-
NiTiHf-based shape memory alloys
H. E. Karaca*, E. Acar, H. Tobe and S. M. Saghaian
NiTiHf-based shape memory alloys have been receiving
considerable attention for high
temperature, high strength and two-way shape memory applications
since they could have
transformation temperatures above 100uC, shape memory effect
under high stress (above500 MPa) and superelasticity above 100uC.
Moreover, their shape memory properties can betailored by
microstructural engineering. However, NiTiHf-based alloys have some
drawbacks
such as low ductility and high slope in stress induced
martensite transformation region. In order to
overcome these limitations, studies have been focused on
microstructural engineering by aging,
alloying and processing. It has been revealed that
microstructural control is crucial to govern the
shape memory properties (e.g. transformation temperatures,
matrix strength, shape recovery
strain, twinning type, etc.) of NiTiHf-based alloys. A summary
of the most recent improvements on
selected NiTiHf-based systems is presented to point out their
significant shape memory
properties, effects of alloying, aging and microstructure of
transforming phases and precipitates.
Keywords: NiTiHf, High temperature shape memory alloys,
Microstructure control, Precipitation hardening, Superelasticity,
Work output
This paper is part of a special issue on Smart Materials
IntroductionShape memory alloys (SMAs) are a unique class
ofsmart materials with the ability of changing their
shapesdepending on the applied temperature, stress and in spe-cial
case of ferromagnetic alloys, magnetic field. Shapememory alloys
can produce very high actuation strains,stresses and work outputs
as they undergo reversiblemartensitic phase transformation.1 In
addition to theirremarkable properties in actuation, vibration
damping,noise reduction and sensing, they are compact,
robust,lightweight, frictionless, quiet, environment-friendly
(nohydraulic liquids), easy to inspect and have low after-market
costs for inspection and maintenance.24 Shapememory alloys are
playing a growing role in supplying keyactuation forces and sealing
functions in oil and gas,automotive, aerospace and biomedical
industries.24 Theability to remain elastic under very large
deformationmakes SMAs potential candidates for superelastic
devicesfor civil structures.5,6 Moreover, their superelasticity,
goodcorrosion resistance, biological and magnetic
resonancecompatibility and high bending resistance resulted in
theiremployment as the biomedical devices in the
orthodontic,orthopaedic, vascular, neurosurgical fields.7,8
Among the various SMA systems, NiTi alloys havegood dimensional
stability, shape memory properties,ductility and workability.
Currently, NiTi alloys are themost commercially viable SMAs and
practically beingused in various medical and engineering
applicationswhere the operating temperature is below 100uC.9 It
hasbeen found that the transformation temperatures (TTs)
of NiTi can be adjusted by tailoring the stoichiometry
orformation of precipitates.10,11 However, the TTs ofbinary NiTi
cannot be increased above 120uC. Thedevelopment of a shape memory
material with proper-ties similar to those of near equiatomic NiTi,
but withhigher strength and TTs, especially above 100uC, isurgently
needed for a broad range of applications in theaerospace,
automotive and oil and gas industries to serveas compact actuators
for flow and clearance controls,actuation tubes for rotors, moving
or morphing surfacesas well as inlet/exhaust configurations, linear
actuatorsand sealants.24
Ternary element addition to NiTi alloys is the mostpromising
method to obtain commercially available hightemperature shape
memory alloys (HTSMAs) in thenear future.12 Ternary element
addition should not onlyincrease the TTs, but also help to maintain
the goodmechanical and shape memory properties of NiTi alloys.It
has been found that the addition of Hf, Zr, Pd, Pt andAu elements
to NiTi increases its TTs.2,9 Among thoseelements, Pd, Pt and Au
are very expensive and will limitthe use of their respective
ternary alloys to some criticalapplications only (i.e. aerospace),
while Zr is associatedwith high oxygen affinity.2,9,13 Among the
potentialHTSMAs, due to its low cost, medium ductility and highwork
output NiTiHf seems to be the most encouragingHTSMA for a wide
range of applications in the critical100300uC temperature
range.12
The TTs of NiTiHf alloys do not increase much up to10 at-% Hf
content, however, at chemical concentrationshigher than 10 at-%,
they tend to increase linearly up to525uC for 30 at-% Hf when Hf is
added at the expense ofTi.1416 Transformation temperatures of
NiTiHf alloysare not notably affected by a change in Ni composition
aslong as the alloys are Ni-lean, but dropped steeply when
Department of Mechanical Engineering, University of Kentucky,
Lexington,KY 40506, USA
*Corresponding author, email [email protected]
2014 Institute of Materials, Minerals and MiningPublished by
Maney on behalf of the InstituteReceived 1 November 2013; accepted
23 June 2014DOI 10.1179/1743284714Y.0000000598 Materials Science
and Technology 2014 VOL 30 NO 13a1530
-
Ni content is increased beyond the equiatomic (50
at-%)composition, consistent with the behaviour of
NiTialloys.1517
The main disadvantages of Ni-lean NiTiHf alloys aretheir large
hysteresis (.50uC), poor ductility at roomtemperature, lack of
cyclic stability due to the high stressfor the reorientation of
martensite and detwinning, thelow strength for slip and poor
formability.2,18 It shouldbe noted that Wojcik19 studied the
possibility of thecommercialisation of the NiTiHf (Hf content less
than10 at-%) alloys and showed that hot rolling can besuccessfully
utilised to produce thin sheets. Anotherdrawback of the alloy is
the absence of stress plateauduring phase transformation that
results in the lack ofsuperelasticity. This behaviour has been
attributed to thesimultaneous occurrence of stress induced
martensite(SIM) and dislocation slip.20,21 To increase the
strengthfor slip, NiTiHf alloys were severely deformed that
resultedin increased recoverable transformation strain,
decreasedirrecoverable strain levels and thermal hysteresis
underconstant stress experiments, as well as improved
cyclicstability.18 However, no superelasticity was observed dueto
large hysteresis and low material strength.
Precipitation strengthening has been used to improvethe
mechanical properties of NiTiHf alloys as a successfulmethod.Meng
et al.22,23 revealed that it is possible to formprecipitates in
Ni-rich NiTiHf alloys and TTs can beincreased drastically to
temperatures above 100uC. Theyhave also reported that coherent
precipitates increase thematrix strength and enhance the thermal
stability.22 Ifthe chemical composition is slightly Ni-rich with
high Hfcontent (15 to 20 at-% Hf), fine nanometer size
precipi-tates which are face centred orthorhombic structure,simply
referred to as the H-phase,24,25 are formed uponaging treatments.
