Work-hardening mechanisms of the Ti Cu Ni Sn Nb ... · A pronounced work-hardening behav-ior after yielding is clearly visible. Fitting plastic defor-mation by a power-law equation

Work-hardening mechanisms of the Ti60Cu14Ni12Sn4Nb10nanocomposite alloy

Amadeu Concustella)

Departament de Fısica, Facultat de Ciencies, Edifici Cc, Universitat Autonoma de Barcelona,08193 Bellaterra, Barcelona, Spain

Jordi SortInstitucio Catalana de Recerca i Estudis Avancats and Departament de Fısica, Facultat de Ciencies,Edifici Cc, Universitat Autonoma de Barcelona, 08193 Bellaterra, Barcelona, Spain

Jordina FornellDepartament de Fısica, Facultat de Ciencies, Edifici Cc, Universitat Autonoma de Barcelona,08193 Bellaterra, Barcelona, Spain

Emma RossinyolServei de Microscopia, Facultat de Ciencies, Edifici Cs, Universitat Autonoma de Barcelona,08193 Bellaterra, Barcelona, Spain

Santiago SurinachDepartament de Fısica, Facultat de Ciencies, Edifici Cc, Universitat Autonoma de Barcelona,08193 Bellaterra, Barcelona, Spain

Annett GebertIFW Dresden, Institute for Metallic Materials, D-01171 Dresden, Germany

Jurgen Eckertb)

IFW Dresden, Institute for Complex Materials, D-01171 Dresden, Germany; and TU Dresden,Institute of Materials Science, D-01062 Dresden, Germany

M. Dolors BaroDepartament de Fısica, Facultat de Ciencies, Edifici Cc, Universitat Autonoma de Barcelona,08193 Bellaterra, Barcelona, Spain

(Received 14 November 2008; accepted 27 April 2009)

The work-hardening mechanisms of the Ti60Cu14Ni12Sn4Nb10 nanocomposite alloy werestudied. This material is composed of micrometer-sized dendrites embedded in ananostructured eutectic matrix and a CuTi2 intermetallic phase. Our study shows that, inthe as-quenched state, the nanostructured eutectic matrix behaves softer than thedendrites. During mechanical deformation, both the dendrites and the eutectic matrixharden, whereas the hardness of the CuTi2 intermetallic phase remains unaltered. Thehigh strength of the dendrites is caused by the interplay between solid solution hardeningand dislocation networks during plastic flow. Interestingly, the mechanical hardening ofthe nanoeutectic matrix is also assisted by a martensitic transformation of the NiTi phase.Transmission electron microscopy studies clearly show that the martensitictransformation of this phase is accompanied with grain size refinement, which also playsa role in the deformation-induced mechanical hardening.

I. INTRODUCTION

Composite materials consisting of micrometer-sizedgrains surrounded by a glassy or nanostructured matrixhave attracted much attention because of their appealing

combination of mechanical properties, i.e., high strength,typical of glassy and nanostructured materials, togetherwith enhanced plasticity.1–5 A recent example of glass-matrix composites studied by Hofmann et al.5 exhibitedtensile ductility exceeding 10%, yield strengths of 1.2–1.5 GPa, and toughness and fracture energy surpassingthose achievable in the toughest titanium or steel alloys.Moreover, the effects of a ductile toughening phase onflow and fracture behavior of semibrittle intermetallicsfor high-temperature applications have been studied in sys-tems such as Nb–Si6,7 and Mo–Si–B.8,9

a)Address all correspondence to this author.e-mail: [email protected]

b)This author was an editor of this journal during the review anddecision stage. For the JMR policy on review and publication ofmanuscripts authored by editors, please refer to http://www.mrs.org/jmr_policy

DOI: 10.1557/JMR.2009.0369

J. Mater. Res., Vol. 24, No. 10, Oct 2009 © 2009 Materials Research Society3146

Recently, a new family of low-density Ti-based com-posites prepared by Cu-mold casting has been reportedto exhibit yield strength >2.2 GPa and plasticity beyond15%.1 In these alloys, micrometer-sized b-Ti dendritesare surrounded by a nanostructured eutectic matrix, amicrostructure that yields the aforementioned good me-chanical properties. The volume fractions of the den-drites, the nanostructured matrix, and the nature of theconstituent phases play an important role in the plasticdeformation and hardening mechanisms of these Ti-based alloys.10 Higher dendrite volume fraction giveshigher fracture angles, lower yield strength, and in-creased plastic deformation.

