SMAs are metallic alloys that can return to a predetermined shape after deformation through a thermal cycle. They exhibit both shape memory effect and superplasticity. SMAs undergo a phase transformation between austenite and martensite phases. The austenite phase remembers the alloy's original shape. When heated through this phase transformation, an SMA can recover from large deformations. Common SMAs include nickel-titanium and copper-based alloys. SMAs find applications where lightweight actuators are needed, such as in biomedical and aerospace devices. Researchers are working to develop high-temperature SMAs for uses over 200°C.

1. DevinRowe
MET 532
Shape Memory Alloys
Introduction
Materialscapable of returningtoa predeterminedformafterbeingdeformedduringuse waslong
thoughtto be science fictionuntil 1932 whenthe Shape MemoryEffect (SME) was firstobservedina
gold-cadmium. Since thenmanytypesof materialshave beenengineeredtoexhibit SMEsuch as
polymers,ceramics,gelsandof course metallicalloys. Shape MemoryAlloys(SMAs)are the most
covetedshape memorymaterialsforobviousreasonsof astheyhave all the traitsof metalsand can
essential “fix”itself afterextensivedeformationduringuse. Andsothe remainderof thisreportwill be
devotedtoSMAsand theiruses
The Basics of SMAs
To be consideredaSMA two unique propertiesmustbe observedinthe metal. Firstisof course the
SME, the metal mustbe able to returnto a presetshape afterexposure toaparticularenvironmental
stimuli inthiscase a thermal-cycle. Howeveritisworthnotingthatthe thermal-cycleisn’tnecessarily a
part of the definitionas othershape memorymaterial returntotheiroriginalshape afterexposerto
certainchemicals, pHchanges andmagneticfields ,it’sjustthata temperature change isthe primary
thingthat causes SMAs to returnto theirshapes. Secondthe alloymustshow traitsof superplasticity,
whichisthe abilityof a material toexhibit large recoverablestrains(uptoaround 15%), while deformed
withinatemperature range thatis characteristicof a specificalloy.
How they work
So howdoesshape memoryworkinSMA’s? Well ingeneral itiscausedbya cycle of phase
transformations. All SMAshave anaustenite andmartensitephase thatare stable atdifferent
temperatures. However,theseare notthe same austenite and martensitephasesthatwe are familiar
withinIron-carbonalloyswhere austenite wasanFCCstructure and martensite wasaBCT structure
broughton by retainedcarbon. Inthe case of SMAs the austenite isstrongerthanthe martensite phase
and the austenite issome sortof cubicstructure while the martensite canvaryfromtetragonal,
orthorhombicand monoclinicstructure. Lookbelow toFigure 1for an illustrationof the how SMAs
work.
Figure 1. A macroscopic illustration of SMAs deformation and recovery process. Figure was takenfrom page 4 of Shape
MemoryPolymers and Textiles.

