phase characterisation and mechanical behaviour of fe b modified cu zn al shape memory alloys

6 which presents the BSE images ofthe boron modified Cu–Zn–Al alloy, is generally characterised by large grain size and a change in grain edge morphol-ogy from sharp to round/rod-like tha

w w w j m r t c o m b r Availableonlineatwww.sciencedirect.com

Original Article

Phase characterisation and mechanical behaviour

of Fe–B modified Cu–Zn–Al shape memory alloys

Kenneth Kanayo Alanemea, ∗, Eloho Anita Okotetea, Nthabiseng Maledib

a r t i c l e i n f o

Received18June2016

Accepted13October2016

Availableonlinexxx

Keywords:

Cu–Zn–Alalloy

Shapememoryeffect

Lathmartensite

Micro-alloying

Mechanicalbehaviour

Phaseanalysis

a b s t r a c t

Themicrostructures,phasecharacteristicsandmechanicalbehaviourofCu–Zn–Alalloys modified with Fe, B, and Fe–B mixed micro-alloying additions has been investigated Cu–Zn–Alalloyswereproducedbycastingwithandwithouttheadditionofthe microele-ments(Fe,BandFe–B).The alloysweresubjectedtoahomogenisation–coldrolling– annealingtreatmentschedule,beforethealloysweremachinedtospecificationsfortensile test,fracturetoughness,andhardnessmeasurement.Optical,scanningelectronmicroscopy andX-raydiffractionanalysiswereutilisedformicrostructuralandphasecharacterisation

ofthealloys.Adistinctdifferenceingrainmorphologywasobservedinthealloysproduced –theunmodifiedalloyhadpredominantlyneedle-likelathmartensitestructurewithsharp grainedgeswhilesignificantlylargertransversegrainsizeandcurveedged/nearelliptical grainshapewasobservedforthemodifiedCu–Zn–Alalloys.Cu–Znwithfccstructurewas thepredominantphaseidentifiedinthealloyswhileCu–Alwithbccstructurewasthe sec-ondaryphaseobserved.ThehardnessoftheunmodifiedCu–Zn–Alalloywashigherthan thatofthemodifiedalloyswithreductionsinhardnessrangingbetween32.4and51.5% However,thetensilestrengthwassignificantlylowerthanthatofthemodifiedalloygrades (28.37–52.74%increaseintensilestrengthwasachievedwiththeadditionofmicro-alloying elements).Similarly,thepercentelongationandfracturetoughness(10–23%increase)of themodifiedalloywashigherthanthatoftheunmodifiedalloygrade.Themodifiedalloy compositionsmostlyexhibitedfracturefeaturesindicativeofafibrousmicro-mechanism

tocrackinitiationandpropagation,characterisedbytheprevalenceofdimpledrupture

©2016BrazilianMetallurgical,MaterialsandMiningAssociation.PublishedbyElsevier EditoraLtda.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http://

creativecommons.org/licenses/by-nc-nd/4.0/)

Theattractivepropertiesofshapememoryalloys(SMAs)have

been explored fora wide range of applicationssuch as in

E-mail:kalanemek@yahoo.co.uk(K.K.Alaneme)

sensing, medical,commercial and other industrial applica-tions [1–3] The current limitation to the of SMAs is that commerciallyavailableSMAsareprincipallymadeoutof

Ni-Tialloyswhichareexpensiveandalsohaveahugeprocessing costandfacilityburden[4].Thishasmadethesearchformore

http://dx.doi.org/10.1016/j.jmrt.2016.10.003

2238-7854/©2016BrazilianMetallurgical,MaterialsandMiningAssociation.PublishedbyElsevierEditoraLtda.Thisisanopenaccess articleundertheCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/)

cost-effectiveSMAsanongoing pursuitbyresearchers

Cu-basedandFe-basedSMAshavebeenidentifiedasthemost

promising low cost alternatives to Ni-Ti alloys The lower

strainrecoveriesofCu(∼5%)andFe(<5%)comparedtothatof

NiTialloy(8%)iscompensatedforbylowerprocessingcostand

amenabilitytofabricationroutesandfacilitiesutilisedfor

con-ventionalmetalprocessing[5–7].Cu-basedSMAsareofspecial

interestbecauseofitsmodestrecoverablestrain(∼5%)which

isclosesttothatofNiTialloy,cheaperproductioncost,and

easeoffabrication,andhighthermalandelectrical

conduc-tivity[8–10].Regrettably,theseCu-basedalloysareassociated

withbrittlenessinducedbycoarsegrainstructure,highelastic

anisotropyandhighdegreeoforderwhichreducesthecold

workabilityofthe alloys[10].Furthermore,Cu-basedalloys

aresusceptibletoageingwhichcouldleadtophase

(martens-ite)stabilisationandchangeintransformationtemperatures

whichaffectsitsshapememorycapacity[3,11,12]

