The effect of precipitated disperse H-phase particles on the thermoelastic B2–B19′ martensitic transformation (MT) under compressive load has been studied on [001]-, [236]-, and [223]-oriented single crystals of Ni51.0Ti36.5Hf12.5 (at %) alloy in the initial (as-grown) state. It is established that, in Ni51.0Ti36.5Hf12.5 single crystals containing disperse H-phase particles with dimensions within 125–150 nm at a volume fraction of ~30%, neither the critical stresses of martensite formation nor superelasticity strain depend on the orientation. Fully reversible B2–B19′ MTs in Ni51.0Ti36.5Hf12.5 single crystals have been observed in tests at external axial stresses up to 1700 MPa and temperatures up to Tt ~ 373 K

Chumlyakov, Yuri I.

Gerstein, G.

Maier, Hans Jürgen

Panchenko, E. Yu.

Sehitoglu, Huseyin

Surikov, N. Yu.

Technical Physics Letters

English

E. Yu. Panchenko

Yu. I. Chumlyakov

N. Yu. Surikov

H. J. Maier

G. Gerstein

H. Sehitoglu

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Superelasticity in high-strength heterophase single crystals of Ni51.0Ti36.5Hf12.5 alloy

Tomsk State University Repository

ISSN 10637850, Technical Physics Letters, 2015, Vol. 41, No. 8, pp. 797–800. © Pleiades Publishing, Ltd., 2015.Original Russian Text © E.Yu. Panchenko, Yu.I. Chumlyakov, N.Yu. Surikov, H.J. Maier, G. Gerstein, H. Sehitoglu, 2015, published in Pis’ma v Zhurnal Tekhnicheskoi Fiziki,2015, Vol. 41, No. 16, pp. 68–76.797In recent years, much research attention has beendevoted to developing highstrength, hightemperature shape memory alloys for applications in car building and aerospace industries. The most promising candidate materials include ternary alloys of the Ni–Ti–Hf system capable of exhibiting thermoelastic B2–B19' martensitic transformations (MTs). In alloysdoped with Hf from 10 to 30 at % (instead of Ti in NiTialloys close to equiatomic compositions, CNi = 49.0–50.0 at %), the B2–B19' MT start temperature oncooling Ms increases to 525°C [1–3]. However, a lowstrength of B2 austenite in NiTiHf ternary alloys leadsto degradation of their functional properties duringthe MT process, thus restricting the field of possibleapplications. It has been suggested that an increase inthe Ni content to CNi ≥ 50.5 at % in NiTiHf alloys (aswell as in NiTi binary alloys [4]) would allow a highstrength state (with austenite plastic flow onset aboveG/100, where G is the shear modulus [5]) due to thedeviation of alloy composition from stoichiometricand the onset of dispersion hardening with precipitation of a large volume fraction (f > 7–8%) of coherentdisperse particles on aging. For effective control overthe B2–B19' MT temperatures and the functionalproperties of alloys, including the shape memoryeffect and superelasticity (SE), it is necessary to studythe influence of the microstructure of aged NiTiHfalloys on the features of thermoelastic B2–B19' MTdevelopment under conditions of cooling/heating andcompressive loading.In this context, we have studied asgrownNi51.0Ti36.5Hf12.5 (at %) single crystals with [001],[236], and [223] orientation for elucidating the possibility of reversible B2–B19' MTs at high stress levels(above 1000 MPa) and determining the orientationdependence of SE in compression strained samples.The use of single crystals for investigation excludes thegrainboundary segregation of disperse phase particlesand allows the influence of crystal orientation on thecritical stresses for martensite formation, reversiblestrain level, and SE temperature interval. Previously,such investigations were performed on single crystalsof NiTiHf alloys with Ni content below 50.5 at % [3].To the best of our knowledge, no data have beenreported in available literature on the orientationdependence of stressinduced MT development inNi51.0Ti36.5Hf12.5 single crystals.Single crystals of NiTiHf alloy with nominal composition Ni51.0Ti36.5Hf12.5 (at %) were grown in ahelium atmosphere using the Bridgman technique.The orientation of asgrown crystals was determinedby Xray diffraction