The formation of fine precipitatesprovides high resistance to
dislocation motion resultingin exceptional strength and stability
limiting residualstrain during transformation under isothermal and
iso-baric conditions.2628
Quaternary alloying and precipitation strengtheninghave also
been used to improve the overall behaviour ofNiTiHf polycrystalline
and single crystal alloys. The shapememory properties of heat
treated Ni45?3Ti29?7Hf20Pd5 (at-%) alloys in single crystalline and
polycrystalline formshave been reported.2933 The replacement of 5%
Pd withNi of Ni50?3Ti29?7Hf20 alloy resulted in a very high
strengthalloy that has high damping capacity of 35 J cm23
in polycrystalline form and 44 J cm23 in [111] orientedsingle
crystals.32-33 Transformation strain of 2% wasobserved in aged
[111] oriented Ni45?3Ti29?7Hf20Pd5 singlecrystals under a
compressive biasing stress of 1500 MPa.31
Moreover, perfect superelastic behaviour with recoverablestrain
of 4?2% was observed in the solutionized conditioneven when
compressive stress levels as high as 2?5 GPawere applied.32
However, it is also known thatNi45?3Ti29?7Hf20Pd5 alloys are
brittle, since they generallyfail after limited plastic deformation
in compression andduring phase transformation in tension in
superelasticityexperiments.34
It has been considered that low workability is one ofthe main
problems with NiTiHf alloys for practical use.Kim et al.35 reported
that an addition of Nb to NiTiHfalloys caused the formation of a
soft Nb-rich b phaseand improved the cold workability, although the
TTsand plastic strain in thermal cycling experiments under
stress were decreased. Cu has been another alloyingelement to
NiTiHf systems where, in general, it improvedthe glass forming
ability and thermal stability of NiTiHfalloys while decreasing
their TTs.36,37 NiTiHfCu alloyshave also demonstrated two-way shape
memory effect.38
It has recently reported that Ni45?3Ti29?7Hf20Cu5 alloyshave the
capability to recover compressive strains of 2%above 100uC and
two-way shape memory strain of 0?8%above 80uC.39
Hsieh and Wu40 investigated the TTs and hardnessvalues of
Ti50?52xNi49?5Zrx/2Hfx/2 (x5020 at-%)
40 andrevealed that TTs can be increased from 50 to 323uCwith
increased Zr and Hf contents. Their shape memoryresponses under
stress (e.g. constant stress thermal cycling,superelasticity) have
not been reported yet.
This article reviews the effects of alloying, aging
andprocessing on the shape memory properties and micro-structure of
NiTiHf-based alloys. Special attention isgiven to recently
developed Ni-rich NiTiHf-based alloys.
Transformation temperatures of NiTiHf-based shape memory
alloysMany studies have been conducted in order to gain
thefundamental understanding on how to change the TTsof SMAs.41 It
is known that chemical compositionalteration is very effective to
change the properties suchas TTs, transformation strain and matrix
strength ofSMAs. Figure 1a shows the effects of Ni content on theMp
(martensite peak temperature) of NixTi902xHf10.
16
It is clear that Mp is insensitive up to 50 at-% Ni andthen
suddenly decreases to below 0uC with increased Nicontent.
Figure 1b shows the change in Mp as a function ofHf.1416,42 It
is clear that Mp does not change up to 3%of Hf and then increases
after 5%. Up to 10% Hf, theincrease ofMp is about 5uC/at-% Hf. As
the Hf increasesbeyond 10%, there is an abrupt increase ofMp by
almost20uC/at-% Hf in NiTiHf alloys and Mp reaches up to400uC for
25% Hf.Figure 2a shows the differential scanning calorimetry
responses of the Ni50?3Ti29?7Hf20 alloys after heat treat-ment
at selected temperatures from 300 to 900uC for3 h.27 Initially, TTs
slightly decreased compared to the asextruded (extruded at 900uC)
material when aged at 300and 400uC. Then, TTs increased with heat
treatmenttemperature up to 700uC and then TTs decreased. Themaximum
Af (austenite finish temperature) was revealedto be 210uC in
Ni50?3Ti29?7Hf20 alloys aged for 3 h at600uC. Figure 2b shows the
change in TTs for Ni45?3Ti29?7Hf20Pd5 polycrystalline specimens
aged for 3 h attemperatures between 400 and 900uC.33 The trend in
TTswith heat treatment temperature was similar to thatof Fig. 2a.
The maximum Af was about 150uC inNi45?3Ti29?7Hf20Pd5 after aging at
600uC for 3 h. Themain reason for the TTs change with aging in the
bothalloys could be attributed to the change in the
chemicalcomposition of matrix33 due to the formation of
preci-pitates that will be discussed in details in the
micro-structure part.
Zarinejad et al.41 revealed a practical relationshipbetween the
chemical composition and TTs by consider-ing the number (ev/a) and
concentration (cv) of valenceelectrons in NiTi-based alloys. The
number of d and selectrons is accepted as the number of valence
electrons
Karaca et al. NiTiHf-based shape memory alloys
Materials Science and Technology 2014 VOL 30 NO 13a 1531
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for an atom in transition metals while the number ofvalence
electrons is considered to be p and s electrons foran atom in
non-transition metals.43 The number ofvalence electrons of alloys
can be calculated with thefollowing equation44
ev
a~fAe
AvzfBe
BvzfCe
Cvz
::: (1)
where fA, fB and fC are atomic fractions of A, B and C
elements, eAv ,eBv ,e
Cv are the related valence electrons for
the elements in an alloy system.
The following equation can be used to determine theaverage
concentration of valence electrons44
cv~ev
et~
fAeAvzfBe
BvzfCe
Cvz
:::
fAZAzfBZBzfCZCz:::(2)
where ZA, ZB and ZC are the atomic numbers ofelements A, B and
C, respectively.
Figure 3 shows the relationships between the Ms(martensite start
temperature) (orMp) and ev/a and cv inNiTiHf-based
SMAs.30,35,37,40,41,44,45 It is clear that the
TTs do not have a clear trend with ev/a while theygenerally
decrease with increasing cv. It is commonlyagreed that higher
electron concentration results inhigher bulk (resistance to volume
change) and shear(resistance to shape change) moduli.44,46 Thus,
theconcentration of the electrons may affect the strengthof atomic
bonds in metallic materials. In general, as theconcentration of
valence electrons increases, the resis-tance to shear also
increases. Thus, further energyprovided by undercooling is
necessary for the martensi-tic transformation resulting in
decreased TTs.
