi

croscopy(TEM),thepresenceofthe𝜃’ Al2CuandMg2SiphasesinDu ralumin.Morerecently,Härteletal.[14]studiedthe2017Aalloyand observedthe𝜃’phaseandanotherphase,ΩAlCu.However,theydidnot mentionthepresenceofQ AlCuMgSi,neitherMg₂Si.Itistobenoted thoughthatthesiliconcontentwas,intheircase,verylow(0.08wt%).

Asshownhere,literaturedoesnotprovideaclearandcomprehensive

reportonprecipitationoccurringinDuralumin.

This article focuses on the observation and identiﬁcation of the

metastablephasesandprecipitationsequencesoccurringinaDuralu

min(A U4G)of1958collectedonanoldaircraftandagedatambient

temperatureformorethan60years.Itwillbecomparedtoamodern

equivalent,the2017Aalloy.Whenlookingattheinternationalstan

dards[10],itisinterestingtonotethat,asopposedtotheA U4G1which

compositionissigniﬁcantlydiﬀerentfromthemodernequivalent2024,

thecompositionofA U4G/2017Ahasbarelyevolvedovertime.Asthe

collectedDuraluminismorethan60yearsold,bycomparingits be

haviourandmicrostructuretothemodernequivalent,itisalsotheoc casiontostudylong termagingonthistypeofalloy.Thisconstitutesa ﬁrstmotivationforstudyingthisalloy.Thesecondmotivationisrelated toCulturalHeritage.Duraluminconstitutesmostoftheairplanes’fuse

lageandstructurebeforeandduringWorldWarII.Preciousspecimens

conservedinMuseumshouldbepreserved.Byunderstandinganddoc

umentingtheintrinsicconstitutionofthealloyanditsbehaviourover time,itwillbeeasiertoanticipateproblemssuchaslackofmechanical resistanceand/orcorrosion.

ThemicrostructureoftheA U4Gand2017Aindiﬀerentstates,as

receivedandafterartificialaging,willbereportedhereafterbycom biningdifferenttechniques:conventionalbrightanddarkfieldTEM,se lectedareadiffractionpatterns(SADP),scanningtransmissionelectron

microscopycoupledwithenergydispersiveX rayspectroscopy(STEM

EDS),withhighangleannulardarkﬁeld(HAADF)imagingorwithelec

tronenergylossspectroscopy(EELS),diﬀerentialscanningcalorimetry (DSC)andsmallangleX rayscattering(SAXS)experiments.Itisshown that,comparedtowhatcouldbeexpectedfromtheliteratureonquater

naryAl Cu Mg Sialloys[15,16]witharangeof2.5 4.5wt%ofcop

per,andaratioofMg:Sicloseto1orwhatwasobservedinthe1950s

and1960sonDuralumin[7,13],acomplexandspeciﬁcprecipitation

sequenceoccursinthesealloys.

2. Materialsandcharacterizationtechniques

2.1. Materials

ThestudiedalloyisaDuralumin,identiﬁedasA U4Gintheshapeof plate(about1mmthick)collectedonanaircraftfrom1958,aBreguet

Sahara765:thesamplewasextractedfromaﬂapofthelandinggear,

a partnotsubjectedtohottemperaturesrelatedtoengine proximity

(seeFig.S1ofsupplementaryinformation).Theequivalentmodernal

loy,ENAW 2017A(labelledhereafter2017A)wasboughtfromBikar

MetalleGmbH.ItwasreceivedinT4state.Ingeneral,for2017Aand

2024alloys,thestatesT3andT4arecommonforpartsoflowthickness (<12mm).Inthesestates,thealloyexhibitsagoodcorrosionresistance, ahightoughnessandagoodfatigueresistance[17].Theplatecollected ontheplanereceivedalsoaT4treatmentuponfabrication:itwasclearly mentionedintheBreguetstandards[18].However,forthisalloy,theas receivedstatecorrespondsinfact,toaT4stateplus10yearsofservice and50yearsinoutdoorsconditions.

For each alloy, the inductively coupled plasma optical emission

spectrometry (ICP OES) measurements, performed byEvans Analyti

cal GroupSAS,is reportedin Table1.It isconﬁrmedthatthecom

positionsaresimilar.Currently,2017Acontainssomezincwhereasold

versionof thisalloydoesnot.TheratiosMg:Siare1.1and1.16for

2017AandA U4G,respectively,whichcorrespondtoreportedvaluesfor Duralumin[7].

Forthestudyofhardeningprecipitation,twostateswerecompared:

as received andpeak agingconditions. Toﬁndpeak agingcondition,

received sampleswereheat treated at 180°C during diﬀerent times.