Recently, Kim et al.11 studied the microscopic defor-mation mechanisms of a Ti66.1Nb13.9Ni4.8Cu8Sn7.2 (at.%)composite alloy with a high volume fraction of den-drites. The compressive stress–strain curve for this com-posite exhibits a continuous stress increase withincreasing strain, indicative of a continuous work hard-ening until failure. In the early stages of deformation, thework hardening of the material was attributed to arrestedslip bands at the interfaces between the dendrites and thenanostructured matrix. Further deformation provoked ro-tation of the dendrites and propagation of shear bandswithin them; however, shear bands were blocked as aresult of their interaction with the arrested slip bands. Athigher strains, transformation of the b-Ti phase into ophase with a plate-like morphology was observed. Final-ly, shear bands in the dendrites were only formedby amorphous and distorted b-Ti. Therefore, the harden-ing behavior of this alloy is attributed mainly to thedendrites, whereas the nanostructured matrix wasclaimed to provide the main contribution to the overallstrength.

Several studies of the deformation behavior in similaralloys with a higher volume fraction of nanostructuredmatrix also showed that the dendrites are mainly respon-sible for the work hardening of these alloys.12,13 Thedendrites work-harden by formation of dislocation net-works, and further deformation results in formation ofshear bands. On the other hand, it has been pointed outthat the nanostructured matrix work-softens during de-formation.12 This effect was mainly ascribed to itsnanocrystalline nature, but there is no microstructuralevidence for this behavior. In fact, previous nanoinden-tation experiments have shown that the nanostructuredeutectic matrix is actually softer than the dendrites in theas-quenched state, but it work-hardens under severeplastic deformation.14,15 Therefore, the determination ofthe constituent phases within the eutectic matrix and adetailed investigation of the deformation-induced micro-structural changes become necessary to further under-stand the mechanical properties of this family of alloys.

Woodcock et al.16 and previous work from theauthors14 showed that the eutectic regions are composed

of b-Ti rods mainly surrounded by cubic NiTi (B2), andin smaller quantities orthogonal NiTi (B19) and mono-clinic NiTi (B190). It is well known that near-equiatomicNiTi alloys are shape memory alloys, and therefore, thecrystal may transform from the above-mentioned austen-itic B2 phase to the martensitic B190. This transforma-tion can occur on cooling or under the influence of anapplied stress and consists in a coordinated “shear-like”distortion undergone by the atomic lattice.

This study uses nanoindentation to monitor the hard-ening mechanisms in each of the constituent phases ofthe Ti–base composite alloy. It is clearly shown that, inthe as-cast state, the dendrites are harder than the eutec-tic matrix. Furthermore, during mechanical deformation,both the dendrites and the eutectic matrix work-harden,whereas the hardness of the CuTi2 intermetallic phaseremains unaltered. The microscopic deformation mecha-nisms related to the hardening of the different constitu-ent phases have been identified and studied usingtransmission electron microscopy (TEM) and x-ray dif-fraction (XRD).

II. EXPERIMENTAL DETAILS

An alloy ingot with the composition Ti60Cu14-Ni12Sn4Nb10 (at.%) was prepared by arc-melting a mix-ture of the pure elements under argon atmosphere.Different cylinders of 3 mm in diameter and 50 mm inlength were obtained by Cu mold casting in an Ar atmo-sphere.

Macroscopic compression tests were carried out atroom temperature using an Instron 8562 testing machine(Norwood, MA) at an approximately constant strain rateof 1 � 10�4/s. Experiments were carried out using flat-faced dies of tungsten carbide (WC). The compressionsamples with an aspect ratio of 2:1 were preparedaccording to ASTM standards. A sample was loadeduntil fracture [see curve (a) in Fig. 1]. To study the

FIG. 1. Compressive true stress–true strain curves of the Ti60Cu14Ni12Sn4Nb10 composite alloy loaded on (a) fracture, (b) e = 12%,

(c) e = 9%, and (d) e = 5%.

