Deformation in the P3 phase (R-phase) of NiTiFe was investigated by in situ neutron diffraction during compressive loading at cryogenic temperatures. At 216 K, upon loading the R-phase detwinned and subsequently underwent a reversible stress-induced transformation to the B19\u27 phase (martensite). At 92 K on the other hand, detwinning was suppressed and the stress-induced martensite formed did not transform back upon unloading. The experiments also directly observed a hitherto theoretically predicted B33 phase. Rietveld refinement of the neutron diffraction spectra were used to determine lattice parameters of the B33 and R-phases. Plane-specific elastic moduli were also determined for the R-phase

Bourke, M. A. M.

Krishnan, V. B.

Rathod, C. R.

Shmalo, S. B.

Vaidyanathan, R.

English

V. B. Krishnan

S. B. Shmalo

C. R. Rathod

M. A. M. Bourke

R. Vaidyanathan

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Applied Physics Letters

Low temperature deformation of the R-phase in a NiTiFe shape memory alloy

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University of Central Florida STARS Faculty Bibliography 2010s Faculty Bibliography 1-1-2010 Low temperature deformation of the R-phase in a NiTiFe shape memory alloy V. B. Krishnan University of Central Florida S. B. Shmalo University of Central Florida C. R. Rathod University of Central Florida M. A. M. Bourke R. Vaidyanathan University of Central Florida Find similar works at: https://stars.library.ucf.edu/facultybib2010 University of Central Florida Libraries http://library.ucf.edu This Article is brought to you for free and open access by the Faculty Bibliography at STARS. It has been accepted for inclusion in Faculty Bibliography 2010s by an authorized administrator of STARS. For more information, please contact STARS@ucf.edu. Recommended Citation Krishnan, V. B.; Shmalo, S. B.; Rathod, C. R.; Bourke, M. A. M.; and Vaidyanathan, R., "Low temperature deformation of the R-phase in a NiTiFe shape memory alloy" (2010). Faculty Bibliography 2010s. 388. https://stars.library.ucf.edu/facultybib2010/388 Appl. Phys. Lett. 97, 091910 (2010); https://doi.org/10.1063/1.3481694 97, 091910© 2010 American Institute of Physics.Low temperature deformation of the R-phase in a NiTiFe shape memory alloyCite as: Appl. Phys. Lett. 97, 091910 (2010); https://doi.org/10.1063/1.3481694Submitted: 16 February 2010 . Accepted: 03 August 2010 . Published Online: 03 September 2010V. B. Krishnan, S. B. Shmalo, C. R. Rathod, M. A. M. Bourke, and R. VaidyanathanARTICLES YOU MAY BE INTERESTED INA Low Hysteresis NiTiFe Shape Memory Alloy Based Thermal Conduction SwitchAIP Conference Proceedings 824, 3 (2006); https://doi.org/10.1063/1.2192327Elastocaloric effect in a textured polycrystalline Ni-Mn-In-Co metamagnetic shape memoryalloyApplied Physics Letters 105, 161905 (2014); https://doi.org/10.1063/1.4899147High cyclic stability of the elastocaloric effect in sputtered TiNiCu shape memory filmsApplied Physics Letters 101, 091903 (2012); https://doi.org/10.1063/1.4748307Low temperature deformation of the R-phase in a NiTiFe shapememory alloyV. B. Krishnan,1 S. B. Shmalo,1 C. R. Rathod,1 M. A. M. Bourke,2 and R. Vaidyanathan1,a1Department of Mechanical, Materials and Aerospace Engineering, Advanced Materials Processingand Analysis Center (AMPAC), University of Central Florida, Orlando, Florida 32816, USA2Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USAReceived 16 February 2010; accepted 3 August 2010; published online 3 September 2010Deformation in the P3 phase R-phase of NiTiFe was investigated by in situ neutron diffractionduring compressive loading at cryogenic temperatures. At 216 K, upon loading the R-phasedetwinned and subsequently underwent a reversible stress-induced transformation to the B19 phasemartensite. At 92 K on the other hand, detwinning