Thesampleswerethencutalongthreeperpendiculardirectionsrela

tivetotherollingdirectionandmeasurementsofmicro hardness(Vick ers)werecarriedoutinthecorrespondingplanes(RP:rollingplane;NP: normalplane;TP:transverseplane).

2.2. Characterizationtechniques

Thealloyswereanalysedbydiﬀerent techniques,which required

adapted preparations.Forhardnessmeasurements aswell asfor the

observation of the microstructure by optical microscope (OM) and

scanningelectronmicroscope(SEM),eachspecimen(inthe3planes)

wasembeddedinanepoxyresinandmechanicallypolishedonwater

lubricatedabrasivepapers(siliconcarbide):P600thenP1200andeven

tually on polishing cloths with diamond paste (from 3μmdown to

1μm).

ForTEMobservations,thepreparationwasthefollowing:tobring

thesamplethicknessdownto25μm,amechanicalpolishingwithpaper

gradeupto2400SiC,wasperformed.Thespecimenswerethenelec

trochemicallythinnedusingaTenupol 5Struersapparatusoperatingat 60Vinasolutionofmethanolandnitricacid(3:1)at−15°C.Observa tionsofthenanostructurewerecarriedoutonaJEOL2010operatingat

200kV.Brightﬁeldimagesweretakenwithanorientationslightlyoﬀ

the[100]Alzoneaxis.ChemicalelementswereidentiﬁedwithaCM20

FEGTEM/STEMmicroscope,operatingat200kVandequippedwithan

energydispersiveX rayspectroscopy(QuantaxEDSsystemwithsilicon

driftdetectorfromBruker).ForEDSmapping,anareaof235×88pix

elswas scannedwithaprobesizeofabout5nmandahighstatistic

(20000cps).

HAADFscanningTEMmicrographsofas receivedandheat treated

sampleswereacquiredusing aJEOLcold FEGJEM ARM200Finstru

mentoperatedat200kV(energyresolution,0.3eV)andequippedwith

aprobeCscorrectorwithaspatialresolutionof0.078nm.Toidentify thephasesandchemistry,energyelectronlossspectroscopy(EELS)spec

trawereacquiredusingaGIFQuantumERimagingﬁlter,between850

and1850eVinorder toincludetheCu L_2,3 (931 951eV),theMg K

(1305eV),theSi K(1839eV)andtheAl K(1560eV)edges.EELSwas

performed onlinescansacrossprecipitatesaswellason someﬁxed

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Fig. 1. DSCcurvesofA-U4Gand2017A,as-receivedstate.Theheatingrateis 20°C/min.2017Acurveisshiftedupwardforabettervisualisation.

DSCexperiments were alsocarried out.For this,sampleswith a

thicknessof1mmandaweightofabout20mgwereprepared.They

weresubjectedtorampheatingbetween−10°Cand550°Cat20°C/min inaMettler ToledoDSC3+.Resultswerecorrectedforbaselineandnor malizedforthesampleweight.

SAXSmeasurementswereperformedonanXeussequipmentpro

posedbyXenocs.TheX raysourceisequippedwithaCu anodeand

theSAXSsetupisequippedwithanti scatteringslitsand6MPilatusde

tector.Thebeamsizeis1mm2_._For_SAXS_experiments,_samples_were

thinneddownto100μm.Inordertoinvestigatethelargeq rangeavail able,3sample to detectordistanceswereconsidered(40cm,1.2mand 2.5m).Inaddition,afixeddetectorcollectedtheintensityscatteredat wideangles,andallowedtoobtainthefirstdiffractionpeaksofthema

terial.Transmissionandbackgroundcorrectionsweremadeusingstan

dardprotocols.Thequantitativeanalysiswascarriedoutaccordingto theapproachdescribedinapreviousarticle[19].

3. Results

3.1. Asreceivedstate

DSCresultsforA U4Gand2017Ainas receivedstatearepresented

inFig.1.Sixdifferentregionscanbeidentified.Peaksaredirectlyre latedtoprecipitatesnucleationandgrowthinthealloy.Thefirstregion (I),fromroomtemperatureupto150°Cisflat.Itisgenerallyassumed

thatinthistemperaturerange,Guinier Prestonzonesshouldappear.