A. Concustell et al.: Work-hardening mechanisms of the Ti60Cu14Ni12Sn4Nb10 nanocomposite alloy

J. Mater. Res., Vol. 24, No. 10, Oct 2009 3147

hardening mechanisms, different specimens were pre-pared by interrupting the mechanical tests at differentcompressive strain (e) levels: 12%, 9%, and 5%, corre-sponding to curves (b), (c), and (d), respectively, inFig. 1. The as-cast and deformed samples were examinedby XRD using Cu–Ka radiation. From the XRD patterns, adata analysis program based on a full pattern fitting proce-dure (Rietveld method) was used to show the presence ofseveral crystalline phases in the as-cast sample.17,18 More-over, the microstructures of as-cast and deformed sampleswere studied using a JEOL JSM-6300 scanning electronmicroscope (SEM; Tokyo, Japan). The samples wereprepared for SEM by grinding with SiC paper, final pol-ishing with a colloidal suspension of 0.04-mm SiO2 parti-cles mixed with H2O2 in the ratio 9:1, and etched in adilute aqueous solution of HF and H2O2 (100:5:2).

Hardening of the different constituent regions, i.e.,dendrites, eutectic regions, and CuTi2, was investigatedby means of nanoindentation using a sufficiently lowforce (1.5 mN) to ensure that the plastic region beneaththe indenter was hardly affected by the surroundingregions. The global hardness of the composite was eval-uated using a load of 500 mN. Careful mechanical pol-ishing of the as-cast and deformed samples was carriedout before nanoindentation tests. These tests were per-formed in laboratory air using a Nanoindenter XP with adiamond pyramidal-shaped Berkovich-type indenter.The experiments were run in load control mode at aconstant loading time of 15 s, applying the maximumforce during 30 s followed by unloading over a constanttime of 15 s. For the lowest applied force, a set of 100indentations was performed. The thermal drift was keptbelow �0.05 nm/s for all the experiments. The hardnessand the reduced elastic modulus (Er) were derived fromthese load-displacement curves using the method of Oli-ver and Pharr.19 The contact area between the diamondindenter and the specimen was calculated from a calibra-tion on a fused-quartz standard material. After nanoin-dentation tests, the specimens were observed by SEM toidentify each individual indent site, as detailed else-where.14

Thin foils for TEM imaging were prepared by ionbeam thinning using a Gatan PIPS. TEM observationswere carried out using a JEOL JEM-2011, equipped withenergy dispersive x-ray (EDX) analysis.

III. RESULTS

The true stress–true strain curve of the as-cast sampleunder compression [Fig. 1, curve (a)] shows excellentmechanical properties with rather low Young modulus(E = 75 GPa), high yield stress (sy = 1200 MPa), highfracture strength (sf = 1930 MPa), and high fracturestrain (ef = 13%). A pronounced work-hardening behav-ior after yielding is clearly visible. Fitting plastic defor-

mation by a power-law equation gives a strain hardeningexponent of 0.24, which is within the typical range formetallic materials.The microstructure of the as-cast alloy [Fig. 2(a)] is

composed of micrometer-sized dendrites, eutectic regions,and darker regions containing an intermetallic CuTi2phase. Previous studies14 have shown that the high cool-ing rates achieved during casting and the presence ofNb and Sn lead to a microstructure composed of b-Ti

FIG. 2. SEM secondary electron image of (a) as-cast alloy and

(b) sample loaded until fracture.


J. Mater. Res., Vol. 24, No. 10, Oct 20093148

micrometer-sized dendrites and metastable eutecticregions consisting of b-Ti rods embedded in NiTiphases. The atomic composition of the different regions,investigated by EDX, is shown in Table I. It should benoted that the b-Ti phase, particularly in the dendrites,contains a reasonable amount of Nb and Sn in dissolu-tion. The microstructure of the alloy compressed untilfracture [Fig. 2(b)] shows that cracks leading to fracturepercolate in the microstructure along the boundaries ofthe different regions, i.e., dendrites, eutectic matrix, andintermetallic phase. This damage mechanism may berelated to the difference in hardness between the inter-metallic phase and the b-Ti dendrites or eutectic matrix,similar to transformation-induced plasticity (TRIP)steels.20