was suppressed and the stress-inducedmartensite formed did not transform back upon unloading. The experiments also directly observeda hitherto theoretically predicted B33 phase. Rietveld refinement of the neutron diffraction spectrawere used to determine lattice parameters of the B33 and R-phases. Plane-specific elastic moduliwere also determined for the R-phase. © 2010 American Institute of Physics.doi:10.1063/1.3481694NiTiFe shape memory alloys are known to exhibit tem-perature and/or stress-induced phase transformations be-tween cubic B2, trigonal P3, and monoclinic B19 phasescommonly designated as austenite, R-phase, and martensite,respectively. The R-phase and martensite have the abilityto detwin and convert between their respective variantswhen deformed phenomena commonly referred to asdetwinning.1,2 For example, cooling a NiTiFe alloy that isaustenitic at room temperature can result in a phase transfor-mation to the R-phase which can subsequently be deformedto detwin and/or form stress-induced martensite. The reversetransformation from detwinned R-phase can occur againstexternal loads below room temperature making these alloysuseful as actuators, e.g., in low temperature thermalswitches. The R-phase transformation in NiTiFe is ofparticular interest owing to its stability and reducedhysteresis.3,4 Macroscopic measurements cannot differentiatebetween elastic and inelastic strains associated with detwin-ning or stress-induced transformation. Therefore, knowledgeon deformation in the R-phase is currently limited especiallywith respect to its elastic behavior and the competition be-tween detwinning and the formation of stress-induced mar-tensite. Furthermore, the reversibility of the R-phase trans-formation to martensite and the formation of any additionalstress-induced phases have not been investigated.With the objective of providing insight into the afore-mentioned issues, in situ neutron diffraction during coolingand mechanical loading was performed on a NiTiFe sampleat the spallation neutron source at Los Alamos NationalLaboratory. This technique has previously provided valuableinsight into the strain, texture, and phase fraction evolutionin bulk shape memory alloy specimens e.g., Refs. 5–7.However, the capability to perform in situ neutron diffractionduring loading and cooling has only recently beenimplemented.8 Here it was used to make in situ neutron dif-fraction experiments on a Ni46.8Ti50Fe3.2 alloy at a minimumtemperature of 92 K and a maximum compressive stress of700 MPa.Cylindrical compression samples 10 mm in diameterand 24 mm length were fabricated by electrical dischargemachining from a vacuum-induction-melted and vacuum-arc-remelted billet. The samples were solutionized at 1023 Kfor 1 h in a vacuum furnace and subsequently oil quenchedto room temperature. The R-phase start, R-phase finish, aus-tenite start, and austenite finish, were determined by differ-ential scanning calorimetry to be 236 K, 223 K, 227 K, and2392 K, respectively. The martensite start temperaturewas undetectable while cooling the sample to 20 K. Mea-surements were performed in “time-of-flight” mode using thecapability for neutron diffraction during cooling and me-chanical loading on the Spectrometer for Materials Researchat Temperature and Stress Ref. 9 at Los Alamos NationalLaboratory. Measurements were made on two samples. Onesample was cooled to 216 K and mechanically loaded andunloaded to a maximum stress of 700 MPa while neutronspectra were recorded at selected stresses. A second identicalsample was cooled to 92 K and mechanically loaded andunloaded to a maximum stress of 420 MPa while neutronspectra were again acquired at selected stresses.Upon cooling from room temperature both samples un-derwent a phase transformation from austenite to R-phase.Figure 1 shows a section of neutron spectra obtained fromthe