Here,intheas receivedalloys,GPZaresupposedlyalreadyformed.A

secondregion(II)containsanendothermicpeakbetween150°Cand

180°Candasmallexothermicpeakaround200°C,ascribedtothedis solutionofGPZandtheformationof𝜃’’phase,respectively.Thethird andfourthregions(IIIandIV)exhibittwolargeexothermicpeaks:one

between 220°C and280°C (III)and onebetween 280°C and320°C

(IV).InliteratureonAl Cu Mg Sialloys,thesepeakswereattributed todiﬀerentphases.ItwasﬁrstobservedbySaulnierandSyre[7]that

anexothermic peakat290°Ccoincidedwith thepresenceof the𝜃’

Al2Cuphasewhereasthepeakat350°Cwasmarkedbytheapparition

ofaneedleshapeprecipitation,supposedlyMg2Si.Morerecentworks [20 22]reportapeakaround250°Casthegrowthofthequaternary(Q’ or𝜆’) AlCuMgSiphaseandapeakaround280°Castheapparitionofthe 𝜃’ Al2Cuphase.Theﬁfthregion(V)isasmallexothermicpeakbetween 350°Cand400°C,associatedbysomeauthorstotheincoherent𝜃 Al2Cu phaseprecipitation[22].However,insteadofaprecipitationpeak,this

shouldercouldcorrespondtothestartofthedissolutionofprecipitates presentinthealloy,whichoccursherebetween450°Cand520°C(re gion VI)andthenproceedstofullsolidsolutionatthesolutionizing temperature.

ThemainobservationfromFig.1isthesimilarityinbetween the

A U4Gand2017Abehaviour.Inthemeasurements,theobservedpeaks

areclosetowhatisreportedfortheDuraluminin1952bySaulnierand Syre[7]andforthe2017AalloybyHärteletal.[14].Similarprecip

itationshouldthusbeexpected.PeakVishoweverslightlyshiftedto

highertemperatures(around400°C)for2017AcomparedwithA U4G

(around350°C).Thiscouldbethesignofanearlierstartforprecipitate dissolutionintheA U4Galloy.

Representativebright ﬁeldTEMimagesinconventionalmodeforthe

as receivedsamplesorientedin[001]AlzoneaxisareshowninFig.2(a)

and(b).Intheas receivedsamples,onlydispersoids(AlMnSi)canbe

observedinbrightﬁeldmode:theyarelargerinsize(125nm±51nmin

average)in2017AthanintheA U4G(57nm±25nminaverage),both

presentingalargestandarddeviation.Diﬀractionpatterns(notshownin Fig.2)donotpresentanyvisiblespotthatcanbeattributedtocoherent precipitation.

AtomicresolvedSTEM HAADFexperimentswereperformedonthe

as receivedalloys,seeFig.3fortheimagesofA U4Gin[001]Alzone axis.Precipitationinthematrixwashardlydetectable.InHAADF mode, seeFig.3(b)and(d),theintensityisproportionaltoZ1.5–1.8_,_Z_being

theatomicnumber[23].Brightdots/areascorrespondtotheheavier

elementandarethusrepresentativeofcopperatoms.TheEDSanalysis

conﬁrmedthepresenceofcopperinhigherconcentrationinthesezones

comparedwiththematrix.Thus,Cu richzones,notverywelldeﬁned

andhavingasizeofafewnanometres,weredetectedhereandtherein thematrix(seeFig.3(a)and(b)).Someclusterswerealsolocatedatthe matrix/dispersoidinterfaces(seeFig.3(c)and(d)).

SAXSmeasurements,performedonA U4Gand2017Aalloysinthe

as receivedstate,areshowninFig.4.TheSAXSdataobservedonboth

specimensshowaverysimilarbehaviourthatcanbedescribedbytwo

maincontributions.Inthelowqrange(0.01<q<0.1Å−1_),_a_clear q−4_contribution_is_observed,_which_can_be_attributed_to_large_precip itatessuchasthedispersoids.Intherange0.1<q<1Å−1_,_a_plateau isobserved,followedbyanintensitydecreaseasqincreases.Wehave modelledthiscontributionbyadistributionofsphericalprecipitatesof uniformelectrondensity(i.e.,composition)withapolydispersitygiven bytheSchultzdistribution(see[24]forthedetailedexpressionofthis

contribution). Aleast square ﬁtof thedatagives meandiametersof

about8Å,whichisconsistentwiththeexistenceofGPZorsmallclus tersinthealloymatrix.Theexactnatureoftheclustersisatthisstage

notdetermined.