Cu–Zn–AlalloysarethemoststudiedCu-basedalloysand

havebeen exploredforuse inapplications suchas

fasten-ers,springs,couplingsandthermalactuatorswheretheuse

ofother alloyswouldbetechnically andeconomically

inef-fective[13,14].Cu–Zn–Alalloyshaverelativelyhigherstrain

recoverycomparedtootherCu-basedalloys.However,

simi-lartootherCu-basedalloygrades,Cu–Zn–Alalloysareprone

tobrittlefailure,andhavelowfractureandfatiguestrength,

andlowworkability[15].Grainrefinementhasbeenproffered

asatechniquefortheimprovementofmechanicalandshape

memorypropertiesofCu-basedSMAs[3,15].Inthisregard,the

useofmicro-alloyingelements[16],mechanicaland

thermo-mechanicalprocessing[17–19];havebeenexploredtoachieve

grainrefinementinCu-basedSMAs.Theuseofmicro-alloying

elementssuchasB,Ti,V,Ce,Fe,Co,Be,Zrhavebeenthemost

reportedoftheseapproaches;andmanyoftheinvestigations

showthattheirpresencehasinfluenceonthemicrostructure,

transformation and shape memory properties of Cu-based

SMAs[5,16,20].BandFearetheleastexpensiveofthese

micro-alloyingelementsandhavebeenreportedtobeveryefficient

grain modifiers [5,21] However there have been divergent

opinions on the optimum concentrations of these

micro-alloying elements and the mechanisms for effecting grain

modificationinCu-basedSMAsingeneralandCu–Zn–AlSMAs

inparticular.Forinstance,Wangetal.[22]reportedthat0.01%

Bwastheoptimumconcentrationrequiredtoachievegrain

refinementinCu–Zn–AlalloybutFerreiraetal.[23]statedthat

Bconcentrationwithintherangeof0.05–0.08%wassufficient

foreffectivegrainrefinement.Feconcentrationforeffective

grainmodificationandenhancedmechanicalpropertieshas

alsobeensubjectofresearchdiscussrecently[24].Thepresent

work,investigatesthephasecharacteristicsand

microstruc-tureofFe,B,andFe–BmodifiedCu–Zn–AlSMAsindetailsand

itseffectonthemechanicalbehaviour

2.1 Alloy and sample preparation

TheCu–18Zn–6Albasedalloyswereproducedinaccordance

withAlaneme[25].Chargecalculationwasusedtodetermine

theamountofCu,ZnandAlrequiredtoproduceCu–18Zn–6Al

base composition containing two different weight percent each (0.05wt.% and 0.1wt.%) ofFe and B.Two other com-positionscontaining mixturesofFeandB(0.05wt.% Band 0.05wt.% Fe)(0.025wt.% Fe and 0.075wt.% B) were equally produced.TheFeandBweightpercentselectedarewithin the range reported in literature [22–24] The charge for Cu–18Zn–6Alalloywithouttheadditionofmicro-alloying ele-ments was also preparedas the control composition The alloys werepreparedvialiquid metallurgyroute,meltedin

acruciblefurnaceandcastintosandmouldsinsertedwith metallic chills.The chemicalcomposition of the Cu–Zn–Al shape memory alloys determined by EDS analysis is pre-sentedinTable1.Aftercasting,thealloysweresubjectedto homogenisationtreatmentat800◦Cfor4handcooledinairin accordancewithAlaneme[25].Thealloyswerethensubjected

to10% coldrollingusingaminiature coldrolling machine; afterwhichthesampleswereannealedat450◦Cfor1hand cooledinairtoremoveinternalstressesdevelopedinthe sam-ples duringthe colddeformationprocess.Thesamplesfor mechanicaltest,microstructuralandphasecharacterisation weremachinedtoprescribedtestspecifications.Thesamples werefinallyannealedat400◦Cfor2hthenwaterquenchedat roomtemperaturetoeliminatestressesgeneratedduringthe machiningprocess

2.2 Microstructural characterisation

2.2.1 Optical microscopy

AZeissopticalmicroscopewasusedformicrostructural inves-tigationofthealloysproducedtoassessthegrainstructure and phase distribution Thespecimens formicrostructural examination were metallographically prepared following a series ofgrinding and polishing process.Subsequently the specimens were etched in a solution of5gferric chloride,

10mlHCl,and95mlethanol,swabbingfor10–20s;afterwhich microstructuralanalysiswasperformed

2.2.2 Scanning electron microscopy (SEM)

Detailedmicrostructuralcharacterisationandcompositional analysiswas carriedout usingaZeisshigh resolutionfield emission gun scanning electron microscope (Zeiss FESEM)

to complement theoptical microscopy analysis.Back scat-tered electron(BSE) mode,secondary electronimagingand energy dispersive spectroscopy (EDS) were used for micro-structuralandfractographicanalysisoftheCu–Zn–Alalloys produced

Table 1 – Chemical composition of the Cu–Zn–Al alloys.