2. Once the SMA isinits shape, the austenite iscooled belowthe transformationtemperature where it
transformsto a thermoelasticmartensite whosestructure hasmanyvariants,typicallyshearedplatelets.
Because the martensiticstructure isself-accommodating,there isnodeformation tothe overall shape
of the componentwhenaustenite changesto martensite.The martensite deformsbyatwinning
mechanismthattransforms the differentvariantstothe variantthatcan accommodate the maximum
elongationinthe directionof the appliedforce.The interfacesbetweenplateletsinthe martensite
phase slipveryreadilyandthe material isdeformedat relativelylow appliedstresses.Howeverthe
austenite phase hasonlyone possible orientation,thuswhenheated,all the possible deformed
structuresof the martensite phase mustreverttothisone orientationof the austenitememoryphase
and the material recoversitsoriginal shape. Howeverthisisnotalwaysa single steptransformation,
dependingonthe thermal and/ormechanicalhistoryof the alloy,the transformationcanbe 2 or
multiple step.
Manufacturing and Processing
The manufacturingprocesscan varydependingwhatelementsare prevalentinthe SMA,butas a
general rule SMAsare producedinmannersverysimilartothe methodscommonto theirbase metal
but withhigherrestrictionsplacedonthe range of alloyingcompositionasthisisa keypart of what
determines the base transformationtemperature of thatalloy. SomostSMA are producedintraditional
mannerslike EAF,butSMAs thatare highin Ti are made in mannerssuchas VacuumArc or Vacuum
InductionmeltingprocessesasTi isveryeasilyoxidized.
As forprocessingthe same general hotandcoldworkingcan be employedasthose usedformost
metals. Howeveritisimportanttonote that the transformation temperatures of SMAsare altered
duringthese processingsteps. Coldworkingisknowntoraise the transformationtemperature of SMAs,
althoughitisn’tfullyunderstoodwhy itisbelievedthatvacanciesanddislocationsinthe lattice are
mechanismsthatcan stabilize martensite. Followingthis idea,one wouldthinkthatannealingwould
bringthe transformationtemperaturesdownbutinrealityitactuallyhelpstofix thematthe higher
temperature broughtonby coldworking. (See Figure 2below.) Thermalcyclingisthe onlymethodaside
fromjust recastingthatisknownto reduce the transformationtemperaturesof SMAsbutjustlike how
coldworkingraisesthe temperature,itisstill notfullyunderstoodwhatthermal cyclingdoestolowerit.
Figure 2. Transformation temperatures with 10%CW followed by annealing at different temperatures. Figure was takenfrom
Shape MemoryAlloys:Processing, CharacterizationandApplications.

3. Thisraisesanotherquestion, howdoSMAsknow whatshape to remember? Well toputitsimplythe
SMAs undergoa heattreatingprocesscalled“training”. Totrainthe austenite,the metal mustbe
cooledtothe martensite phase first. The metal isthenbentandfastenedintoshape. Thistime when
the martensite isheatedtoaustenite,thiscauses internalstrainsinitslattice thatnormallywouldn’t
developbecauseshiftingduringthe recoveryof the shape wouldpreventthembutnow developsince
the austenite canno longercorrectitself. Bytime the entire SMA hasreachedthermal equilibriumwith
the annealingtemperature of the furnace,these internal strains have beenremovedfromthe lattice so
the processis finishedwitharapidcoolingandreversiontomartensite. The temperatureforthis
processiscarefullyselectedbasedonthe SMAscomposition. Asone wouldexpectthe SMEisn’t
perfect,the SMA will graduallybegintolose itsshape memorywithrepeatedthermal-cyclingassome
recrystallizationbeginstake effect,butrepeatingthe initial annealingprocesswillrestore the initial
shape memoryof the SMA.
One-way vs Two-way
All SMAsdisplaythe abilitytorememberashape uponheatingintothe austenite range whichisknown
as the One-WayShape MemoryEffect(OWSME),but some SMAs can alsorememberanothershape
uponcoolingto martensite whichisknownasTwo-WayShape MemoryEffect(TWSME).(See Figure 3
belowfora macroscopicviewof the twodeferentshape memories).
Figure 3. Macroscopically Mechanism of One vs Two Way Shape Memory Effect: One Way, (a) Martensite,(b)Loaded and
Deformedinmartensite phase T≤ Mf, (c) Heatedabove T G As (austenite),(d) Cooling to martensite T≤ Mf. TwoWay, (a)
Martensite state, (b)Several deformation withan irreversible amount, (c) Heated, (d)Cooled
Unlike OWSME andsuperplasticitywhichare naturallyoccurringpropertiesinSMAs,aTWSME must be
“trained”intothe material ina similarmannertohow the metal was givenitsinitial shapeforOWSM.
In OWSMthe austenite wasthe only phase thatrememberedashape,inTWSMthe martensite now
remembersashape as well. Toaccomplishthisplasticstrains are inducedduringthissecondarytraining
that generate internal stressesand material asymmetrieswhichproduce preferential formationof
specificmartensitevariants.Asaresult,the material returnstoabiasedmartensite state when