As stated above, even though there are some guidelinesin
predicting the TTs of NiTiHf-based alloys, therelationship between
the nominal chemical compositionand TTs is not completely
established since there aremany other factors that may alter TTs
such as precipita-tion and grain size effects.37,4345 For instance,
if theprecipitates are fine and interparticle distances are
small,nucleation of martensite could be more difficult andrequire
additional undercooling, resulting in decreasedTTs. Transformation
temperatures are also sensitive to
1 Mp temperature as a function a Ni and b Hf contents in NiTiHf
alloys1416,42 (chemical compositions are in at-%)
2 Transformation temperatures of a Ni50?3Ti29?7Hf20 and b
Ni45?3Ti29?7Hf20Pd5 alloys after heat treatment of 3 h at
selected
temperatures27,33
Karaca et al. NiTiHf-based shape memory alloys
1532 Materials Science and Technology 2014 VOL 30 NO 13a
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local chemical composition changes due to formation
ofprecipitates. Moreover, it is known that internal stressfields
increase the TTs in SMAs.37,4345
Crystal structure and microstructure ofNiTiHf-based alloysThe
shape memory and superelastic properties of NiTi-based alloys are
significantly influenced by the mic-rostructure, such as the
precipitation size, interparticledistance and martensite
morphology. In order to obtaingood shape memory and superelastic
properties, it isimportant to strengthen the matrix to prevent the
intro-duction of dislocations during the martensitic
transforma-tion. One of the well-known procedures to improve
thestrength of the matrix is the precipitation hardening.Aging of
NiTiHf alloys produces several precipitates thataffect the
martensite morphology that will be discussed indetails.
Crystal structure of NiTiHf alloysIn general, the crystal
structures of austenite andmartensite phases in NiTiHf alloys are
cubic (B2) andmonoclinic (B199), respectively, which are similar to
thosein NiTi binary alloys. Zarinejad et al.47 investigatedthe
effect of Hf on the lattice parameters of the B199martensite in
NiTiHf alloys. The lattice parameters a, b, cand b angle of the
martensite are plotted in Fig. 4 asa function of Hf content for
Ni(1002x)/2Ti(1002x)/2Hfx,Ni502xTi50Hfx and Ni50Ti502xHfx (x5520
at-%) alloys.The addition of Hf increased all the lattice
parameters forthe Ni(1002x)/2Ti(1002x)/2Hfx and Ni502xTi50Hfx
alloys. Onthe other hand, when Ni is constant, the increase in Hf
inthe Ni50Ti502xHfx alloy increased a, c and b but decreasedb.
Potapov et al.45 also observed a similar dependence oflattice
parameters on the Hf content for Ni49?8Ti50?22xHfx(x5825 at-%)
alloys where the increase in Hf while Niwas kept constant to 49?8%
slightly decreased the latticeparameter b, while it increased a, c
and b of B199martensite. It was also reported that the addition of
Hfincreased the lattice parameter of B2 austenite.45 Thevolume
change during transformation was smaller than0?5% which was similar
to that in NiTi binary alloys(y0?3% or less).48,49 It should be
noted that in somestudies, NiTiHf alloys with more than 15 at-% Hf
in
Ni48?5(Ti51?52xHfx)50 and between 20 and 30 at-% Hf of
Ni50(Ti502xHfx)51 were reported to have orthorhombic
B19 martensite.
Precipitation characteristics and their effects onmartensite
morphologyNi-lean NiTiHf-based alloys
Konig et al.52 fabricated NiTiHf thin films with a
widecomposition range by magnetron sputtering method
andinvestigated their TTs, precipitate structure and thermalcycling
properties. Multilayer thin films (individual layersy15 nm thick)
were sputtered from elemental targetsand annealed at 550uC for 1 h
in order to transform theirmultilayer structure into alloys. Figure
5 depicts thecomposition regions in which different precipitates
areformed.52 The relative intensity of one characteristic X-ray
diffraction peak belonging to the phase of interest wasplotted
colour-coded within a section of the NiTiHfternary phase diagram.
Four different precipitates, i.e.HfNi(Ti), Ti2Ni(Hf), Hf2Ni(Ti) and
Laves phase, wereconfirmed in Ni-lean composition regions. They
con-cluded that the observation of reversible phase transfor-mation
was limited by the formation of Ti2Ni(Hf),HfNi(Ti) and/or Hf2Ni(Ti)
precipitates. These precipi-tates restricted the transforming
region to compositionswith Ni contents abovey40 at-% and Hf
contents belowy30 at-%.The Ti2Ni(Hf) precipitates have also been
observed by
many other researchers in Ni-lean NiTiHf alloys.17,36,5355
It has been reported that the volume fraction of theTi2Ni(Hf)
precipitates decreased with increasing the Nicontent, although the
Ti2Ni(Hf) precipitates were stillobserved in slightly Ni-rich
compositions.17,23 Fine Ti2Ni(Hf) precipitates strengthen the
matrix and improveshape memory and superelastic properties of
NiTiHf-based alloys.36,53 The effects of aging temperature and
timeon the formation of Ti2Ni(Hf) precipitates were investi-gated
by Meng et al. in Ni49Ti36Hf15
53 and Ni44Ti36Hf15Cu5
36 alloys. The size of the precipitates increased withincreasing
aging temperature and time. Figure 6a and bshows the bright-field
transmission electron microscopy(TEM) images of the Ni44Ti36Hf15Cu5
ribbons annealedat 500 and 700uC for 1 h, respectively.36 According
to theselected area diffraction (SAD) pattern (Fig. 6c) takenfrom
the specimen annealed at 500uC, the precipitate was
3 Ms as a function of a ev/a and b cv in NiTiHf-based shape
memory alloys30,35,37,40,41,44,45
Karaca et al. NiTiHf-based shape memory alloys
Materials Science and Technology 2014 VOL 30 NO 13a 1533
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confirmed to be Ti2Ni(Hf). The diameter of the precipi-tates was
estimated to be 2040 nm when the annealingtemperature was 500uC.