2.3 X-ray diffraction analysis

The analysis of the crystalline phases present and their

intensity was carried out on the Cu–Zn–Al alloys using

X-ray diffractometry (XRD) The samples were prepared for

XRD analysisfollowing standard procedures.APANalytical

EmpyreandiffractometerwithPIXceldetectorandFefiltered

Co-K␣assourceofradiation,wasusedfortheanalysis.The

analysiswasperformedfromangle2spectralrangeof0◦to

120◦.ThephaseswereidentifiedusingX’PertHighscoreplus

software

2.4 Mechanical properties

2.4.1 Hardness measurement

Thehardnessvaluesofthealloyswereevaluatedona

hard-nesstestingmachineusingaDigitalRockwellhardnesstester

adoptinga HRA(60kgf) scale.Thesamplepreparationand

testingprocedurewasperformedinaccordance withASTM

E18-16 standard [26] Five hardnessindentswere madeon

eachspecimenandreadingswithinthemarginof±2%were

takenforthecomputationoftheaveragehardnessvaluesof

thespecimens

2.4.2 Tensile testing

Thetensilepropertiesoftheproducedalloyswereevaluated

bytensile testing usingauniversaltesting machine

Speci-mensforthetestweremachinedtotensiletestspecifications

of5mmdiameterand30mmgaugelength.Thespecimens

weremountedonthetestingplatformandpulled

monoton-icallyatastrainrateof10−3/suntilfracture.Thespecimen

preparationand testingprocedurewere inaccordance with

ASTME8/E8M-15astandard[27].Foreachalloycomposition,

threerepeattensile tests wereperformed toguaranteethe

reliabilityofthedatagenerated.Thetensileproperties

eval-uatedfromthetestare theultimatetensilestrengthand%

elongation

2.4.3 Fracture toughness

Thefracturetoughnessofthealloyswasdeterminedusing

cir-cumferentialnotchtensile(CNT)testinginaccordancewith

Alaneme[28]Thespecimensforthetestweremachinedto

gaugelengthof27mm, gaugediameterof6mm(D),notch

diameterof4.2mm(d),andnotchangleof60◦.Thespecimens

werethensubjectedtotensileloadingtofractureatroom

tem-peratureusinganInstronuniversaltestingmachineoperated

ataquasi-staticstrainrate of10−3/s.Thefractureload(P f)

obtainedfromtheCNTspecimens’load–extensionplotswas

usedtoevaluatethefracturetoughnessusingtheempirical

relation[29]:

K1C= Pf

D3/2



1.72

d



−1.27



(2.1)

whereDanddarethespecimendiameterandthediameterof

thenotchedsectionrespectively.Planestrainconditionsand,

byextension, the validityofthe fracture toughness values

Fig 1 – Optical micrograph of the unmodified Cu–Zn–Al alloy.

obtainedwasdeterminedusingtherelationsinaccordance withNathandDas[30]:

D≥K

1C

y

2

(2.2)

ThreerepeattestswereperformedforeachCu–Zn–Alalloy compositiontoensurerepeatabilityandreliabilityofthedata generated

3.1 Microstructures of the Cu–Zn–Al alloys

3.1.1 Optical microscopy

Opticalmicrographsoftheunmodifiedandmodifiedalloysare presentedinFigs.1–4.Fig.1showstheopticalmicrographof theunmodifiedalloy,characterisedbyneedle-likelath struc-turewhichwasacknowledgedtobelathmartensitestructure ThelathstructureobservedinFig.1issimilartothelath mar-tensitestructuresinCu–Zn–Alalloysreportedintheworksby