on a DRON3 instrument usingFeKα radiation. The samples had the shape of parallelepipeds with dimensions 3 × 3 × 6 mm. The microstructure of single crystals was studied by transmissionelectron microscopy (TEM) on a JEOL 2010 instrument at an accelerating voltage of 200 kV. Mechanicaltests were performed on an Instron 5969 electromechanical testing machine at a stain rate of  = 4 ×10⎯4 s–1. Characteristic B2–B19' MT temperaturesMs = 190 ± 2 K (start of the forward transition on cooling) and Af = 235 ± 2 K (finish of the reverse transitionon heating) were determined from the temperaturedependence of electric resistivity. Investigation onε·Superelasticity in HighStrength Heterophase Single Crystals of Ni51.0Ti36.5Hf12.5 AlloyE. Yu. Panchenkoa *, Yu. I. Chumlyakova, N. Yu. Surikova, H. J. Maierb, G. Gersteinb, and H. Sehitogluca Tomsk State University, Tomsk, 634050 Russiab Institut für Werkstoffkunde, Leibniz Universität Hannover, 30823 Garbsen, Germanyc Department of Mechanical Science and Engineering, University of Illinois, Urbana, IL 61801, United States*email: panchenko@mail.tsu.ruReceived February 24, 2015Abstract—The effect of precipitated disperse Hphase particles on the thermoelastic B2–B19' martensitictransformation (MT) under compressive load has been studied on [001], [236], and [223]oriented singlecrystals of Ni51.0Ti36.5Hf12.5 (at %) alloy in the initial (asgrown) state. It is established that, inNi51.0Ti36.5Hf12.5 single crystals containing disperse Hphase particles with dimensions within 125–150 nmat a volume fraction of ~30%, neither the critical stresses of martensite formation nor superelasticity straindepend on the orientation. Fully reversible B2–B19' MTs in Ni51.0Ti36.5Hf12.5 single crystals have beenobserved in tests at external axial stresses up to 1700 MPa and temperatures up to Tt ~ 373 K.DOI: 10.1134/S1063785015080283798TECHNICAL PHYSICS LETTERS  Vol. 41  No. 8  2015PANCHENKO et al.a Supra VP55 scanning electron microscope equippedwith an energydispersive Xray (EDAX) microanalyzer showed that asgrown Ni51.0Ti36.5Hf12.5 singlecrystals contained a small volume fraction (<5%) ofHf and Tirich dendrites and micronsized hafniumoxide particles.It was established that asgrown Ni51.0Ti36.5Hf12.5single crystals contained a large volume fraction (up to30%) of precipitated Hphase (Fig. 1). The Hphaseparticles with Ni0.52Ti0.19Hf0.29 composition possess afacecentered orthorhombic lattice with parametersa = 1.264 nm, b = 0.882 nm, and c = 2608 nm [6, 7].The microdiffraction pattern exhibits satellites characteristic of Hphase particles in four crystallographicvariants: 1/4[210]B2 (two variants) and 1/3[110]B2 (twovariants). Particles precipitated on cooling in asgrown NiTiHf single crystals had the following parameters: length d = 125–150 nm, thickness b = 30–40nm, and interparticle distance λ = 35–100 nm.Figures 2 and 3 show, respectively, the dependencesof critical stresses σcr of martensite formation on testtemperature T and the typical stress–strain curves σ(ε)in the temperature interval ΔTSE of SE observation forNiTiHf single crystals oriented along [223], [236], and[001] directions. The critical stresses for B2–B19' MTonset at T > Ms increase in accordance with the Clapeyron–Clausius relation [4](1)dσcr/dT ΔS/εtr,–=where εtr is the trasnsformation strain and ΔS is theentropy change during the MT.The orientational dependence of α = dσcr/dTaccording to Eq. (1) is determined by the transformation strain εtr. Theoretical estimations of the maximum lattice strain resource εtr0 for B2–B19' MT (withallowance for the complete detwinning of B19' martensite) in compression strained singlephase NiTiHfcrystals with [223] and [236] orientations yield|εtr0|[236] = 8.0% and |εtr0|[223] = 7.0%, respectively.These values are significantly greater than that for[001] orientation, |εtr0|[001] = 1.0% [3]. Based on thesetheoretical estimations of εtr0, the value of α[001] =dσcr/dT for the crystal with [001] orientation must beseven to eight times as large as that for crystals