Figure 3 shows the XRD patterns from the as-castsample and the sample compressed up to 12%. It isworth mentioning that the b-Ti reflections in the as-castsample are asymmetric. The asymmetry is caused by theslightly different lattice parameters of the b-Ti phasescomposing the dendrites and the eutectic rods,14 as alsoreported by Woodcook et al.16 from TEM studies. Fits(not shown) to the experimental data from the full-pattern fitting procedure show that the as-cast sample isalso composed of tetragonal CuTi2 (I4/mmm), cubicNiTi (B2), and small amounts of orthorhombic NiTi(B19) and monoclinic NiTi (B190) phases. After deform-ing the alloy up to 12%, the cubic NiTi reflectionscompletely disappear from the diffraction pattern[Fig. 3(b)]. However, the monoclinic NiTi reflectionsbecome more visible and intense. Furthermore, in thedeformed sample, there is broadening of the peaks cor-responding to all constituent phases. Increase of micro-strains and reduction of grain size are responsible for thisobserved broadening.21 The parent phase of all Ti–Niand Ti–Ni-based alloys has a cubic B2 (ordered BCC)structure. Depending on composition and thermome-chanical treatment, the B2 phase may take differenttransformation paths. B2–B190 transformation occurs inquenched Ti–Ni alloys; B2–R–B190 transformationoccurs in aged Ti–Ni (with Ti3Ni4 precipitation) orcold-worked Ti–Ni and in ternary Ti–Ni–Fe(Al) alloys;and B2–B19–B190 transformation occurs in Ti–Ni–Cualloys, which is the present case. As mentioned previous-ly,22 the end product of all martensitic transformationsin Ti–Ni-based alloys is B190 martensite. Therefore,

B190 martensite can be considered as the ground state ofall Ti–Ni and Ti–Ni-based alloys.

Figure 4(a) shows the evolution of hardness of thealloy on compression measured with a force of 500 mN.As already evidenced by the compression tests (seeFig. 1), the alloy work hardens with increasing compres-sive strain. Low-force nanoindentation measurementswere performed to investigate the hardening behavior ofthe individual constituent phases. In the as-cast state, thedendrites are harder than the eutectic matrix as shown inFig. 4(b). The intermetallic CuTi2 behaves as the hardestconstituent phase because dislocation movement is hin-dered by its complex structure. Figure 4(b) also showsthe evolution of hardness of each constituent duringcompression. Although CuTi2 remains with similar hard-ness during compression, the dendrites and eutectic ma-trix show work hardening. Interestingly, the eutectic

TABLE I. Atomic composition (evaluated from EDX analysis) of the

different phases composing the nanocomposite alloy.

Ti Cu Ni Sn Nb

Dendrites 58 10 3 5 23

Eutectic matrix 64 12 10 2 12

Eutectic rod 72 10 3 1 13

FIG. 3. (a) XRD patterns of Ti60Cu14Ni12Sn4Nb10 alloys in the as-

cast state or deformed to 12% strain. (b) Enlargement of the XRD

patterns showing that the diffraction peaks of the NiTi B2 phase

disappear after deformation. The main peaks are assigned to the dif-

ferent phases.



matrix hardens faster than the dendrites, both reachingsimilar hardness value at fracture.

The high stress level achieved during compression re-sulted in high deformation of the dendrites [Fig. 5(a)].Dark-field images of different sets of hkl planes [Figs. 5(b)

and 5(c)] show that the dark bands in Fig. 5(a) arecaused by the formation of bend-contours. Bend-contours occur when a particular set of diffracting planesis not parallel everywhere; the planes rock into, andthrough, the Bragg condition, leading to the formationof bend-contours.23 The high residual stresses accumu-lated in the dendrites may be responsible for the forma-tion of bend-contours. Furthermore, differences incontrast after the bend-contours and the observed distor-tions are provoked by the high amount of dislocationscreated during deformation.An example of the microstructure of the eutectic re-

gion in the as-cast state is shown in Fig. 6. The rod of b-Ti phase has a mean thickness of 65 nm. Low contrast isobserved in the matrix, which is mainly formed by theB2 NiTi phase, mainly because of defect contrast.Figure 7(a) is a bright-field TEM image of the eutectic

matrix in a specimen deformed up to 10%. The NiTiregions show grain refinement induced by deformation,as evidenced in Fig. 7(b), which is a dark-field image fromthe diffraction spot marked in the inset, corresponding toan interplanar distance of d = 2.1 � 0.1 A. Figure 7(c)shows a high-resolution TEM image (HRTEM) from aboundary between a b-Ti rod and NiTi matrix in a de-formed specimen. The filtered image clearly shows that ahigh amount of dislocations accumulates in the boundarybetween the b-Ti rod and the NiTi phase.