Xu[31]andDasgupta[32].ThealloyswithBadditions(Fig.2) wereobservedtohaveundergoneachangeinthe martens-itemorphologyfromsharpedgedneedle-likelathstructureto curveedgedrodlikestructurewiththegrainsrelativelylarger than theunmodifiedCu–Zn–Alalloy(Fig.1 Therealsoisa slightincreaseingrainsizewithincreaseinBcomposition (from0.05to0.1wt.%)ifFig.2(b)iscomparedwithFig.2(a) Thisimpliesthatnucleationandgrowthconditioninthealloy

isinfluenced bytheBconcentration TheadditionofFeto theCu–Zn–Alalloys(Fig.3)showastructurewithnear ellip-ticalphasemorphologyandsignificantlyreducedneedle-like martensiteappearance.Thegrainsizedidnotappeartobe sensitivetoFeconcentration(Fig.3 andb)aswiththecaseof

Baddition(Fig.2 Themixed(Fe–B)composition(Fig.4)was observedtohaveacombinationofnearellipticalgrainshape withcurveedgedneedlelikelathmartensitestructure.The observationsmadeareclearindicatorsthatthemicro-alloying elements(Fe,B,andFe–B)actuallymodifythegrainsizeand morphologyofCu–Zn–Alalloys

Fig 2 – Optical micrographs of (a) 0.05B modified Cu–Zn–Al

alloy and (b) 0.1B modified Cu–Zn–Al alloy.

3.1.2 SEM observations

The scanning electron microscopy images of the

unmodi-fiedandmodifiedCu–Zn–Alalloys(Figs.5–8)showstructural

featureswhicharecompletelyinconformitywiththe

observa-tionsmadefromthoseoftheopticalmicrographsofthealloys

(Figs.1–4).Itisobservedfrom Fig.5(a)thatthe unmodified

alloymicrostructureconsistsofpredominantlylath

martens-ite structure Fig 6 which presents the BSE images ofthe

boron modified Cu–Zn–Al alloy, is generally characterised

by large grain size and a change in grain edge

morphol-ogy from sharp to round/rod-like (that is compared with

Fig.5 Table2presentsthechange intransverse lath/grain

thicknessfortheunmodifiedandmodifiedCu–Zn–Alalloys

produced.Table2showsthat thegrainsizeincreases with

B addition from 13␮m for the unmodified composition to

about 28–38␮mforthe Bmodifiedcomposition Alsograin

sizeisconfirmedto besensitivetothe concentrationofB,

as the averagetransverse grain sizeincreased from 28␮m

forthe 0.05wt.%BmodifiedCu–Zn–Alalloy compositionto

38␮mforthe0.1wt.%BmodifiedCu–Zn–Alalloygrade.This

suggeststhat nucleationandgrain growthisinfluenced by

thepresenceandconcentrationofBinthe Cu–Zn–Alalloy

Similar grain morphologies are observed forthe Fe

modi-fied(Fig.7)and the Fe–B modifiedCu–Zn–Al alloys(Fig 8

The change in the grain morphology observed on the Fe

Fig 3 – Optical micrographs of (a) 0.05Fe modified Cu–Zn–Al alloy and (b) 0.1Fe modified Cu–Zn–Al alloy.

modifiedCu–Zn–Alalloy(Fig.7)wasquitesignificant, evolv-ingtowardsanellipticalgrainshape.Thenear-ellipticalgrain morphologyobservedisinagreementwithresultsfrom pre-viousstudies[23,24]onFemodifiedCu–Zn–Alalloys.Also,it can beseenfrom Table2thatalthoughgrainsizeincrease occurswiththeadditionofFeasmicro-alloyingelementin Cu–Zn–Alalloy,thegrainsizeincreasewasinvarianttotheFe concentrationsusedasgrainmodifiers(thatis0.05wt.%Fe and0.1wt.%Fehadsimilargrainsize).Fig.7(c)showsa rep-resentative EDS profilefor the Fe modified Cu–Zn–Al alloy showingpeaksofCuZnandthepresenceofFeinthealloy

Table 2 – Average martensite lath thickness in Cu–Zn–Al alloys.

martensitelath thickness(␮m)

F(Cu–Zn–Al–0.05B–0.05Fe) 27±1.23

G(Cu–Zn–Al–0.025B–0.075Fe) 26±0.04

Fig 4 – Optical micrographs of (a) 0.05Fe–0.05Fe modified

Cu–Zn–Al alloy and (b) −0.025Fe – 0.075B modified

Cu–Zn–Al alloy.

Itwas alsoobserved thatgrain size increaseand

substan-tially modified grain edges (to near elliptical grain shape)

occurredwiththeadditionofFe–Basmicro-alloyingadditions

inCu–Zn–Alalloy.However,similartotheFemicro-alloying

additions,thegrain sizeincreasewas invarianttotheFe–B

concentrationsused

Fig 5 – Back scattered electron images of the unmodified

Cu–Zn–Al alloy.