orientedin the [223] and [236] directions. However, in contrastto singlephase NiTiHf singlecrystals and aged crystals with nanosized (d < 75 nm) Hphase particles [3],the experimental values of α = dσcr/dT = 8.3–10.1 MPa/K in heterophase Ni51.0Ti36.5Hf12.5 singlecrystals with all studied orientations are close (Fig. 2).This degeneracy of the orientational dependence ofcritical stresses for martensite formation is related tothe absence of the orientational dependence of reversible strain |εSE| for SE manifestations. The maximumvalue of this stress determined from σ(ε) curves incrystals with [223] and [236] orientations amounted to|εSE| = 1.4 ± 0.3% and was significantly lower than theoretical estimations of |εtr0| = 7–8% and close to theestimated value of |εSE| = 1.0 ± 0.3% for [001]orientedcrystals. An increase in the preset strain above |εSE|leads to elastic deformation of the stressinduced martensite, which is accompanied by a sharp increase inthe level of axial stresses and does not contribute to100 nm(a) (b)(c)100 nm200B2110B2000Fig. 1. Microstructure of asgrown Ni51.0Ti36.5Hf12.5 single crystals: (a, b) brightfield and darkfield TEM images,respectively; (c) the corresponding selected area diffraction pattern, zone axis is [001]B2; the darkfield image wasrecorded using the reflection indicated by the arrow in thediffraction pattern.16001400120010008006004002000450400350300250200T, K|σ|, MPaΔTstab ΔTSE8.3 MPa/K8.6 MPa/K10.1 MPa/KAfMs[236][223][001]Fig. 2. Temperature dependences of the critical stressesof martensite formation in compressionstrainedNi51.0Ti36.5Hf12.5 single crystals oriented along [001],[236], and [223] directions: (ΔTstab) temperature intervalof stressinduced martensite stabilization; (ΔTSE) temperature interval of SE.TECHNICAL PHYSICS LETTERS  Vol. 41  No. 8  2015SUPERELASTICITY IN HIGHSTRENGTH HETEROPHASE SINGLE CRYSTALS 799|εSE| (Fig. 3). Fully reversible thermoelastic B2–B19'MTs have been observed for external axial stresses upto 1700 MPa. At higher loading (σ > 1700 MPa), thedevelopment of MTs is accompanied by irreversiblestrain.The temperature interval of SE also weaklydepends on the orientation of heterophaseNi51.0Ti36.5Hf12.5 single crystals and amounts to ΔTSE ~75 K (Fig. 2). The development of thermoelastic MTin crystals of all orientations studied proceeds with ahigh strainhardening coefficient of θ = δσ/δε = (12–15) × 103 MPa and the magnitude of stress hysteresisreaches Δσ = 400–500 MPa (Fig. 3).Data for the temperature interval of Af < T < Af +60 K reveal the stabilization of stressinduced B19'martensite and the absence of reversible MTs uponunloading despite the fact that B19' martensite isunstable at T > Af (Fig. 2). This stabilization of stressinduced martensite is related to a high level of energydissipation during the stressinduced MTs, which ismanifested by a large value of stress hysteresis Δσ:(2)Here, σcr(Ms) is the critical stress at T = Ms and TSE1 isthe temperature of SE onset for which thermoelasticMTs are fully reversible upon unloading. For example,in crystals with [223] orientation, we have observedΔσ ≈ 500 MPa and σcr(Af) < 300 MPa. Therefore,stressinduced martensite is stabilized (Tstab = 60 K)and the SE is manifested at T > Af (TSE1 = Af + ΔTstab)and σcr(TSE1) > 500 MPa (Figs. 2 and 3).Small values of reversible strain in the SE state andthe absence of its orientational dependence in hetσcr TSE1( ) σcr Ms( ) TSE1 Ms–( )dσcr/dT+ Δσ.