4. temperaturesare cooledfromAustenitefinishedtemperature (Af) toMartensite finishtemperature (Mf)
rather thantwinnedmartensiteconfiguration.
There are three knownwaystoinduce a TWSME ina SMA andtheyinclude shape memorytraining,
pseudoelastictraining,andthermal cyclingtrainingunderaconstantstress.Shape memorytraining
involvesrepeatedcyclesof deformationof the alloybyastress-inducedmartensitictransformationand
recoveryof the deformationbyareverse transformationinducedbyheatingundernostress.
Pseudoelastictrainingissimplyamechanical cyclingprocessinpseudoelasticitythroughthe stress-
inducedmartensitictransformationandthe reverse transformationthatproceedsagainstthe external
stress.Thermal cyclingtraininginvolvessubjectingthe alloytorepeatedthermaltransformationcycles
underthe influence of anexternal biasstress.Amongthesethree trainingproceduresconstant-stress
thermal cyclinghasappearedtobe the mosteffectivemethodinintroducingTWSME.
Types of SMAs
There are several differenttypesof SMAs. Asmentionedbeforethe firstSMA discoveredwasagold-
cadmiumalloyin1932, but as one wouldexpectthismetal isfartoosoft andexpensive forcommercial
use. Otheralloysbasedonironand zincwhere discoveredbutonlyCopper-basedandNickle-based
SMAs have beencommerciallyexploitedtoanysignificantextentasthese twodisplaythe greatest
potential torecoverfromstrains.
Copper-based and Nickel-based SMAs
Copper-basedshape memoryalloysexhibithigheractuationtemperatures(approximatelyinthe range -
200 to +200°C) thanNiTi alloysandare sometimesthe onlychoice forhightemperature applications,
(i.e.>100°C) buttheirlack of strengthand oxidationresistance oftenmakesNickelbasedSMAsthe
metal of choice.
Applications
SMAs are oftenemployedinthe medical fieldasstintsastheyare capable of bendingwiththe body
withminimal discomforttothe patient. Buttheyhave alsofoundtheirwayintodentistry and
optometry. IndentistrySMAsSME are usedto dowork,in braceswhenthe wiresare originallysetthey
are inthe deformedmartensitestate butuponheatingtothe ambienttemperatureof yourmouththey
cross the Af temperature andare nowtryingto returnto itsoriginal setshape pullingyourteethasthey
do. SMAs inoptometryare usedinthe constructionof framesfor glasses. Ordinarymetal or plasticeye
glassesframeswouldbeenof breakafteraccidentsthatoccur duringeverydaylifebutSMAswould
allowthe framestobendthenreturnto normal immediatelyafterwardswiththe use of athermal cycle.
How theydidthisissimple,theyengineerthe allowsAs temperaturebelownormal roomtemperatures.
What thisdoesiswhena loadis applied anddeformationoccursthe austenite changestoastress
inducedmartensite,whenthe loadisreleasedthe martensiteimmediatelychangesbacktoaustenite
and regainsitsshape. SMAsare alsousedinwide veritiesof othermiscellaneousapplications such
mechanical dampeners,andstructural supportsbutinmostrecentyearsSMAs are beingsoughtforuses
inhightemperature applications.