Fig 6 – Back scattered electron images of (a) 0.05B modified Cu–Zn–Al alloy and (b) 0.1B modified Cu–Zn–Al alloy.

Themicrographssuggestthatalthoughsamecoolingrate wasadoptedforallalloycompositionsproduced,the nuclea-tionandgraingrowthrateswerealteredbythepresenceof themicro-alloyingelements.Theincreaseingrainsize char-acteristic of the Fe, B, and Fe–B modified Cu–Zn–Al alloys investigated differsfrom the observationsmade in similar studies[5,23,24]whereinadditionsofFeandBwereobserved

tocausegrainrefinement.Grainrefinementobservedin Cu-basedalloyshasbeenattributedtochangesinsolidification conditionsofthematerialasaresultofmicro-alloying ele-mentsadditionreducingthecoarsenessofthemicrostructure

[31,33].Thegrainsizeincreaseobservedinthisstudycanbe related toresultsobserved elsewhere.Some micro-alloying elementssuchasTi-Bhavebeenreportedtodelaynucleation

of certain phases while enhancing grain growth simulta-neously [34] The formation of martensite in the forward transformationhasbeenreportedtobeasaresultof nuclea-tionandgraingrowthintheparentphase;hencetherateof nucleationinfluencesthetypeandmorphologyofmartensite formed[35]

3.2 Analysis of X-ray diffraction

TheX-raydiffractionanalysisofselectedalloycompositions

ispresentedinFig.9.Cu–ZnandCu–Alwereidentifiedasthe crystallinephasespresentinthealloy.TheCu–Znphasewas

Fig 7 – Back scattered electron images of (a) 0.05Fe modified Cu–Zn–Al alloy, (b) 0.1Fe modified Cu–Zn–Al alloy, and (c) representative EDS profile of the 0.1Fe modified Cu–Zn–Al alloy.

determined to befcc structurefrom analysingthe pattern

ofthediffractingcrystal planesfortheentire2diffraction

angles [36] TheCu–Al phase was determined to have bcc

structurefollowingsimilarbasis.ThepredominanceofCu–Zn

peaks in the alloys analysed was in agreement with EDS

results(Fig.7c),andXRDpatternsofCu–Zn–Alalloysreported

inliterature[37].Thecrystallinephasesidentifiedaretypical

ofCu–Zn–Alalloyswithshapememorycapacity.Guerioune

etal.[9]reportedthattheCu–ZnphaseinCu–Zn–Alalloyis

ametastablephasewhichexhibitsshapememoryeffectand

theCu–AlformsasaresultoftheaffinityofCuforAl.The

micro-alloyingelementshadnoeffectontheXRDpeaksas

diffractionsbythesamecrystalplanesareobservedtooccur

withthesameintensityanddiffractionangles(2)

3.3 Mechanical behaviour of the Cu–Zn–Al alloys

3.3.1 Hardness

The hardness values of the unmodified and modified

Cu–Zn–AlalloysarepresentedinFig.10.Itisobservedthatthe

hardnessoftheunmodifiedCu–Zn–Alalloyishigherthanthat

ofthemodifiedalloys withreductionsinhardnessranging

between32.4and51.5%.ThemodifiedCu–Zn–Alalloy

compo-sitions,isobservedtohaveasignificantdecreaseinhardness

withincreaseinBconcentration.However,theFemodified

Cu–Zn–Alalloygradehadmarginalincreaseinhardnesswith

increaseinFeconcentration.ItisalsonotedthattheCu–Zn–Al

alloysmodifiedwithFe–Bhadthehighesthardnessvaluesin comparisonwithothermodifiedCu–Zn–Alalloycompositions Thevariationinhardnesscanbeattributedtothevariation

ingrainsizeandmorphologyofthealloysasobservedfrom

Figs 1 to8 Thehardness trendis alsoobserved to corre-latewellwiththetransversegrainsizedeterminedforallthe Cu–Zn–Alalloygradesproduced.Inthisregards,thehardness increasewithdecreaseinthegrainsizedetermined(Table2

Thefinelathmartensitestructureisprincipallyresponsible forthehighhardnessoftheunmodifiedalloysasindentation resistanceimproveswithfinergrainsize[25]

3.3.2 Ultimate tensile strength

Theultimatetensilestrengthresultsoftheunmodifiedand modifiedCu–Zn–AlalloysarepresentedinFig.11.Thetensile strengthoftheunmodifiedalloyisobservedtobesignificantly lowerthanthatofthemodifiedalloygrades.Tensilestrength increasewithintherangeof28.37–52.74%wasobservedwith theuseofFe,BandFe–Basmicro-alloyingelementsforthe Cu–Zn–Al alloy.Thelowtensile strengthoftheunmodified alloyisattributedtothefinelathmartensitestructure.The edgesoftheneedlelikelathstructureserveassitesforstress concentrationandtriaxialstressstateinthemicrostructure Stressconcentrationandtriaxialstressstateconstrains yield-ingofthematerialandfacilitatesmicro-cracknucleationand propagationwhichculminatesinfractureevenwhenthe nom-inalappliedstressonthematerialisrelativelylow[31].The