After annealing at 700uC, thesize of the precipitates increased to
y150 nm. The fineprecipitates formed in the ribbon after annealing
at 500uCstrengthened the matrix and prohibited plastic
deforma-tion, which resulted in a perfect superelastic shape
recoveryafter deformation to 3?5% strain. On the other hand,
theribbon annealed at 700uC showed an incomplete shaperecovery due
to the lower strength of the matrix with largeprecipitates.
It is important to note that the size of the
Ti2Ni(Hf)precipitates are very effective to control the
martensitemorphology. It was found that (001)B199 compoundtwins
were dominant when the material containedhomogeneously distributed
Ti2Ni(Hf) precipitates with2040 nm in diameter (Fig. 6a). Similar
martensitemorphology has been observed in a Ti-rich NiTi thinfilm
with a homogeneous distribution of fine Ti2Niprecipitates.56 When
the annealing temperature was700uC, {011}B199 type I twins became
dominant andthe martensite variants showed mainly spear-like
andmosaic-like morphologies as shown in Fig. 6b. Marten-site
domains with (001)B199 compound twins were alsoobserved around the
coarse Ti2Ni(Hf) precipitates. Thespear-like and mosaic-like
morphologies have beenreported as typical morphologies of the
martensite inHf-added NiTi alloys.57,58
Ni-rich NiTiHf-based alloys
Meng et al.23,59 have reported that Ni4(Ti, Hf)3 pre-cipitates
were formed in Ni-rich NiTiHf alloys similar tothe Ni4Ti3
precipitation in NiTi binary alloys. However,recently, it has been
reported that a new precipitate whichhas a more complicated
structure than that of Ni4(Ti,Hf)3forms in Ni-rich NiTiHf
alloys24,25,60 and improvestheir shape memory and superelastic
properties due toprecipitation strengthening.26,27,33 Initially,
Han et al.61
reported a precipitate with a face-centred orthorhombiclattice
with a space group of F 2/d 2/d 2/d in an agedNi48?5Ti36?5Hf15.
There are six different variants in thisorthorhombic precipitate
with habit planes of (100)P//{001}B2 and long axes of
[001]P//,-110.B2. However, theydid not provide an atomic structure
model for theobserved precipitate.
Recently, Yang et al.25 proposed an atomic structuremodel which
contains of 192 atoms in an orthorhombicunit cell for the observed
precipitate in Ni-rich NiTiHfalloys. The orthorhombic precipitate
phase was namedas H-phase and Fig. 7a shows the unit cell of
thisprecipitate.25 In order to refine the structure model, abinitio
density functional theory calculations have alsobeen performed to
relax the structure model.24,25 Selectedarea diffraction patterns
obtained from a single large H-phase precipitate in a Ni52Ti28Hf20
alloy are shown inFig. 7bd.25 All the SAD patterns revealed the
orienta-tion dependence between the precipitate and austenite
B2
4 Lattice parameters a a, b b, c c and d b of B199 martensite as
a function of Hf in NiTiHf alloys47
Karaca et al. NiTiHf-based shape memory alloys
1534 Materials Science and Technology 2014 VOL 30 NO 13a
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phase (the diffraction spots are indexed according to
theaustenite phase). There were additional reflections at
1/3positions along ,110.B2
* in reciprocal space as shownby arrows, which was a
characteristic of the H-phase. Thecomposition of the proposed
H-phase was Ni50Ti16?7Hf33?3, whereas it has been indicated by
energy dispersivespectroscopy analysis that the Ni content of the
H-phaseprecipitate was always slightly richer than that of
thenominal composition of Ni-rich NiTiHf alloys in contrastto the
proposed Ni content of 50 at-% .24,25,62 Thereforethe formation of
H-phase precipitates depleted Ni fromthe matrix and increased TTs
as shown in Fig. 2. Yanget al.25 observed anti-site defects within
the precipi-tate which may slightly change the composition of
theprecipitate, and proposed that the H-phase did not have
a unique composition. The effects of the alloy composi-tion on
the H-phase precipitation were investigated bySantamarta et al.24
They concluded that the H-phaseprecipitates grew faster in alloys
with higher Ni contentsince the precipitates were richer in Ni
content comparedto the nominal composition of the alloys.
Similarly, for afixed Ni content, the growth of the H-phase became
fasterwhen the Hf content was increased.
The control of the size and interparticle distance of H-phase
precipitates is important to obtain good shapememory and
superelastic responses. It has been reportedthat the aging
temperature and time significantly affectedthe size and
interparticle distance of the precipitatesformed
inNi-richNiTiHf-based alloys.23,24,33 Figure 8acillustrates the
representative microstructure of Ni50?3Ti29?7
5 Composition regions in which different precipitate phases
exist. The relative intensity of an X-ray diffraction peak for
each phase is plotted colour-coded within a section of the
ternary NiTiHf diagram for a HfNi(Ti), b Ti2Ni(Hf), c
Hf2Ni(Ti), and d Laves phase (colour code: red5high;
green5medium; blue5low intensity)52 Figure 5 will be repro-
duced to be mono on the printed version
6 a typical bright-eld image of martensite in Ni44Ti36Hf15Cu5
ribbon annealed at 500uC for 1 h and SAD pattern takenfrom region
W, electron beam//[1-10]M,T; b typical martensite structure in the
ribbon annealed at 700uC for 1 h and theSAD pattern taken from
region D, electron beam//[2-11]M1,M2//[ -2 -11]M3; c SAD pattern
obtained from Ti2Ni(Hf) type preci-pitates formed in ribbon
annealed at 500uC for 1 h, electron beam//[110]Ti2Ni(Hf)
36
Karaca et al. NiTiHf-based shape memory alloys
Materials Science and Technology 2014 VOL 30 NO 13a 1535
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7 a unit cell of unrelaxed orthorhombic model of the H-phase.25
SAD patterns of the b [001]B2, c [1-11]B2, d [1-10]B2 and e[110]B2
zone axes obtained from a single large particle in a Ni52Ti28Hf20
alloy.
24 The small arrows and circles mark the
additional reections arising from the precipitate
8 Bright-eld images of the Ni50?3Ti29?7Hf20 alloy a extruded at
900uC, b aged at 550uC for 3 h and c aged at 650uC for3 h.27
Bright-eld images of the Ni45?3Ti29?7Hf20Pd5 alloy aged at d 550uC
and e 650uC for 3 h.