samples at 300, 216, and 92 K in the nominally unloadedcondition holding stress of 8 MPa. At 300 K, the 110austenitic peak is seen which subsequently split with coolingto 300 and 112 R-phase peaks as a result of elongationalong the 111	 crystallographic direction of the B2 phase.10The splitting increased with cooling due to a change in thetrigonal angle of the R-phase unit cell.11 Similarly, splittingof the austenite 210 peak to R-phase 303 and 411 peakswere observed. The peaks were identified based on a=3.01120.0002 Å for the cubic austenite phase at 300 Kand a=b=7.3004 c=5.2878 0.0002 Å for the trigonalR-phase at 92 K in the unloaded condition as determined byRietveld refinement of neutron spectra12.Compressive loading of the R-phase at 216 and 92 Kproduced different results. On examining a section of neu-tron spectra corresponding to 111 R-phase and 011 mar-aElectronic mail: raj@mail.ucf.edu.APPLIED PHYSICS LETTERS 97, 091910 20100003-6951/2010/979/091910/3/$30.00 © 2010 American Institute of Physics97, 091910-1tensite peaks acquired during loading and unloading up to700 MPa at 216 K in Fig. 2a, a stress-induced R-phase tomartensite phase transformation was observed during load-ing as the 111 R-phase peak decreased in intensity and the011 martensite peak increased in intensity. Upon unloading,the martensite reverted to the R-phase as seen in the reversetrend in the 111 R-phase and 011 martensite peaks. Incontrast, for the sample cooled to 92 K, the stress-inducedmartensite formed from R-phase upon loading remained inthe nominally unloaded condition Fig. 2b. This differencein behavior between testing at 216 and 92 K is attributed tothe thermodynamic hysteresis typically associated with mar-tensitic transformations wherein the reverse transformationto R-phase from martensite occurs at higher temperatureshigher than 92 K in this case.13 Furthermore, as expectedthermodynamically, martensite is favored at lower tempera-tures resulting in stress-induced martensite forming at around65 MPa at 92 K and at around 450 MPa at 216 K.The solid lines in Figs. 3a and 3b are presented toinvestigate detwinning in the R-phase and correspond to theratio of normalized intensities of individual R-phase diffrac-tion peaks with respect to the unloaded state. The informa-tion is presented for peaks selected for the statistical qualityof their fits as previously established during the analysis ofneutron diffraction spectra acquired in situ during loading inNiTi.12 The observation that certain reflections increase innormalized intensity with loading and certain reflections de-crease in normalized intensity with loading up to 400 MPa at216 K Fig. 3a is indicative of texture evolution associatedwith detwinning in the R-phase at 216 K. In contrast, Fig.3b which represents deformation in the R-phase at 92 Kdoes not show this trend and the intensities remained around2.1 2.11 2.12 2.13 2.14 2.15 2.1601020304050normalizedintensity(arbitraryunits)R-phase at 216 KR-phase at 92 Kd-spacing (Å)B2 phase at 300 K{300} {112}{300} {112}{110}FIG. 1. Color online Section of normalized neutron diffraction spectraacquired at 300 K, 216 K, and 92 K, respectively, showing phase transfor-mation from cubic austenite B2 to trigonal R-phase in NiTiFe. The reflec-tions are from lattice planes perpendicular to the length of the sample.2.95 3 3.05 3.1 3.15020406080100120140normalizedintensity(arbitraryunits)30 MPa50 MPa30 MPa100 MPa200 MPa300 MPa390 MPa400 MPa450 MPa500 MPa600 MPa700 MPa600 MPa300 MPa150 MPad-spacing (Å){111}R-phase{011}martensit eloadingunloading2.95 3 3.05 3.1 3.150100200300400500600d-spacing (Å)125 MPa8 MPa68 MPa90 MPa425 MPa225 MPa350 MPa286 MPa350 MPa175 MPa227 MPa210 MPa155 MPa160 MPa114 MPa152 MPa130 MPa8 MPanormalizedintensity(arbitraryunits){111}R-phase{011}martensiteloadingunloading92 K216 KFIG. 2. Color online Section of