Fig 8 – Back scattered electron images of (a) 0.05Fe–0.05B

modified Cu–Zn–Al alloy, and (b) 0.025Fe–0.075B modified

Cu–Zn–Al alloy.

improvedtensilestrengthinthemodifiedcompositionsisas

aresultofthechange inthegrainmorphologydespitethe

obviousincreaseingrainsize.Theround/ellipticalgrainedges

reducesthetendencytoactasstressconcentrators,andthus

necessitatetheapplicationofarelativelyhigherstressto

facil-itateyieldingandincreasemaximumstressbearingcapacity

Thisimpliesthatthestressappliedtothemodifiedalloyis

moreuniformlydistributedincontrasttotheunmodifiedalloy

wherehigherstress concentrationoccurs atthe lathedges (tips)

3.3.3 Percent elongation

ThepercentelongationoftheCu–Zn–Alalloyswasobserved

to increase with the addition of the micro-alloying ele-ments (Fig 12).Thisimpliesthat theductilityofthealloys improved with B, Fe, and Fe–B addition The elongation

is also observed to be sensitive to the concentration of the microelement as reduction in percentage elongation occurred with increasein the concentrationofthe respec-tive micro-alloying elements (B, Fe and Fe–B).The Fe and Fe–B modified Cu–Zn–Al alloy compositions were observed

to have higher elongation values The unmodified alloy had the lowest elongation (6.7%) compared to 12.6% and 11.7% for 0.05Fe and 0.05Fe–0.05B modified alloys, respec-tively Improvementinductilitydue toadditionsofFe and

B to Cu–Zn–Al alloys has been reported by Alaneme [25]

andFerreiraetal.[23].Thisimprovementisanindicatorof improvedplasticityandhencetheworkabilityoftheCu–Zn–Al alloy

3.3.4 Fracture toughness

Thefracturetoughnessvaluesofthe unmodifiedand mod-ifiedCu–Zn–AlalloysarepresentedinFig.13.Itisobserved thatthefracturetoughnessoftheCu–Zn–Alalloyincreased withintherangeof10–23%withtheuse ofFe,B,andFe–B

asmicro-alloyingadditions.TheFemodifiedCu–Zn–Alalloy compositionswereobservedtohavethehighestimprovement (∼23%increase)ofallthemodifiedCu–Zn–Alalloysproduced

Inessence,thisimpliesthattheresistanceofCu–Zn–Alalloys

tocrackpropagationisimproved.Thegrainmodificationfrom lath martensite (needle-like)grainedgesto round/elliptical grain edgemorphology forthemodifiedCu–Zn–Al alloysis responsiblefortheimprovedfracturetoughness.The ellipti-calgrainedgemorphologyhelpsreducestressconcentration whichreducestendencyforcracknucleationandpropagation

[38]

3.3.5 Fracture surface observation

Fig 14 shows representative secondary electron imagesof the tensile fracture surfaces ofthe unmodified and modi-fiedCu–Zn–Al alloysinvestigated.Thefracturefeaturesare

Fig 9 – X-ray diffraction analysis of the unmodified alloy (A) and three modified alloy compositions (0.05B modified Cu–Zn–Al alloy) (B), (0.05Fe modified Cu–Zn–Al

alloy) (D) and (0.05Fe–0.05B modified Cu–Zn–Al alloy) (F).

Fig 10 – Hardness values of the unmodified and modified Cu–Zn–Al alloys.

Fig 11 – Ultimate tensile strength of unmodified and modified Cu–Zn–Al alloys.

indicativeofafibrous micro-mechanismtocrackinitiation,

propagationandfracturecharacterisedbytheprevalenceof

dimpledruptureparticularlyforthemodifiedalloy

composi-tions(Fig.14(b)and(c)).Acombinationofsphericaldimples

and grain facetswere observed on the fracture surface of

the unmodified Cu–Zn–Al alloy, indicating mixed fracture modes The fractographic features observed are in agree-mentwithfracturesurfacecharacteristicsofCu–Zn–Alalloys modified with micro-alloying elements reported by other authors[24,33]

Fig 12 – Percentage elongation of the unmodified and modified Cu–Zn–Al alloys.