5. High Temperature Shape Memory Alloys (HTSMA)
HTSMAs have become a growinginterestinaerospace,automotive,processcontrol andenergy
industries inrecentyears. Asthe usesfora shape changingalloysare essentiallyinfinite the most
applicationdesiredisinactuators,asSMA basedactuators are far lighterandmore efficientthanother
traditional actuatingunits. ButSMAsare limitedbytheirrespectivetransformationtemperatures
makingthemreallyonlyeffective atlowertemperaturesusuallyinthe range of ±100o
C formost NiTi
SMAs and ±200o
C for Copper-basedSMAs. Unfortunatelymaydesiredapplicationswouldneedworkat
much highertemperaturesforexample inaerospace actuatorswouldneedtooperate between200-
1000o
C and inautomotive actuatorsinandaround the engine needtoworkataround 100-300o
C. But
inrecentyearsresearchhas beenperformedtoincrease the transformationtemperatureforthese
alloyswithafocus onalloyingadditionstoNiTi andCuAl SMAs.
Nickel basedSMAsare by far the mostimportantSMA showingthe greateststrengthtoductilityratioof
all the SMAs alongwithgoodcorrosion resistance. AccordingtoresearchersatNASA the additionof Pt
or Pd to NiTi have beenknowntopushthe upperlimitsof transformationtemperature to1050o
C, which
at face value soundspromising,butlaterstatedthe datacollectedforthose particular systemswere
understressfree conditionsandtofindoutif theycouldperformasactuators subsequenttestsmustbe
performed. Asthere report stated,“..the mere exhibitionof shape memoryatelevatedtemperaturesis
not sufficientwhenconsideringthesematerialsforactuatorapplications.”,whichisverytrue as
actuator isexpectedtobe able to dowork soevenif the material doeshave shape memory,if itcan’t
exertenoughforce ontoanobjectwhile changingshape itisuselessasan actuator. Furtherliterature
researchand experimentationinthe same documentreportedthatPt/Pdadditionstoof about15-30
at% at the expense of Ni showpromisingworkpotential withamaximumof about10.5J/cm3
. However
otherfactors needtobe take intoconsiderationwiththe additionof otheralloyelementsthe yield
strengthsof the austenite phase andthe twinnedmartensitechange asshowninFigure 4 below.
Figure 4. Effect of Pd content on the yield strength of martensite and austenite. Figure was takenfrom Challenges and
Progress in the Development of High-Temperature Shape MemoryAlloys Based onNiTiXCompositions for High-Force Actuator
Applications.
Figure 4 showsthe yieldingbehaviorof 50o
C above the Af and below the Mf,as you can see once the
additionof Pdhitsabout37% the yieldstrengthof bothphasesare aboutequal and the entire planon
usingthe HTSMAs hingesonthe martensite yieldingbutnotthe austenite. Thispossibledilemmaaside
anotherproblemmaybecome important, the heavyadditionof PdandPt to these alloysmaymake

6. themtoo expensivetobe practical but whenbalancedagainstall the weighttheywouldbe savingby
replacingbulkypneumaticand hydraulicactuatorsonspace shuttlesandotherdevice where weightis
keyit mayevenout. Thisthe reasonthat CopperbasedSMAsare of interest.
CopperbasedSMAsare an attractive alternative tonickel basedonesastheymuchcheaper and display
a betterSME thaniron basedSME, howevertheirtransformationtemperatureisonlyataround120o
C
for themto be of any use in HighTemperature applicationsthe upperlimitswill needtobe pushedabit.
Like withNiTi alloy additionstoestablishedCuAlsystemshave shownthatthe transformation
temperaturescanbe raised. Itis fairlywell knownthatCu-Al-Ni canmeethightemperature
transformationneedsbutbecause of theirlackof ductilityinpolycrystalline form,onlysingle crystals
wouldworkmakingitcostlyagain. OtherCu-Al-Xsystemshave beenfoundthatshow higher
transformationtemperaturessuchas Cu–Al–Agalloys whichshowedamartensitictransformation
temperature higherthan300o
C. Cu–Al–Fe alloyscanbe consideredtofunctionabove 200°Cwith
relativelygoodSME.Cu–Al–Nballoyswere alsostudied asCu–Al-basedHTMSAs,exhibitingthe
combinedpropertiesof highmartensitictransformationtemperature(around300°C),12.7% tensile
strainand 90% shape memoryratio. Buta recentarticle from SmartMaterialsand Structures
mentionedresearchintoanewCu–Al-basedHTMSA,aCu-Al-Tasystem. Preliminaryresultsprove tobe
promisingasthe alloyshowsa transformationtemperature ashighas450o
C witha 100% recoverywith
up to 2.5% strainon the material.Furtherresearchneedstobe done intothe work capabilitiesof this
alloysoit isn’tcertainyetwhetherthiswill be the metal we have beenwaitingforinSMAs.
OtherHTSMAs such as Ni–Ti–Zr/Hf,Ni–Al,Ni–Mn,Zr–,Ru–Ta/Nballoys have beendevelopedbutstill
face some keyissue. Forinstance,Ni–Al alloysare consideredtobe unstable;Ni–Ti–ZrandNi–Mn-
basedalloysare toobrittle foractual production. Sountil new material ishasbeenbroughtintofocus
Ni-Ti-Pt/PdandCu-Al-Taalloysare ourbestbetforHTSMAs.

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