normalized neutron diffraction spectraacquired during loading and unloading at a 216 and b 92 K in NiTiFe. At216 K the R-phase reversed from martensite upon unloading but not at 92 K.The reflections are from lattice planes perpendicular to the length of thesample.0.60.70.80.911.11.21.30 50 100 150 200 250 300 350 400{111}{112}{300}{222}{303}{411}normalizedintensity(arbitraryunits)compressive stress (MPa)216 K92 K-0.001-0.0008-0.0006-0.0004-0.000200.00020.00040.50.60.70.80.911.11.20 10 20 30 40 50 60 70 80{111}{222}{411}{322}{333}{111}{222}{411}{322}{333}strainnormalizedintensity(arbitraryunits)compressive stress (MPa)intensity (left axis)strain(right axis)FIG. 3. Color online The solid lines are the normalized R-phase peakintensity ratios relative to the unloaded state with loading at a 216 andb 92 K. The dashed lines in b are the strains determined from changes ind-spacing of various R-phase reflections at 92 K. The reflections in a arefrom lattice planes parallel and in b perpendicular to the length of thesample and are, respectively, selected for clarity and in order to determineplane-specific moduli.091910-2 Krishnan et al. Appl. Phys. Lett. 97, 091910 2010the normalized valued of unity until about 65 MPa. Atstresses higher than 65 MPa, the R-phase underwent a stress-induced transformation to martensite, and hence a decreasein the normalized intensities of the various reflections wasobserved. Thus at 216 K and at stresses below 500 MPa, thefree energy of the R-phase is lower than that of the mono-clinic phase and hence detwinning is favored as the deforma-tion mode to accommodate stress. However, at stressesabove 500 MPa the system lowers its energy by undergoinga stress-induced transformation to martensite, which revertsto the R-phase upon unloading Fig. 2a. At 92 K on theother hand, detwinning is suppressed and a direct stress-induced transformation to martensite is observed that doesnot revert due to the thermal hysteresis associated with mar-tensitic transformations. The fact that detwinning is limitedat stresses lower than 65 MPa at 92 K enabled the compari-son of the macroscopic modulus determined from extensom-etry with the plane-specific elastic moduli determined fromchanges in d-spacing relative to a stress free d-spacing withstress.12 The dashed lines in Fig. 3b represent these strainswith increasing stress for various R-phase planes. The modu-lus from extensometry was determined to be 90.9 GPa andcompared well with the values of 95.6 GPa, 113.8 GPa, and105.2 GPa for the 111, 322, and 411 planes, respec-tively. At stresses above 65 MPa, the onset of stress-inducedtransformation results in redistribution of strain among vari-ous grains resulting in a nonlinear and anisotropicresponse.7,12Figure 4 shows a section of neutron spectra recordedfrom the sample at 92 K as it is loaded from the nominallyunloaded condition 8 MPa to 210 MPa. The spectra havebeen normalized and are overlaid. The maximum value ofthe x-axis corresponds to that of the d-spacing of the 111R-phase peak on the right and the minimum value to that ofthe 100 martensite peak on the left. As the sample is loadeda peak appears in the middle and shifts with stress. The shiftif hypothetically attributed to elastic straining due to appliedstress results in an unrealistic modulus of 6.3 GPa. The po-sition of this reflection is consistent with that of the 012reflection in a base-centered orthorhombic B33 NiTi struc-ture space group Cmcm predicted from density functionaltheory calculations.14 This structure does not possess aunique crystallographic relationship with the parent phaseand is expected to only exist in the absence of shear andhydrostatic stresses applied or residual. The experimentallyobserved shifting is thus attributed to a change in the mono-clinic angle to 98° as the symmetry changes from B33 tothat of B19 with