Fig 13 – Fracture toughness of the unmodified and modified Cu–Zn–Al alloys.

Fig 14 – Representative secondary electron images of the tensile fracture surfaces of the Cu–Zn–Alloys (a) unmodified Cu–Zn–Al alloy, (b) 0.05B modified Cu–Zn–Al alloy, and (c) 0.05Fe–0.05B modified Cu–Zn–Al alloy.

The microstructure, phase characteristics and mechanical

behaviour of Cu–18Zn–6Al alloys containing two different

weightpercenteach(0.05wt.%and0.1wt.%)ofFeandB,and

two other compositions which have mixtures ofFe and B

(0.05wt.%Band0.05wt.%Fe)(0.025wt.%Feand0.075wt.%B)

wereinvestigated.Theresultsshowthat:

1 Needle-like lath martensite structure with sharp grain edges observed for the unmodified Cu–Zn–Al alloy was transformed to grains with significantly larger trans-verse grain size and curve edged/near elliptical grain shape for the Fe, B, and Fe–B modified Cu–Zn–Al alloys

2 Cu–Znwithfccstructurewasthepredominantcrystalline phaseidentifiedinthealloyswhileCu–Alwithbcc struc-turewasthesecondphaseobserved

3 ThecrystallinephasesidentifiedaretypicalofCu–Zn–Al

alloyswithshapememorycapacity

4 ThehardnessoftheunmodifiedCu–Zn–Alalloywashigher

thanthatofthemodifiedalloyswithreductionsin

hard-nessrangingbetween32.4and51.5%

5 ThetensilestrengthoftheunmodifiedCu–Zn–Alalloywas

significantlylowerthanthatofthemodifiedalloygrades

Thepercentelongationandfracturetoughnessofthe

mod-ifiedalloywerehigherthanthatoftheunmodifiedalloy

grade

6 Themodified Cu–Zn–Al alloysmostly exhibited fracture

featuresindicativeofafibrousmicro-mechanismofcrack

initiationandpropagationcharacterisedbytheprevalence

ofdimpledrupture

Conflicts of interest

Theauthorsdeclarenoconflictsofinterest

r e f e r e n c e s

[1] DarjanC[Masterthesis]Shapememoryalloys.Universityof

Ljubljana;2007

[2] BarbarinoS,SaavedraF,AjajRM,DayyaniI,FriswellMI.A

reviewonshapememoryalloyswithapplicationsto

morphingaircraft.SmartMaterStruct2014;23:19

[3] DarRD,YanH,ChenY.Grainboundaryengineeringof

Co–Ni–Al,Cu–Zn–Al,andCu–Al–Nishapememoryalloysby

intergranularprecipitationofaductilesolidsolutionphase

ScrMater2016;115:113–7

[4] AlanemeKK,OkoteteEA.Reconcilingviabilityand

cost-effectiveshapememoryalloyoptions–areviewof

copperandironbasedshapememorymetallicsystems.Eng

SciTechnolIntJ2016,

http://dx.doi.org/10.1016/j.jestch.2016.05.010

[5] DasguptaR,JainAK,KumarP,HussainS,PandeyA.Roleof

alloyingadditionsonthepropertiesofCu–Al–Mnshape

memoryalloys.JAlloysCompd2014,

http://dx.doi.org/10.1016/j.jallcom.2014.09.047

[6] CladeraA,WeberB,LeinenbachC,CzaderskiC,ShahverdiM,

MotavalliM.Ironbasedshapememoryalloysforcivil

engineeringstructures:anoverview.ConstrBuildMater

2014;63:281–93

[7] EvirgenA,MaJ,KaramanI,LuoZP,ChumlyakovYI.Effectof

agingonthesuperelasticresponseofasinglecrystalline

FeNiCoAlTashapememoryalloy.ScrMater2012;67:475–8

[8] GómezdeSalazarJM,SoriaA,BArrenaMI.Corrosion

behaviourofCu-basedshapememoryalloysdiffusion

bonded.JAlloysCompd2005;387:109–14

[9] GueriouneM,AmiourY,BounourW,GuellatiO,BenaldjiaA,

AmaraA,etal.SHSofshapememoryCuZnAlalloys.IntJ

SelfPropagHighTempSynth2008;17(1):41–8

[10]AminiR,MousavizadSM,AbdollahpourH,GhaffariM,

AlizadehM,OkyayAK.Structuralandmicrostructuralphase

evolutionduringmechano-synthesisof