3.2. Artiﬁcialagingtreatmentat180°Candidentiﬁcationofprecipitates Fig.5presentstheVickershardnessasafunctionofagingtimeforthe

twostudiedalloys:A U4Gand2017A.OnlyRPhardnesswasplottedas

measurementsonotherplanesgavesimilarresults.TheA U4Gisharder

thanthemodernalloy2017Abyabout10 HVthroughouttheaging

curveexceptforlongover agingtimeswheretheA U4Ghardnessdrops

faster.Inbothcases,theconditionscorrespondingtothemaximumpeak hardnesswerereachedafter8hat180°C.Thesepeakageingconditions wereselectedfortheinvestigationofthemicrostructure.

Fig.6showstheSAXSmeasurementsafteragingfor8hat180°C.

Onceagain,bothalloysbehavesimilarly.Fromthesemeasurements,it

isseenthatthesignal,intherangeof0.1<q<1Å−1_,_disappeared_in favourofasignalintherange0.04<q<0.2Å−1_,_which_translates_into thedisappearanceofsmallclustersandtheformationoflargerparticles. Thiscontributiontothescatteredintensitycanbereproducedbyusing

aﬂatcylindermodel.Dimensionsofprecipitatesare:4nmthick and

70nmlongforA U4Gand3nmthickand40nmlongfor2017A.

Themicrostructurescorresponding tothispeak agedcondition in

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Fig. 2. TEM bright ﬁeld images of as-received al-loys(a)2017A;(b)A-U4Gshowingmainly AlMnSi-dispersoidsinthematrix.

Fig. 3. STEMimagesofas-receivedA-U4Galloyin[001]Al

zoneaxis;(a)and(c)Annularbrightﬁeld;(b)and(d)high angleannulardarkﬁeld.Images(a)and(b)ononesideand(c) and(d)ontheothersidecorrespondtotheexactsamearea. Aclusterisindicatedwiththewhitearrowattheinterface dispersoid/matrix.

Fig. 7: bright ﬁeld images and corresponding selected area electron

diﬀractionpatternsarepresented.Fig.8showstheSTEM EDSmapping

fortheA U4Galloy.

After8hat180°C,averydenseprecipitationisobservedinsidethe matrix(Fig.7)andatgrainboundaries(Fig.8).Inbothalloys(Fig.7(a)

and(c)), precipitateslyingin {100}Al planes areobserved:some as

plateletsandothersasrods,withprecipitationondislocations.Asthe

observedprecipitationissimilarinbothA U4Gand2017Aalloys,ad

vancedanalysesresultsarethenonlyshownforA U4Galloy.

InFig.8,precipitatesatgrainboundaries wereidentiﬁedbyEDS:

theyare(Mg,Si) precipitatesandcopper precipitates.OntheseEDS

maps,dispersoidsofAlMnSinaturearepresent.Aprecipitationinthe matrixisalsovisiblebuthardlyidentiﬁable.

Asamatter of fact,diﬀerent precipitatespopulationsco exist in

sidethegrainsandtheiridentiﬁcationwasmadepossiblebycombin

ingdiﬀerentelectronmicroscopytechniques.FromSADPtakenalong

[001]_Alzoneaxis,showninFig.7(b)and(d),thepresenceof𝜃’ Al2Cu phasewasevidencedthankstothestrikesalong<001>Aldirectionsand faintspots(110)_𝜃’at 1₂ of(220)Al[25].Theycorrespondtothreevari

antsofplate shapesemi coherentprecipitatesgrowinginthe{100}Al planes:oneface onandtwoedge on.The𝜃’ Al2Cuprecipitatenature

wasconﬁrmedthankstoSTEM HAADFasshowninFig.9(b).Similarly

toHAADFimagestakenonas receivedstates,brightdotscorrespondto

copper richatomiccolumns.Seenedge on,thecrystalstructureofthis

plateletisbody centredtetragonalaspreviouslyshownbyBourgeois

etal.[26]andShenetal.[27].Thepresenceofpre𝜃’−1(Al₂Cu)pre cipitatescoherentonthe{100}Alplanes,wasalsorevealedwithSTEM HAADF,asshowninFig.9(c)):thesetypesofprecipitateswererecently identiﬁedbyLiuetal.[28].Theyconsistofthreeparallelcopperplanes,

eachseparatedbytwoaluminiumplanes.OnSADP,diﬀractionpatterns

ofthepre𝜃’−1coincidewithdiffractionpatternsof𝜃’ Al2Cu.Bothtypes arethus,difficulttodifferentiate.Atpeak agedconditions,thisphase, whichcorrespondstothefirststageof𝜃’ Al2Cuformation,isnotpre dominant:mostofthepopulationhasevolvedtothe𝜃’ Al2Cuphase.