33 Inset in d is the enlar-
gement of area D. The SAD patterns shown in d and e were taken
from the area D and E, respectively
Karaca et al. NiTiHf-based shape memory alloys
1536 Materials Science and Technology 2014 VOL 30 NO 13a
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Hf20 alloys in as extruded and aged conditions.27 The
bright-field image of the as extruded Ni50?3Ti29?7Hf20alloy is
shown in Fig. 8a. Precipitate formation was notconfirmed in the as
extruded condition. Figure 8b and cshows TEMmicrographs of the
extruded Ni50?3Ti29?7Hf20alloy aged at 550 and 650uC for 3 h,
respectively. Fine andcoherent H-phase precipitates were formed in
the 550uCaged specimen. When the aging temperature increasedfrom
550 to 650uC, the precipitate size increased fromabout 20 to 4060
nm. The interparticle distance alsoincreased after aging at 650uC
for 3 h compared with the550uC for 3 h case. Figure 8d and e shows
the H-phaseprecipitates and B199 martensite in slightly
(NizPd)-richNi45?3Ti29?7Hf20Pd5 alloys in aged conditions.
33 The sizeof the spindle-shaped H-phase was increased when
theaging temperature was increased from 550 to 650uC.The fine and
coherent H-phase precipitates in the Ni50?3Ti29?7Hf20 and
Ni45?3Ti29?7Hf20Pd5 alloys aged at 550uCimproved the shape memory
and superelastic propertiesdue to precipitation strengthening.
However, the alloysaged at 650uC exhibited relatively poor shape
memoryand superelastic properties due to large precipitate
sizes.
The martensite morphology in Ni-rich NiTiHf-basedalloys is
affected by the size and interparticle distance ofH-phase
precipitates. The martensite variants in the asextruded
Ni50?3Ti29?7Hf20 alloy show spear-like mor-phology and high density
of twins can be seen inside themartensite plates (Fig. 8a). Han et
al.57,58 have reportedtwo types of martensite morphologies;
spear-like andmosaic-like in NiTiHf alloys and they also revealed
thateach martensite lath is consisted of (001)B199 compoundtwins.
If the precipitates were small and interparticledistance was short,
the growing martensite plates canabsorb all the precipitates during
growth as it can beseen in the 550uC aged Ni50?3Ti29?7Hf20 alloys
(Fig. 8b).The large martensite plates were related by the
{011}B199type I twinning mode, which was confirmed by the
SADpattern shown in Fig. 8b taken at the interface of theplates. It
should be noted that no internal twins wereobserved in the large
martensite plates in the 550uC agedspecimen. On the other hand,
when the precipitates werebig and interparticle distance was large,
martensiteplates can be formed between the precipitates and
thethickness of the plates was controlled by the
interparticledistance of the precipitates (Figs. 8ce). In
Ni45?3Ti29?7Hf20Pd5, the SAD patterns were taken from the area Dfor
the 550uC aged specimen (Fig. 8d) and from the areaE for the 650uC
aged specimen (Fig. 8e). It was revealed
that the main twinning mode observed in the martensitewas
(001)B199 compound twin in both aging conditions.It was suggested
that the internal twinning type was notaffected by the size of the
H-phase precipitates if themartensite plates are formed between the
precipitates.
Addition of Nb and Pd to NiTiHf alloys
In NiTiHf-based alloys, the lattice invariant shear (LIS)of the
martensitic transformation depends on the alloycomposition. The
(001)B199 compound twins have beenfrequently observed in martensite
plates and consideredas the LIS in NiTiHf alloys.57,58 However,
recently, it wasfound that the ,011.B199 type II twin was the LIS
in a(NizPd)-rich Ni45?3Ti39?7Hf10Pd5 alloy which was homo-genised
at 900uC followed by furnace cooling.30 TheNi45?3Ti39?7Hf10Pd5
alloy exhibited less hardening duringtransformation compared to a
Ni45?3Ti29?7Hf20Pd5 alloywhich has (001)B199 compound twins.
Figure 9a shows a bright-field TEM image for
theNi45?3Ti39?7Hf10Pd5 alloy
30 which consisted of two phases,B2 austenite and B199martensite
at room temperature. Inthe SAD pattern taken from the austenite
phase (Fig. 9b),there were diffuse streaks along the ,110.B2
* directionsin reciprocal space. The diffuse streaks could be
attributedto the formation of very small precipitates during the
slowfurnace cooling process from the homogenisation tem-perature.
Sandu et al.63 also observed similar diffusestreaks in an aged
Ni-rich NiTiZr alloy. The SAD patterntaken from the martensite
phase (Fig. 9c) indicated thatthe internal twins formed in the
martensite variants werethe ,011.B199 type II twins. Compared to
the (001)B199compound twin, lower density of twins is found when
theLIS is the,011.B199 type II twin. It is noted that the LISin
NiTi binary alloys is known as the ,011.B199 type IItwin and the
(001)B199 compound twin has been observedin NiTi alloys as a
deformation twin.64 The (001)B199compound twin has been also found
in nanocrystallineNiTi alloys65 and in aged Ni-rich NiTi alloys
with fineNi4Ti3 precipitates.
66 These results suggested that the LISin NiTiHf-based alloys
depends on the alloy compositionand the size and interparticle
distance of precipitates.
Kim et al.35 reported that addition of Nb to NiTiHfalloys causes
the formation of a soft Nb-rich b phase andimproves the cold
workability. The stability of shapememory properties is improved by
the precipitation of theb phase, although the shape recovery strain
decreases bythe addition of Nb. Figure 10 shows the
back-scatteredscanning electron images of (Ni49?5Ti35?5Hf15)Nb
alloys.
35
9 a bright-eld image of Ni45?3Ti39?7Hf10Pd5 alloy homogenised at
900uC followed by furnace cooling, b SAD pattern takenfrom B2
austenite phase and c SAD pattern taken from martensite phase
indicating B199 monoclinic structure30
Karaca et al. NiTiHf-based shape memory alloys
Materials Science and Technology 2014 VOL 30 NO 13a 1537
-
In Fig. 10a, the Ti2Ni type precipitate can be seen in
theNi49?5Ti35?5Hf15 ternary alloy with a slightly dark contrast.The
b phase, which appears white on the images, wasobserved even after
1% Nb addition (Fig. 10b), indicatingthat the solubility limit of
Nb in the matrix was less than1%. The amount of the b phase
increased with increasingNb content. When 15% Nb was added, it
exhibited a fullylamellar microstructure as shown in Fig. 10c,
which is a
characteristic of eutectic solidification. This fine
lamellarstructure strengthened the matrix and prohibited
plasticdeformation during transformation.