nanocrystalline/amorphousCuAlMnalloypowders.Adv

PowderTechnol2013;24:1048–53

[11]DiánezMJ,DonosoE,CriadoJM,SayaguésMJ,DíazG,Olivares

L.StudybyDSCandHRTEMoftheagingstrengtheningof

Cu–Ni–Zn–Alalloys.MaterDes2016;92:184–8

[12]ZenginR,KayalN.Structuralandmorphological

investigationsonshape,memoryCuZnAlalloys.ActaPhys

PolA2010;118(4):619–22

[13]AsanovicV,DelijicK,JaukovicN.Astudyoftransformations

of␤-phaseinCu–Zn–Alshapememoryalloys.ScrMater 2008;58:599–601

[14]BujoreanuLG,LohanNM,PricopB,CimpoesuN.Thermal memorydegradationinaCu–Zn–Alshapememoryalloy duringthermalcyclingwithfreeaircooling.JMaterEng Perform2011;20(3):468–75

[15]LiuJ,HuangH,XieJ.Therolesofgrainorientationandgrain boundarycharacteristicsintheenhancedsuperelasticityof Cu71.8Al17.8Mn10.4shapememoryalloys.MaterDes 2014;64:427–33

[16]KumarP,JainaK,HussainS,PandeyA,DasguptaR.Changes

inthepropertiesofCu–Al–Mnshapememoryalloydueto quaternaryadditionofdifferentelements.RevMatér 2015;20(1):284–92

[17]WeiW,WangSL,WeiKX,AlexandrovIV,DuQB,HuJ MicrostructureandtensilepropertiesofCu–Alalloys processedbyECAPandrollingatcryogenictemperature.J AlloysCompd2016;678(5):506–10

[18]ZhouS,LvW,LiP,GongY,TaoJ,ChengL,etal.Mechanical propertiesanddeformationkineticsofbulkCu–Al–Znalloy subjectedtorollingandannealing.MaterSciEngA 2014;609(15):217–21

[19]GongYL,RenSY,ZengSD,ZhuXK.Unusualhardening behaviourinheavilycryo-rolledCu–Al–Znalloysduring annealingtreatment.MaterSciEngA2016;659(April):165–71

[20]LanziniF,RomeroR.Thermodynamicsofatomicorderingin Cu–Zn–Al:aMonteCarlostudy.ComputMaterSci

2015;96(PartA):20–7

[21]RochaLimaEP,BotelhodeAraújoPC,deQuadrosNF, SanguinettiFerreiraRA.Proceedingsof5thnorth-northeast congressofmechanicalengineering.Fortaleza-CE;1998

[22]WangFT,ChenFX,WeiZG,YangDZ.Theeffectof microelementsonthegrainrefiningandgraingrowth behaviourofCuZnAlshapememoryalloys.ScrMetallMater 1991;25:2565–70

[23]FerreiraSR,RochaLE,QuinoFA,QuadrosNF,OlimpioAO, YadavaYP.MicrostructuralevolutioninaCuZnAlshape memoryalloy:kineticsandmorphologicalaspects.Mater Res2000;3(4):119–23

[24]AlanemeKK,SulaimonAA,OlubambiPA.Mechanicaland corrosionbehaviourofironmodifiedCu–Zn–Alalloys.Acta MetallSlov2013;19(4):292–301

[25]AlanemeKK.Indentationresistanceandcorrosionbehaviour

ofFe–MnmodifiedCu–Alalloysinselectedindustrialand biologicalfluids.ActaMetallSlov2014;20(4):366–74

[26]ASTME18-16.StandardtestmethodsforRockwellhardness

ofmetallicmaterials.WestConshohocken,PA:ASTM International;2016.http://www.astm.org

[27]ASTME8/E8M-15a.Standardtestmethodsfortensiontesting

ofmetallicmaterials.WestConshohocken,PA:ASTM International;2015.http://www.astm.org

[28]AlanemeKK.Fracturetoughness(K1C)evaluationfordual phasemediumcarbonlowalloysteelsusingcircumferential notchedtensile(CNT)specimens.MaterRes

2011;14(2):155–60,

http://dx.doi.org/10.1590/S1516-14392011005000028 [29]DieterGE.Mechanicalmetallurgy.Singapore:McGraw-Hill; 1988

[30]NathSK,DasUK.Effectofmicrostructureandnotcheson thefracturetoughnessofmediumcarbonsteel’.JNavalArch MarEng2006;3:15–22

[31]XuJW.EffectsofGdadditiononmicrostructureandshape memoryeffectofCu–Zn–Alalloy.JAlloysCompds 2008;448:331–5

[32]DasguptaR.AlookintoCu-basedshapememoryalloys: presentscenarioandfutureprospects.JMaterRes 2014;29(16)