OtherprecipitatesintheshapeofrodswereobservedintheA U4G

alloyafter8hat180°C,asshowninthebrightﬁeldimagesofFig.7(a) andintheHAADFimageofFig.9(a).Therods,seenend on,areoriented along<001>Aldirections.Noclearidentiﬁcationbeingpossiblewith

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Fig. 4. SAXScurvesfor2017AandA-U4Gintheas-receivedstate.Themodel wassuperimposedtotheexperimentalcurves.

Fig. 5. Hardnessmeasurementonrollingplaneforold(A-U4G)andmodern (2017A)alloysheattreatedat180°C.

SADP,thisphasewasidentiﬁedthankstoSTEM EELS.Rodsappeared

tobetheQ AlCuMgSiphaseoroneofitsprecursors(Q’).Thecomposi tionfoundforthisphasebySTEM EELSwasinaverage73±10Al 10±4 Cu 12±6Mg 5±5Siat%.Aluminiumcontentforthisphaseisoveresti

matedaspartofthematrixisalsoprobed.Moreover,asobservedon

theSTEM HAADFimage(Fig.10(b))andconﬁrmedbySTEM EELS,the

precipitateshaveacore shellstructurewithacorerichinsiliconand

magnesiumandacopper richshell.Thiscore shellstructureobserved

herecoincideswithresultsreportedbyMatsudaetal.in2007[29]and Biswasetal.in2014[21].

InFig.9(b),oneofthequaternaryprecipitateisseenedge on,co precipitatedwitha𝜃’ Al2Cuprecipitate.Inthisorientation,bySTEM

HAADF,theQ’(Q) AlCuMgsiphase canbe recognizedbytheCu rich

columns (highintensity due tohighatomic number) surrounded by

lowerintensitycolumns(Si,AlandMg)similartothecross sectionstruc tureviewedalong<100>AlzoneaxisshowninDingetal.[30].

Eventually,athirdphase is present,detectedthankstothespots

at1/3and2/3ofthe[022]AlorientationinSADP(Fig.7(b)and(d)). STEM HAADFintwoorientations[110]Aland[112]Al(seeFig.11)was necessarytoidentifythisphaseastheΩAl₂Cu.Thisphaseisknownto

haveanorthorhombicstructure,spacegroupFmmmwitha=0.496nm;

b=0.859nmandc=0.848nmandtoprecipitateinthe{111}Alplanes

[31].MeasurementsofCu CudistancesonSTEM HAADFimagesalong

Fig. 6. SAXScurvesfor2017AandA-U4Gatpeak-agingstate:after8hat 180°C.Themodelwassuperimposedtotheexperimentalcurves.

[001]Ω,[100]Ωand[010]Ωcorrelatewiththeexpectedlatticeparame ters.Theseprecipitatesgrowpreferentiallyondispersoids(AlMnSi).

Alltheobservationsprovidedaclearevidenceonthephasespresent

uponartiﬁcialaging: atgrain boundaries,copperandMg Si precipi

tateswereformedwhileinthematrix,𝜃’ Al2Cu,Q’(Q) AlCuMgSiand

ΩAl2Cuwereclearlyidentiﬁed.

4. Discussion 4.1. Hardnessevolution

ThehigherhardnessofA U4Gcomparedto2017Aintheas received

state(about10HVdiﬀerence)couldbeexplainedbydiﬀerentfactors.

Whenconsidering precipitationat nanoscale,SAXS experiments pro

videdevidenceofthepresenceofclustersinas receivedstatewhether

inA U4Gorin2017A.DSCconﬁrmedthepresenceofthispopulation

intheas receivedstate,asadissolutioneventoccurredaround180°C. These clusters,withadiameterin therangeof8Å,aretheprincipal

causeofhardnessinDuralumin.However,itwasshownthatthenano

precipitationisidenticalinbothalloys,whichrulesoutthisfactoras theprimarycauseinthehardnessdiﬀerence.Secondly,dispersoidscon tributepartlytohardnessbycontrollingthegrain size.Itwasshown

thattheyaresmallerandmorenumerousinA U4Gthanin2017A.This

couldbeanexplanationforthediﬀerenceinhardness.However,since

thegrainsizeiscloseinbothalloys(seeFig.S2inSupplementaryin

formation), thecontributionofdispersoidsshould notbe signiﬁcant.

Thethirdexplanationcomesfromtheplates’conditions.Theoldplate

wasbentwhencollected.Thisactiongeneratedstrainhardening,with

ahigherdensityofdislocations(seeFig.S3inSupplementaryinforma tion).Thepresenceofthesedislocationsisbelievedtobethemainfactor inthehigherhardnessoftheA U4Galloyintheas receivedstate.