>=erophase Ni51.0Ti36.5Hf12.5 single crystals containingdisperse Hphase particles can be explained usinganalysis of the following factors. The first is a composition effect according to which the MT takes placeonly in the B2 matrix, while Hphase particles exhibitno transformation, so that(3)Here, |εtr1| is the theoretically calculated transformation stain of the composite and f ~ 30% is the volumefraction of Hphase particles. Estimations show thatthe maximum theoretical reversible strain in NiTiHfcrystals containing disperse Hphase particles is|εtr1|[236] = 5.6%, |εtr1|[236] = 4.9%, and |εtr1|[001] = 0.7%,so that this factor alone cannot account for small values of reversible strain and the absence of orientationaldependence of SE in composites studied.The second factor consists in the fact that, if distance λ between Hphase particles is below 60 nm, theaustenite phase between these particles can stabilize sothat the B2–B19' MT does not take place. Previously,this phenomenon of B2–B19' MT suppression hasbeen observed in NiTi polycrystals with average grainsize below 60 nm [8]. In the present work, nanocomposite crystals with Hphase particle sizes d > 100 nmand interparticle distances within λ = 35–100 nm didnot exhibit complete transformation of B2 austeniteinto B19' martensite. The regions with distancesbetween particles λ < 60 nm exhibited no MT transformation, which led to a decrease in the overall reversible strain in NiTiHf crystals containing disperseHphase particles.εtr1 εtr0 1 f–( ).=150010005000|σ|, MPa[223] [236] [100]8642 86420 86420 10|ε|, %|ε|, %|ε|, %ΔσTt = 298 K Tt = 298 K Tt = 298 KFig. 3. Stress–strain curves σ(ε) measured at Tt = 298 K in compressionstrained Ni51.0Ti36.5Hf12.5 single crystals oriented along[223], [236], and [001] directions.800TECHNICAL PHYSICS LETTERS  Vol. 41  No. 8  2015PANCHENKO et al.The third factor is that, in B2 austenite regions withλ > 60 nm, the Hphase particles have various orientations relative to applied external stresses and can act asthe sites of preferential martensite nucleation in several variants irrespective of the direction of crystalcompression axis. In the vicinity of the particle–matrix interface, the internal stress fields arising due tothe lattice misfit and a difference between elastic moduli of the particle and matrix lead to a local variant of“unoriented” martensite crystal that differs from themain variant formed under the action of externalstresses in martensite shear systems with a maximumSchmid factor [9]. The formation of this “unoriented”martensite and its hindered untwinning in heterophase single crystals is a physical reason for thedegeneracy of the orientational dependence of the values of |εSE| and critical stress σcr of martensite formation.The multivariant character of MT development instructurally inhomogeneous crystals described aboveleads to elastic energy relaxation and increased energydissipation during MT development as a result of variation of the interaction between martensite crystals.This is manifested by a growth in stresses necessary forthe MT development under load—that is, under conditions of high θ = δσ/δε values, wide stress hysteresisΔσ, and reduced (or completely degenerate) orientational dependence of the functional properties ofNiTiHf single crystals.Acknowledgments. This study was supported in partby the Russian Science Foundation, project no. 142900012.REFERENCES1. H. E. Karaca, S. M. Saghaian, B. Basaran, G. S. Bigelow, R. D. Noebe, and Y. I. Chumlyakov, ScriptaMater. 65, 577 (2011).2. D. Angst, P. Thoma, and M. Kao, J. Phys. IV 5, C8747(1995).3. S. M. Saghaian, H. E. Karaca, H. Tobe, M. Souri,R. Noebe, and Y. I. Chumlyakov, Acta Mater. 87, 128(2015).4. K. Otsuka and X. Ren, Progr. Mater. Sci. 50, 511(2005).5. T. Zhu and J. Li, Progr. Mater. Sci. 55, 710 (2010).6. R. Santamarta, R. Arrotyave, J. Pons, A. Evirgen,I. Karaman, H. E. Karaca, and R. D. Noebe, ActaMater. 61, 6191 (2013).7. F. Yang, D. R. Coughlin, P. J. Phillips, L. Yang,A. Devaraj, L. Kovarik, R. D. Noebe, and M. J. Mills,Acta Mater. 61, 3335 (2013).8. T. Waitz, V. Kazykhanov, and H. P. Karnthaler, ActaMater. 52, 137 (2004).9. Yu. Chumlyakov, I. Kireeva, E. Panchenko, V. Kirillov,E. Timofeeva, I. Kretinina, Yu. Danil’son, I. Karaman,H. Maier, and E. Cesari, Russ. Phys. J. 54 (8), 937(2012).Translated by P. Pozdeev

Superelasticity in high strength heterophase single crystals of Ni51.0Ti36.5Hf12.5 alloy

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Superelasticity in high strength heterophase single crystals of Ni51.0Ti36.5Hf12.5 alloy

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