stress. Rietveld refinement of the acquiredspectra resulted in the following lattice parameters for theB33 phase: a=2.911, b=9.393, c=3.806, and 0.001 Å===90° which compared well with the aforementionedtheoretical predictions.In summary, by recourse to capability of neutron diffrac-tion during loading at cryogenic temperatures at a spallationneutron source insight is provided into deformation in thetrigonal R-phase in bulk shape memory NiTiFe. At 216 K,there is direct evidence of detwinning in the R-phase fol-lowed by a reversible stress-induced martensitic transforma-tion as the R-phase transforms to martensite and transformsback upon unloading. At 92 K, however, detwinning is sup-pressed and the stress-induced martensite formed does notrevert to the R-phase due to hysteresis in the transformation.The elastic modulus of the R-phase from atomic measure-ments was consistent with that from extensometry. A nascentpeak was observed during loading of the R-phase at 92 Kthat shifted with loading and was attributed to that of the B33martensite, a phase that hitherto has only been predicted intheory. These findings have practical implications for usingthe R-phase in cryogenic actuator applications, wherein alimiting temperature exists below which detwinning is sup-pressed in the R-phase as was seen here at 92 K, therebynot providing recoverable strains necessary for the alloy toexhibit shape memory behavior.R.V. acknowledges funding from NSF CAREER underGrant No. DMR-0239512, NASA Grant No. NAG3-2751and SRI. The authors thank B. Clausen, D. Brown, and T.Sisneros at LANL for experimental assistance. This work hasbenefited from the use of the Lujan Neutron Scattering Cen-ter at LANSCE, which is funded by the Office of Basic En-ergy Sciences DOE. LANL is operated by Los Alamos Na-tional Security LLC under DOE Contract No. DE-AC52-06NA25396.1K. Otsuka and X. Ren, Prog. Mater. Sci. 50, 511 2005.2V. B. Krishnan, R. M. Manjeri, B. Clausen, D. W. Brown, and R. Vaidy-anathan, Mater. Sci. Eng., A 481–482, 3 2008.3J. L. Lemanski, V. B. Krishnan, R. Mahadevan Manjeri, W. U. Notardo-nato, and R. Vaidyanathan, Adv. Cryog. Eng. 52A, 3 2006.4V. B. Krishnan, C. Bewerse, W. U. Notardonato, and R. Vaidyanathan,Adv. Cryog. Eng. 54, 3 2008.5S. Rajagopalan, A. L. Little, M. A. M. Bourke, and R. Vaidyanathan,Appl. Phys. Lett. 86, 081901 2005.6C. R. Rathod, B. Clausen, M. A. M. Bourke, and R. Vaidyanathan, Appl.Phys. Lett. 88, 201919 2006.7S. Qiu, V. B. Krishnan, S. A. Padula II, R. D. Noebe, D. W. Brown, B.Clausen, and R. Vaidyanathan, Appl. Phys. Lett. 95, 141906 2009.8T. R. Woodruff, V. B. Krishnan, B. Clausen, T. Sisneros, V. Livescu, D. W.Brown, M. A. M. Bourke, and R. Vaidyanathan, Rev. Sci. Instrum. 81,063903 2010.9M. A. M. Bourke, D. C. Dunand, and E. Ustundag, Appl. Phys. A: Mater.Sci. Process. 74, s1707 2002.10T. Goryczka and H. Morawiec, J. Alloys Compd. 367, 137 2004.11K. F. Hane and T. W. Shield, Mater. Sci. Eng., A 291, 147 2000.12R. Vaidyanathan, M. A. M. Bourke, and D. C. Dunand, J. Appl. Phys. 86,3020 1999.13R. J. Salzbrenner and M. Cohen, Acta Metall. 27, 739 1979.14X. Huang, G. J. Ackland, and K. M. Rabe, Nature Mater. 2, 307 2003.2.88 2.9 2.92 2.94 2.96 2.988 MPa68 MPa90 MPa114 MPa132 MPa152 MPa155 MPa160 MPa175 MPa210 MPa0.40.60.811.21.41.61.82d-spacing (Å)R-phase{111}martensite{100}normalizedintensity(arbitraryunits)FIG. 4. Color online Section of normalized neutron diffraction spectraacquired at 92 K during loading, showing stress-induced phase transforma-tion from R-phase to B19 martensite. The 012 peak of an hitherto theo-retically predicted B33 martensite is observed.091910-3 Krishnan et al. Appl. Phys. Lett. 97, 091910 2010

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Low temperature deformation of the R-phase in a NiTiFe shape memory alloy

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