Morphologies of reoriented martensite andstress induced
martensiteAcar et al.67 have reported the morphology of
thereoriented martensite in a Ni45?3Ti34?7Hf15Pd5 alloy.
10 Back-scattered scanning electron images of a
Ni49?5Ti35?5Hf15, b (Ni49?5Ti35?5Hf15)Nb1 and c
(Ni49?5Ti35?5Hf15)Nb15 alloys35
11 Bright-eld image (TEM) of a as homogenised
Ni45?3Ti34?7Hf15Pd5 and b 8% deformed alloy with corresponding
SAD
pattern.67 Bright-eld image of Ni49Ti36Hf15 c deformed to 8% at
250uC and d deformed to 16% at 250uC.21 The SAD
pattern shown in c was taken from area II
Karaca et al. NiTiHf-based shape memory alloys
1538 Materials Science and Technology 2014 VOL 30 NO 13a
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Transmission electron microscopy observation was car-ried on a
homogenised sample after 8% compressivedeformation at 15uC (below
martensite finish tempera-ture, Mf). Figure 11a and b shows the TEM
micrographsobtained from the as homogenised and deformed
samples,respectively. There are fine twins in the martensite
platesin the as homogenised sample (Fig. 11a). In the
deformedsample (Fig. 11b), thicker martensite plates were formedby
the reorientation of martensite variants as compared tothe as
homogenised sample. The thick martensite platesare considered to be
favourable martensite variants understress. The inset in Fig. 11b
is an SAD pattern taken fromthe interface between the martensite
plates A and B. It wasrevealed that the fine twins in the
martensite plates are(001)B199 compound twins and the boundary
between theplates A and B is close to the {111}B199 type I twin
plane(so called {111}B199-type boundary).
36 It is consideredthat the {111}B199-type boundary can move
under stresswithout significant detwinning of fine (001)B199
compoundtwins in martensite plates.
Dalle et al.68 investigated the morphology of
reorientedmartensite of annealed (800uC for 1 h)
Ni49?8Ti42?2Hf8after 10% tensile deformation. They observed
finer(001)B199 compound twins in the deformed material com-pared to
the as annealed material with self-accommo-dated martensite. They
suggested that the detwinning ofthe (001)B199 compound twins is
difficult and proposedthat, instead of the detwinning, a
supplementary (001)B199mechanical twinning could take place during
deformationby a mechanism of the repetition of the dislocation slip
onthe (001)B199 plane.
Meng et al.20,21 investigated the morphologies of theSIM
inNi49Ti36Hf15 which were solution treated at 1000uCfor 1 h and
deformed in tension at 250uC. Figure 11cshows the typical
morphology of the preferentiallyoriented SIM variants and the SAD
pattern taken fromthe area II for the 8% deformed Ni49Ti36Hf15.
21 (001)B199compound twins were mainly observed in the SIM
plates.The SAD pattern revealed that the SIM plates were
twin-related with {011}B199 type I mode, which was similar tothe
thermally transformed martensite.57,58 The preferen-tially oriented
SIM variants were disappeared and severalmartensite variants were
intersected into each other after
deformation. Figure 11d shows the
variant-crashed/var-iant-intersected morphology after deformation
of 16%.The interfaces of the martensite variants are blurred in
thevariant-crashed/variant-intersected morphology. They notedthat
the stress induced martensitic transformation anddislocation slip
occurred simultaneously during loadingand suggested that the
introduction of dislocations in-creases the martensite variants
with the variant-crashed/variant-intersected morphology.
Mechanical behaviour of NiTiHf-basedshape memory alloysThe
relatively high degree of brittleness or poor cyclicstability in
NiTiHf alloys are the main obstacles fortheir commercial high
temperature applications. It hasbeen observed that ductility of
NiTiHf alloys could beimproved by deformation at higher
temperatures in Ni-lean NiTiHf alloys. Ni49Ti36Hf15 alloys failed
after 7% ofbending deformation at room temperature while they
didnot fracture until 30% tensile strain at 260uC.42,69
Material properties of NiTiHf-based alloys can becontrolled by
aging at different temperatures and time asillustrated in Fig. 12.
Meng et al.53 illustrated that yieldstrength of Ni49Ti36Hf15 can be
adjusted by aging at700uC while ductility was constant as shown in
Fig. 12a.The strength of matrix was improved after 20 h butfurther
increase in aging time decreased the strength ofalloy which can be
related to the size, interparticledistance and volume fraction of
Ti2Ni(Hf) precipitates.Figure 12b shows the hardness (HV) of
Ni50?3Ti29?7Hf20
and Ni45?3Ti29?7Hf20Pd5 alloys as a function of agingtemperature
for 3 h aging. The increase in the hardness inthe both alloys can
be attributed to formation of nano-size coherent precipitates that
minimises the dislocationmotion. The decrease in hardness at high
aging tempera-tures in the both alloy systems can be linked to
formationof semi-/non-coherent precipitates and larger
interparticledistance due to over-aging and thus the lack of
precipita-tion strengthening which was also demonstrated in Fig.
8.It is also clear that Ni45?3Ti29?7Hf20Pd5 is harder in naturewhen
is compared to Ni50?3Ti29?7Hf20.Initially, SMA properties of
Ni-lean NiTiHf alloys
were mainly investigated due to low TTs of Ni-rich
12 a effect of aging time on yield strength and elongation of
Ni49Ti36Hf1553 and b hardness values of Ni50?3Ti29?7Hf20 and
Ni45?3Ti29?7Hf20Pd5 alloys as a function of aging
temperature
Karaca et al. NiTiHf-based shape memory alloys
Materials Science and Technology 2014 VOL 30 NO 13a 1539
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NiTiHf alloys.23,59 Shape memory effect with 3% recover-able
strain or 80% recovery of 6% applied strain isobserved in
compression and bending while 80% reco-very of 2?5% applied tensile
strain is observed inNi49Ti36Hf15.