IV. DISCUSSION

A. Microstructure of the as-cast state

The microstructure of the alloy studied in this work, i.e.,micrometer-sized dendrites surrounded by a matrix com-posed of eutectic regions and a third phase, identified astetragonal CuTi2, is similar to previous studies on alloyswith similar compositions, also cast by rapid quenchingtechniques, i.e., Cu-mold casting.1,16 Nevertheless, al-though an agreement exists in the phase forming the den-drites, ascribed to b-Ti, some discrepancies are found inthe phases composing the eutectic matrix. According to

FIG. 4. (a) Dependence of the composite hardness on compressive

strain evaluated from indentations using a maximum load of 500 mN.

(b) Dependence of the hardness of the constituent phases on compressive

strain evaluated from indentations using a maximum load of 1.5 mN.

FIG. 5. (a) Bright-field image TEM image of a dendrite in a deformed sample to 10%, (b) dark-field image from the diffraction spot marked in

the selected-area diffraction pattern in the inset corresponding to the (211) planes, and (c) dark-field image from the diffraction spot marked in the

selected-area diffraction pattern in the inset corresponding to the (222) planes.



XRD analyses, in combination with SEM and TEMimages, and in agreement with Woodcock et al.16 andprevious work from the authors,14 b-Ti and NiTi B2 main-ly compose the eutectic regions, and in smaller quantity,NiTi B19 and NiTi B190. As it is well known, a martensitictransformation occurs from the cubic B2 phase to themonoclinic B190. XRD results show that, despite the highcooling rate, martensitic transformation of NiTi productphases (orthogonal and monoclinic) from the austeniticparent phase (cubic) was not completed. This is becauseof the fact that the thermally induced martensitic phasetransformation behavior is suppressed altogether for grains<60 nm.24 Finally, Woodcock et al.25 suggested thatthe tetragonal CuTi2 phase is the equilibrium phase in theequilibrium Ti60Cu14Ni12Sn6Ta10 alloy, whereas the for-mation of the metastable b-Ti–NiTi eutectic is favoredby both the high cooling rate and the addition of Nb andSn, which kinetically exclude the formation of the equilib-rium phase.

B. Deformation and work-hardening of theb-Ti dendrites

Although b-Ti dendrites work-harden during deforma-tion, their hardening is less pronounced than in the eutecticmatrix, as shown in Fig. 4(b). Solution of Nb and Sn in theb-Ti dendrites results in high hardness of this phase al-ready in the as-cast state.26 Broadening of the XRD peakscorresponding to b-Ti phase indicates that microstrains inthe dendrites may increase during deformation,21 whereasFig. 5 shows that plastic flow is accommodated through

dislocation networks, evidenced in the change of contrastand distortion of the bend contours. Despite the high de-formation observed inside the dendrites (Fig. 5), the lowwork-hardening rate of the dendrites is intrinsic to theirbody-centered cubic structure and its composition. Indeed,body-centered cubic crystals have a relatively low work-hardening rate at small strains mainly compared withStage 2 in face-centered cubic crystals.27 Furthermore,the initial stage of work hardening in solid-solution alloysis known to have a low work-hardening rate. Alloyingelements in solid solution raise the critical shear stress forglide on all possible systems proportionately, so higherlocalized stresses must be created compared with the puremetal to produce the patches of secondary slip. As a result,the initial solution hardening is likely to constrain thework hardening of the dendrites because of the increasein its yield stress. Studies in Ta alloys clearly showed thatan increase in the solute concentration (either intersitialor substitutional) reduced the rate of work hardening inStage 2 or the complete elimination of the separate stagesof work hardening.28 An increase in the interstitial concen-tration resulted in the grown-in dislocations becomingtightly locked. The addition of substitutional atoms lockedthe grown-in dislocation such that grown-in dislocationacted as barriers or reduced the number of operativesources on the secondary slip systems.

FIG. 7. (a) Bright-field TEM images of the nanostructured matrix in a

deformed sample to 10%, (b) dark-field image from the diffraction

spot marked in the selected-area diffraction pattern, and (c) high-

resolution TEM image from the boundary of a rod of b-Ti in the

eutectic matrix and the filtered image of the marked area using the

(101) planes of b-Ti.

FIG. 6. TEM image of the as-cast sample showing the eutectic matrix

formed by b-Ti and the B2 NiTi phase.