Forover ageingconditions,thehardnessoftheA U4Galloydrops

fasteruntilreachingthehardnessofthemodernalloy.Itcanbehypoth esized thatafterartiﬁcialageing,thehistoricalalloylosespartofits additionaldislocationdensitythroughfasterrecovery.Theﬁnalpartof thehardnesscurvecouldalsobeexplainedbethefactthatalloysage fasterwhenadditionaldislocationsareintroduced,becausethedisloca

tionnetworktransportssoluteatomsthroughpipediﬀusion[32]and

thusacceleratesgrowthandcoarseningof𝜃’precipitates,knowntonu cleateondislocations[33].Thisgrowthandcoarseningwillinducea decreaseinhardness.

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Fig. 7. TEM bright-ﬁeld images andcorresponding SADPin[001]Al zoneaxisaftertreatmentat180°C–

8hfor2017A:(a)and(b) andforA-U4G:(c) and (d).(110)𝜃’spotsandstrikesalong[001]Aldirections

forthe𝜃’phaseareindicatedwitharrows.Spotsat 1/3and2/3of(022)AlcorrespondingtoΩphaseare

surrounded.

Fig. 8. STEM-EDSmappingaftertreatmentat180°C–8hinA-U4Gshowingprecipitationatgrainboundaries:MgSiandCuaswellasinthematrix.

4.2. Nano structureintheas receivedstate

About thenano clusters presentin theas received states, several

pointscanbemade.Thecopper richclustersobservedbyatomically

resolvedTEMimagesinHAADFmodewerenotsuﬃcientlynumerous

tocorrelatewiththemeasuredSAXSsignal.Electronmicroscopyisthus

notfullysucceedinginobserving andanalysingnanoclusters.Onan

otherhand,atthisstage,ourexperimentscannotdiﬀerentiatebetween GPZ(ﬂatclustersgrowingintheplanesparalleltothe<100>Alaxes)

anddisorderedclusters.GPZareusuallyfoundinthemodelAl 4%Cu

alloyagedatroom temperature,asitwasﬁrstlyprovedbyGuinierand

PrestonbyXRDin1938.Lambot[11],whostudiedbothAl 4%Cuand

DuraluminbyX rayscattering,provedthatthescatteringsignalwasper fectlyidenticalinbothalloys,concludingthusthatGPzonesarepresent inDuraluminagedatroomtemperature.Itistobenotedthattheirstud ieswerecarriedoutonsinglecrystals.Inthepresentcase,SAXSsignal

isanaverageofthemeasurementsmadeonseveralgrains:theinfor

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Fig. 9. (a)STEMBright-ﬁeldimageofA-U4G alloy in [001]Al zone axisshowing𝜃’-Al2Cu

(blackarrows)andQ-AlCuMgSirods(white ar-rows),(b)HAADFimagesof𝜃’-Al2Cuplatelets

seenedge-onwithco-precipitation of Q’(Q)-AlCuMgSiand(c)pre-𝜃’−1Al2Cudisksseen

edge-on.

Fig. 10. (a)STEMBright-ﬁeldimageofA-U4G alloyin[001]Alzoneaxis(b)STEM-HAADF

im-ageofQ-AlCuMgSirodsseentop-on.

Furtherinvestigationisrequiredtoconﬁrmthestructuralnatureofthese clusters.

4.3. Uponartiﬁcialaging identiﬁcationofprecipitation

Uponartiﬁcialaging(at180°C for8h),itwasdemonstratedthat

threepopulationsofprecipitatescoexistinthematrix:𝜃’ Al2Cu,Q’(Q)

AlCuMgSiandΩAl2Cu.LinkingbacktheTEMobservationswithDSC

resultsandthankstoresultsreportedintheliteratureforAl Cu Mg

Sialloys,itcanbe hypothesizedthatregionIII(between220°Cand

280°C)isrelatedtothegrowthofQ’(Q) AlCuMgSiphaseandregionIV (between280°Cand320°C)tothegrowthofthe𝜃’ Al2Cuphase.Asfor theΩAl2Cuphase,havingthesamestoichiometryasthe𝜃’ Al2Cuphase andifitsgrowthiscontrolledbydiﬀusion,therecouldbesuperimposi tionofpeaksintheDSCcurve.

The𝜃’ Al2Cuphase,foundinlargequantity,isnotsurprising.Itcon

stitutesthemainhardeningphasein the2xxxfamilyalloysandwas

reportedinallstudiesonDuraluminor2017Aalloy[7,11,13].