20,69 The poor shape memory effect isattributed to high stress
(y500 MPa) for martensitereorientation and high slope in SIM
transformation region(no plateau region observed) confirmed by
tensile experi-ments.69,70 Although no superelasticity is observed
in Ni-lean NiTiHf alloys,21,23 0?88% strain for two-way shapememory
effect has been observed.71 Unstable cyclicbehaviour is a major
problem in Ni-lean NiTiHf alloyswhere it has been observed that TTs
were decreased by40uC during stress free thermal cycling of
Ni49Ti41Hf10after 20 cycles.14
In Ni-rich NiTiHf alloys, almost perfect dimensionalstability
with 3% strain under a compressive stress of500 MPa was observed as
illustrated in Fig. 13a.26 It canbe seen from Fig. 13b that the
stressstrain curve ofsolutionised Ni-lean Ni49Ti36Hf15 at
temperature aboveAf showed high slope in SIM transformation region
anddeformation was not fully recovered upon unloadingwhich was
similar to that in cold worked TiNi,72 TiPd73
and as extruded or overaged Ni-rich NiTiHf27 alloys.Aging can
improve the shape memory and materialproperties of Ni-rich NiTiHf
alloys. Figure 13c showsthe superelasticity responses of as
extruded and agedNi50?3Ti29?7Hf20 alloys.
27 Perfect superelastic behaviourwith 4% recoverable strain was
revealed at 240uC afteraging at 550uC for 3 h in Ni50?3Ti29?7Hf20.
The improve-ment in superelastic behaviour with aging can be
attributedto the presence of coherent and fine H-phase
precipitates(as discussed in the microstructure part and shown
inFig. 8), which strengthen the matrix. Poor superelasticresponse
after aging at 650uC for 3 h can be attributed toloss of the
coherency of the coarsened precipitates. It isworth to note that
beside the fully recoverable strain, Ni-rich NiTiHf exhibited high
yield strength at hightemperature and the ClausiusClapeyron (CC)
slopeswere between 7 and 13 MPa uC21. It should also be notedthat
almost fully recoverable strain with small amount ofplastic
deformation under 1000 MPa with low plastic andperfect superelastic
behaviour was obtained in Ni-richNi50?3Ti29?7Hf20 along the [111]
orientation.
28
Another method to improve the shape memoryproperties of NiTiHf
has been the quaternary alloying.Recently, several studies29,3133
have revealed the effectsof Pd addition on the mechanical
properties of Ni50?3Ti29?7Hf20. It was shown that perfect
superelastic curves(with negligible plastic strain) at stress
levels as high as2 GPa were possible for aged polycrystalline
Ni45?3Ti29?7Hf20Pd5
33 for a temperature window of 50130uC asillustrated in Fig.
14a. However, full strain recovery wasnot observed in over aged
materials due to the formationof large precipitates as shown in
Fig. 8 and high degree ofhardening was observed during
transformation. Whenthe Hf content was decreased to 10% in
Ni45?3Ti39?7Hf10Pd5, high slope in SIM transformation region wasnot
observed that can be attributed to the formation ofdifferent type
and density of twinning as discussed in themicrostructure section
(see Fig. 9). It was mentionedbefore that the ,011.B199 type II
twin was the LIS ina (NizPd)-rich Ni45?3Ti39?7Hf10Pd5 alloy
30 in contrastto the (001)B199 compound twin in
Ni45?3Ti29?7Hf20Pd5alloys.33 Type II twins could be held
responsible for thelack of hardening in the transformation region
of Ni45?3Ti39?7Hf10Pd5 alloys containing 10% Hf compared
toNi45?3Ti29?7Hf20Pd5 alloys that has (001)B199 compoundtwins. The
growth ofmartensite variants with thin (001)B199compound twins is
more difficult in contrast to,011.B199
13 a thermal cycling under stress response of Ni-rich
Ni50?3Ti29?7Hf20,26 b superelastic behaviour of hot rolled
Ni-lean
Ni49Ti36Hf1520 and c superelastic behaviour of Ni-rich
Ni50?3Ti29?7Hf20
27
14 Stressstrain responses of a Ni45?3Ti29?7Hf20Pd533 and
b Ni45?3Ti39?7Hf10Pd530 at temperatures above Af
Karaca et al. NiTiHf-based shape memory alloys
1540 Materials Science and Technology 2014 VOL 30 NO 13a
-
type II twins. Thus, the required energy to complete theSIM
transformation increased and resulted in high slopein SIM
transformation region.Another alloying addition to NiTiHf alloys
has been
Cu where it generally improved the two-way shapememory effect
and thermal stability of NiTiHf alloyswhile decreased their
TTs.3638 It has been reported thatNi45?3Ti29?7Hf20Cu5 can recover
compressive strains ofy2?2% under 700 MPa at temperature above
100uC andcan produce 0?8% two-way shape memory strain attemperature
above 80uC.39 Perfect superelasticity responsewas not observed in
Ni45?3Ti29?7Hf20Cu5 due to its high CC slope (about 1425 MPa uC21)
and work hardeningcoefficient in addition to low yield stress for
plasticdeformation.39
It can be seen from Fig. 15 that the Ni-rich Ni50?3Ti29?7Hf20
alloys had higher TTs and strength than the Ni-leanNi49?5Ti35?5Hf15
alloys. This can be attributed to thedifference in precipitates
types where H-phase particleswere observed in Ni-rich and Ti2Ni(Hf)
precipitates wereformed in Ni-lean NiTiHf after aging. NiTiHfNb
hashigher transformation strain but lower strength than
Ni-rich NiTiHf. Also, as the Nb content was increasedthe TTs
were decreased and the shape memory propertiesof
(Ni49?5Ti30?5Hf15)Nb5 alloys became more stable as theplastic
strain was decreased due to the fine lamellarstructure that
strengthen the matrix as shown in Fig. 10.Moreover, the shape
recovery ratio was increased as Nbcontent increased. Addition of Pd
to Ni50?3Ti29?7Hf20decreased the TTs and transformation strain
while itimproved the strength of the alloy.
Cycling instability is a major concern of the NiTiHfalloys for
high temperature applications. Figure 16 showsthe thermal cycling
under 200 MPa experiments of Ni49?8Ti42?2Hf8 in homogenised and
equal channel angularextruded at 650uC conditions.18 It is clear
that severeplastic deformation improved the thermal cyclic
stabilityand decreased the thermal hysteresis of Ni-lean
Ni49?8Ti42?2Hf8 alloys. Moreover recoverable strain increasesand
irrecoverable strain decreases. Formation of preci-pitates
influences the cyclic degradation resistance inNiTiHf27 where small
coherent precipitates generallyimprove the thermal cyclic stability
while larger precipi-tates do not affect the stability.