C. Strain hardening of the eutectic matrix

Nanoindentation experiments have shown that part ofthe strain hardening of the overall material is caused bythe strain hardening of the eutectic matrix. b-Ti rodsaccumulate dislocations in the grain boundaries, result-ing in hardening of the eutectic matrix. However, it isalso interesting to note that, according to XRD patterns,the alloy deformed up to 12% is almost depleted of theNiTi B2 phase and B190 diffraction peaks start to appear.In agreement with this observation, uniaxial tensile testsshowed that the superplastic plateau stress for nearlyequiatomic NiTi alloy is �500 MPa.29 The strains at theonset of the austenite to martensite transformation and atcompletion of this phase change are 0.015 and 0.045,respectively. Therefore, the eutectic matrix in these Ti-base composite materials may indeed undergo a marten-sitic transformation because the yield stress in thesealloys is higher than the plateau stress for martensitictransformation and fracture strain is larger than the com-pletion strain. This indicates that stress-induced marten-site phase transformation takes place during mechanicaltesting, a mechanism contributing to the overall strainhardening of the eutectic matrix.

Further evidence for this martensitic transformationcomes from recent nanoindentation experiments in NiTi,which have shown shape memory behavior for Berko-vich indentations <80 nm in depth,30 overcoming theenergy barrier necessary for martensite phase transfor-mation in nanocrystalline NiTi during mechanical defor-mation.

Furthermore, the high stresses reached during com-pression tests in the eutectic matrix lead to furthernanostructuring of the NiTi regions of the eutectic ma-trix, as shown in Fig. 7. Waitz et al.24 have shown thatsevere plastic deformation of the NiTi alloy leads to ananostructured material. The complex interaction amongslip dislocations, martensite, and mechanical twins in theNiTi phases may be responsible for the grain refinementto the nanoscale, similarly to a nanostructured b-Ti alloyprocessed by severe plastic deformation.31

Consequently, the enhanced flow strength of the NiTicrystals after deformation is driven by a fundamentalinteraction between the martensitic phase transformationand the dislocation behavior, which at the same timeleads to further nanostructuring of the NiTi phase of theeutectic matrix. Analogous behavior has been observedduring phase transformation in plastically deformed bulkTRIP steels,32 where stress-assisted martensitic transfor-mation induced during deformation at constant tempera-ture causes an increase in the strength.

V. CONCLUSIONS

The hardening behavior of the Ti60Cu14Ni12Sn6Nb10composite alloy has been studied by means of different

experimental techniques. The mechanical properties andthe contribution to the strain hardening of the differentconstituent phases have been successfully monitored bynanoindentation. The microscopic mechanisms responsi-ble for the deformation of the different constituentphases have been identified by XRD analyses and TEMstudies. Contrary to most previous studies where thedendrites were assumed to be intrinsically softer thanthe eutectic matrix, here we showed that the eutecticmatrix is actually softer than the dendrites, and it pro-gressively strain-hardens on plastic deformation.After yielding, the dendrites and the nanostructured

matrix suffer strain hardening, but the mechanisms aredifferent in both regions. On the other hand, the interme-tallic CuTi2 phase does not work-harden during defor-mation. In the nanostructured matrix, martensitic stress-induced transformation occurs during loading in theNiTi B2 phase, and further deformation provokes grainrefinement of the transformed martensitic NiTi B190phase. In contrast, dislocations are homogeneouslydistributed in the dendrites and become arrested at theinterfaces, causing the work hardening of the dendrites.Initial solid-solution hardening of the dendrites causedby Nb and Sn solution is responsible for the moderatework hardening of the b-Ti dendrites despite the highstresses achieved during compression.

ACKNOWLEDGMENTS

We thank Prof. A.L. Greer for fruitful discussions. Wealso acknowledge the technical support from the Micros-copy Service of the Universitat Autonoma de Barcelonaand from MATGAS. A.C. received financial supportfrom MICINN Grant EX-2007-0391. Partial financialsupport from DURSI (2005-SGR-00401), MEC (MAT2007-61629), EU (MRTN-CT-2003-504692), and theINTAS Project (03-51-3779) is also acknowledged.

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Work-hardening mechanisms of the Ti Cu Ni Sn Nb ... · A pronounced work-hardening behav-ior after yielding is clearly visible. Fitting plastic defor-mation by a power-law equation