The presence of the Q AlCuMgSi phase or one of his precursors

(Q’)wasevidenced,inbotholdandmodernalloys.Althoughnotsys

tematicallyrevealedinprevious studiesof Duralumin,theseﬁndings

conﬁrmtheequilibriumdiagramsdatareportedbyMondolfoin1943

[12]whichpredictsthatwithMg:Siratio<1.73,hardeningconstituents shouldleadtotheAl2CuandAl4CuMg5Si4equilibriumphases.Thesil

icon contentisindeedhigherthanthatrequiredfortheformationof

Mg₂Si(Mg:Si=1.08):theexcessof silicon willcombinewithcopper

andmagnesiumtoformthequaternaryphase.Theglobalstoichiomet

riccompositionfortheQphasehasbeen,however,acontroversialmat ter.Mondolfopredictstwopossiblequaternaryphases:Al₄CuMg₅Si₄or Al4CuMg4Si4.Inmorerecentliterature,mainlyforalloysofthe6xxx family(Al Mg Siwithadditionsofcopper)[15],butalsofor2014al loys[20],thestoichiometryreportedinbulkalloyswereAl5Cu2Mg8Si6, Al₄Cu₂Mg₈Si₇, Al₃Cu₂Mg₉Si₇ etc.However, the nanometre scale of theseprecipitatesmakesthequantitativeanalyseschallenging.Biswas

et al.[21]usedatomprobe analysisandprovedthat theQprecipi

tatesevolved,uponaging,fromacopper richcomposition(44Al 22Cu

16Mg 16,5Siat%)toamagnesiumandsilicon richcomposition(28Al

9Cu 37Mg 26Siat%.)after4hat260°C.Inaveryrecentpaper,Ding

etal.[30]wereabletoprovide,bycombiningatomprobetomography

andatomiccolumnintensityquantiﬁcationinSTEM HAADFimages,the

reasonwhyvaryingcompositionwerereportedinpastwork:itwasin

deedprovedtobeduetotheoccupancychangeoftheatomiccolumns.

ThestoichiometryoftheQphaseanditsprecursor,theQ’phasewere

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Fig. 11. STEMBright-ﬁeldimagesandHAADF imagesofA-U4Galloyin(a)and(b):[110]Al

zoneaxis;(c)and(d):[1̄12]Alzoneaxis

show-ingΩ-Al2Cuplatelets(whitearrows)growing

ondispersoids.

matrixeﬀectscannotbeexcludedwithEELS,theelementalcomposi

tionfoundherebyEELSinDuralumin(73±10Al 10±4Cu 12±6Mg

5±5Siat%)isinallcasesmuchleanerinmagnesiumandsiliconthan whatisusuallyreported.Thisresultsshouldbecorroboratedwithother

techniquestoconﬁrmthetendency.Theheterogeneousnucleationof

𝜃’ Al2CuphaseandtheQ’(Q) AlCuMgSiphase,observed inFig.9(b), wasalsoreportedbyotherauthors[21,34].Itcouldbecausedbythe strainﬁeldofQ phaseprecipitatesonwhichthe𝜃’ Al2Cunucleate.

Anotherphase,ΩAl₂Cu,ishereobserved.TheΩphaseisusually

foundinhighdensityinAl Cu Mg Agquaternaryalloyswithahigh

Cu:Mgratio[25,31].Itpresents oneofthehigheststrengtheningpo

tentialin aluminiumalloys. Inparticular, Al Cu Mg Agalloys have

beendevelopedforelevated temperatureaerospaceindustryapplica

tions[35]duetotheenhancedthermalstabilityoftheΩphase.Some

authorsprovedthatsilverwasnotrequiredfortheformationofΩpre cipitatesalthoughitspresenceincreasestheirdensity[36].TheΩphase

wasindeedobservedin2024and2124alloysbyWangetal.[37]and

wasactuallyfoundin2017Aalloyaswell[14].In2024and2014al

loys,theΩphasewasfoundtoformpreferentiallyonmanganese based

dispersoids.Thisheterogeneousgrowthisclearlyevidencedinourre

portedresults.TounderstandwhysuchaphaseappearedinDuralumin,

itisinterestingtodiscussitsnucleationprocess.InAl Cu Mg Agalloys,

theprocesshasbeenwidelyinvestigated:uponaging,magnesiumand

silverco cluster,thencopperatomsaggregatetothe{111}Al planes. Duringcoarsening,whencopperconcentrationbecomesclosetoAl₂Cu, silverandmagnesiumstarttomigratefromtheinteriortotheinterface oftheprecipitate.Theinterfacialstructurewasprovedtobeadouble