Work output, damping capacity andpotential applications of
NiTiHf-basedalloysShape memory alloy based actuators can be
employed aslight weight and energy efficient alternatives of
hydraulicor pneumatic systems2 in automotive, aerospace
anddown-hole energy exploration industries. Alloys withhigher work
output values can be used to decrease therequired weight or size of
actuators.
The maximum work output levels of various NiTiHf-based SMAs as a
function of their average operatingtemperature range are shown in
Fig. 17a. Work outputcan be calculated as the mathematical
multiplication ofreversible transformation strain and applied
stress inconstant-stress thermal cycling experiments. NiTi
alloyshave work output densities of about 1218 J cm23,74
whileNiTiPd andNiTiPt alloys have work output capabilities of69 and
13 J cm23 respectively75 at temperatures above150uC. The work
output of Ni-rich NiTiHf polycrystallinealloys was found to be 1820
J cm23.27 Ni45?3Ti29?7Hf20Cu5 alloys can generate work outputs of
around 1415 J cm23 while NiTiHfNb alloys have work output
levels
15 Thermal cycling under constant compressive stress of
500 MPa results for NiTiHf-based SMAs27,33,35
16 Straintemperature response of Ni49?8Ti42?2Hf8 under 200 MPa a
homogenised and b equal channel angular extruded
at 650uC using route 2C under 200 MPa18
Karaca et al. NiTiHf-based shape memory alloys
Materials Science and Technology 2014 VOL 30 NO 13a 1541
-
of 1718 J cm23 above 100uC and 150uC, respectively.35,39
On the other hand, Ni45?3Ti29?7Hf20Pd5 alloys can generatehigher
work outputs of 3235 J cm23 (up to 120uC)compared to other
NiTiHf-based SMAs, while uppertemperature capability is somewhat
limited compared tothe above mentioned NiTiHf-based alloys.33
Figure 17b shows the damping capacities/absorbedenergies of
NiTi-based alloys as a function of transfor-mation stress. Damping
capacities can be calculatedfrom the area between the loading and
unloading curvesin a superelastic cycle and can be explained as the
abilityto repeatedly dissipate unwanted energy from a system.SMA
based mechanisms could be employed under highnumber of cycles in
real practical applications. Thus,stability of a superelastic curve
is essential for dampingapplications. A HTSMA that could absorb
large energywill be very appealing for high temperature
dampingdevices. In addition to the high work output, NiTiHf-based
SMAs have high damping capacities. They couldbe employed in
aircraft engines as a damper for acousticenergy and construction
for countering seismic move-ments in impact damping devices.The
damping capacity of Ni45?3Ti29?7Hf20Pd5 alloys is
3034 J cm23 stemming from its outstanding mechanicalhysteresis
(around 900 MPa) and good superelastic strainof 4%.33 In related
systems, the damping capacity is 16,1820, 38, and 54 J cm23 for
NiTi, NiTiHf, NiTiNb andNbTi/NiTi nanocomposites, respectively.7678
The Ni45?3Ti29?7Hf20Pd5 alloy has similar damping capability
toNiTiNb alloys that are often used in coupling applica-tions.
However, it should be noted that Ni45?3Ti29?7Hf20Pd5 alloys have
the ability to operate at much higherstresses (y2 GPa) than the
NiTiNb systems. Dampingcapacities of NiTiHfCu and NiTiHfNb were not
com-pared since full recoverable superelastic cycles have notbeen
reported in literature.
ConclusionsFrom the present review of NiTiHf-based alloys, it is
clearthat NiTiHf-based alloys are attractive candidates forhigh
temperature, high strength and damping applications.Their TTs and
strength can be adjusted by heat treatments.They could show perfect
superelasticity above 100uC andshape memory effect under high
stress levels. However,some of their drawbacks such as low
ductility, high slope in
SIM transformation region and low cyclic stability are
stillremained to be improved.
It has been shown that microstructural control bycomposition
alteration and aging is essential in tailoringshape memory and
mechanical properties (e.g. TTs,strain, hysteresis and strength) in
NiTiHf-based alloys.Based on the composition and precipitation
character-istics (e.g. precipitate size and interparticle
distance), themain microstructural features such as twin type,
marten-site morphology can be adjusted that would affect theshape
memory and mechanical properties. In Ni-leanNiTiHf-based alloys,
the size of the Ti2Ni(Hf) precipi-tates are effective to control
the martensite morphology.(001)B199 compound twins are dominant
when Ti2Ni(Hf)precipitates are small (about 2040 nm) and
homoge-neously distributed while {011}B199 type I twins
becomedominant with increasing the size of Ti2Ni(Hf) precipi-tates.
The martensite morphology in Ni-rich NiTiHf-based alloys is
affected by the size and interparticledistance of H-phase
precipitates. When the precipitatesare small and interparticle
distance is short, martensiteplates can absorb the precipitates
during their growth andthey are mainly twin-related with the
{011}B199 type Imode. On the other hand, when the precipitates are
bigand interparticle distance is large, martensite plates canbe
formed between the precipitates. The thickness of theplates is
governed by the interparticle distance of theprecipitates. The
formation of fine H-phase precipitatessignificantly improves the
shape memory and superelasticproperties due to precipitation
strengthening.
Pd addition decreases the TTs of NiTiHf alloys while thematrix
strength was increased by solid solution strengthen-ing.
Ni45?3Ti29?7Hf20Pd5 alloys can show perfect superelasticresponse
under extremely high compressive stress levels of2 GPa with
negligible plastic deformation. NiTiHf(Pd)alloys have high work
outputs and damping capacitiesreaching up to 3035 J cm23 owing to
their good strain,high strength and large mechanical hysteresis. Nb
additionto NiTiHf alloys improves the cold workability and
thestability of shape memory properties while decreases theshape
recovery strain by the precipitation of the b phase.Detailed
studies are needed to gain the fundamental
understanding on processingcompositionmicrostruc-tureproperty
relationships and reveal the true potentialof NiTiHf-based alloys.
Currently, they are the most
17 Comparisons of a work outputs and b damping capacities for
typical NiTi-based SMAs
Karaca et al. NiTiHf-based shape memory alloys
1542 Materials Science and Technology 2014 VOL 30 NO 13a
-
promising alloys for high temperature and
strengthapplications.
Acknowledgement
This work was supported by the NASA EPSCORprogram under grant
no. NNX11AQ31A.
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