layercomposedofAginhexagonalstructureandMgandCubelowthe

centreofthehexagon[38].Magnesiumisconsiderednecessaryforthe

nucleationoftheΩphase.Asforsilver,itsroleistoacceleratetheaggre gationofmagnesiumatomsbyretainingvacanciesandtoassistinthis waythemagnesiumdiﬀusion[39].Siliconisalsoanelementthatinﬂu encesthepresenceofΩphase.FromPolmearetal.[35],silicon(and iron)shouldremainlow(lessthan0.10wt%)tominimisetheformation

oflowmeltingpointeutecticsatgrainboundariesandtomaximiseΩ

precipitation.Infact,highlevelsofsiliconinterferewithprecipitationof Ωbecausesiliconinteractswithmagnesiumandthusreducestheforma tionMg Agco clusters,whichgenerallyfacilitatenucleationofΩ.This wasalsoobservedintheworkbyÜnluetal.[40].However,theauthors showedthatitisrathertheMg:Siratiothathadtobeconsidered:some

Ωprecipitationwasfoundinalloyswithsiliconlevelsashighas0.5

and0.65wt%withcorrespondingMg:Siratiosof3.0and2.23.Inthe

presentalloys,theobservationthatΩformsprincipallyondispersoids

intheDuraluminand2017Acanbeexplainedbythepresenceofcop

perclustersatthematrix/dispersoidinterfaceintheas receivedstate.

Moreover,itdoesnotformasaphasedispersedinthematrixbecause

oftheabsenceortheverylowdensityofmagnesiumclustersinthema trix.Themagnesiumclustersareinthiscaseimpededbysilicon,which ratherfacilitatestheformationoftheQ AlCuMgSiphase.

Conclusion

AnoldDuralumin(A U4G)anditsmodernequivalent(2017A)were

studiedwithdiﬀerentcharacterizationtechniquestoclarifytheprecip itationprocesses.Themainﬁndingsare:

TheprecipitationsequenceinDuraluminandmodern2017Aupon

artiﬁcialaging issimilar. Itconsistsin 𝜃’ Al2Cu,Q’(Q) AlCuMgSi andΩAl₂Cuinthematrix.Atgrainboundaries,variousprecipitates

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arefound:copper basedand(Mg,Si)precipitateswithunknownsto ichiometry.

TheQ AlCuMgSiphaseoroneofitsprecursors(Q’)isobservedin

thematrixbutthe𝛽 Mg2Siphase,previouslyreportedinDuralumin, isnotpresent.Thisphasefacilitatesthenucleationofthe𝜃’ Al2Cu phasethroughstrainﬁeld.

TheΩphaseisobservedanditgrowspreferentiallyondispersoids.

Itpresumablynucleatesdue tothepresenceof copperclustersat

thematrix/dispersoidinterface.Thisphaseisaminorphasedueto theabsenceorrareoccurrenceofmagnesiumclustersinthematrix.

ThenucleationmechanismsfortheΩphaseondispersoidsinthese

Al Cu Mg Sialloysremainstobeevidenced.

Intheas receivedstate(naturallyaged),thepresenceofclustersis evidenced,withcopperasthemainelementbutthestructuralnature oftheseclustersisyettobedetermined.

Despitesomediﬀerencesinthemicrostructure,suchasthedisper

soids’sizeanddensity,Duraluminexhibitsasimilarnanoprecipita tionasitsmodernequivalentinthenaturallyagedconditions,andit behavesalsosimilarlyuponartiﬁcialaging.Theseobservationslead

totheconclusionthattheconditionsexperiencedbythecollected

plate(10yearsofserviceplus50yearsatambienttemperature)had

nosigniﬁcantimpactonthenanostructure.

Overall,theseresultsconstituteabaseforassessingtheverylong termagingofaluminiumalloysinaeronautics.

DeclarationofCompetingInterest

None

Acknowledgments

TheauthorswhichtothankPierreRoblinfromFRFERMAT,Uni

versité deToulouse,CNRS,Toulouse,France,forSAXSmeasurements,

TeresaHungria Hernandez,fromRaimondCastaingMicroanalysisCen

tre,ToulouseforSTEM(HAADF,EDSandEELS)experimentsandthe

volunteersofLesAilesAnciennesToulouseforprovidinguswiththeplate collectedontheBreguetSahara765aircraft.

ThisstudyhasbeenpartiallysupportedthroughthegrantNanoXn°

ANR 17 EURE 0009intheframeworkofthe“ProgrammedesInvestisse

mentsd’Avenir”.

Supplementarymaterial

Supplementarymaterialassociatedwiththisarticlecanbefound,in theonlineversion,atdoi:10.1016/j.mtla.2019.100429.

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