Ando  Keisuke

Ishida  Kiyohito

Ito  Wataru

Kainuma  Ryosuke

Oikawa  Katsunari

Omori  Toshihiro

MATERIALS TRANSACTIONS

English

Omori, Toshihiro

Ito, Wataru

Ando, Keisuke

Oikawa, Katsunari

Kainuma, Ryosuke

Ishida, Kiyohito

Tohoku University Repository (TOUR) / 東北大学機関リポジトリ

FCC/HCP Martensitic Transformation andHigh-Temperature Shape Memory Properties inCo-Si Alloys著者 Omori  Toshihiro, Ito  Wataru, Ando  Keisuke,Oikawa  Katsunari, Kainuma  Ryosuke, Ishida Kiyohitojournal orpublication titleMaterials Transactionsvolume 47number 9page range 2377-2380year 2006URL http://hdl.handle.net/10097/52049FCC/HCP Martensitic Transformation and High-TemperatureShape Memory Properties in Co-Si AlloysToshihiro Omori1, Wataru Ito2;*, Keisuke Ando2;*, Katsunari Oikawa2,Ryosuke Kainuma3 and Kiyohito Ishida21Tohoku University Biomedical Engineering Research Organization, TUBERO Aoba-yama Materials Science Branch,Sendai 980-8579, Japan2Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan3Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, JapanThe microstructure, martensitic transformation temperatures and shape memory properties of Co-Si binary alloys were investigatedby means of optical and transmission electron microscopy, diﬀerential scanning calorimetry and a shape memory test. The " martensite phasewith a hexagonal close-packed (hcp) structure was observed to coexist with the face-centered cubic (fcc)  parent phase at room temperature.The ="martensitic transformation temperatures linearly increased with Si content up to 4mol%, but the trend ﬂattened beyond 4mol%Si whilethe transformation hysteresis and intervals monotonously increased. The Co-Si alloys exhibited shape recovery at high temperatures up to 900Cand a relatively high thermal stability up to 600C. These results suggest that Co-Si alloy system has a possibility for high-temperature shapememory alloys. [doi:10.2320/matertrans.47.2377](Received March 23, 2006; Accepted July 3, 2006; Published September 15, 2006)Keywords: cobalt-silicon, " martensite, martensitic transformation, shape memory eﬀect, high-temperature shape memory alloy1. IntroductionSince AuCd alloys were revealed to exhibit a shapememory phenomenon,1,2) many sorts of shape memory alloys(SMAs) have been reported.3) Although TiNi alloys, whichare located as the most important alloy system for practicalapplications, have been utilized for pipe couplings, sensors,medical uses, etc., TiNi-based commercial SMAs are limitedfor the use at temperatures below 100C. High-temperatureshape memory alloys (HTSMAs), which can operate attemperatures higher than 100C, are currently receivingmuch attention due to their potential applications in theautomotive industry and other ﬁelds. To date, manyHTSMAs, such as TiNi(Hf, Zr, Pd, Pt), NiAl, NiMnGa, Zr-based, NbRu and Co-based alloys, have been reported.4)It is known that the  phase with a face-centered cubic(fcc) structure at high temperatures transforms to the "martensite phase with a hexagonal close-packed (hcp)structure during cooling by the movement of 1=6h112ipartial dislocations on every alternate {111} planes at about400C in pure Co, and several reports on the shape memoryeﬀect (SME) in Co, Co-Ni and other Co alloys have beenpublished.5–8) Recently, the present authors have found thatferromagnetic Co-Al alloys exhibit a good SME at temper-atures higher than 200C associated with the =" martensitictransformation,9,10) which means that Co-based alloys arepromising candidates for HTSMAs. Although an additionof Al is eﬀective to enhance the SME in Co alloys,9) themartensitic transformation temperatures decrease by theaddition of Al. On the other hand, Si addition is a suitablechoice for development of HTSMAs operating at highertemperatures because the =" transformation temperaturesincrease with Si content in the Co-Si alloys.11) However, theshape memory properties in Co-Si alloys have not beenreported. In this study, the martensitic transformation and theshape memory properties of Co-Si alloys were investigated.2. ExperimentalPure Co and Co-Si alloys with nominal compositions ofCo96Si4, Co92Si8 and Co86Si14 were prepared by an inductionfurnace under an argon atmosphere from pure cobalt (99.9%)and silicon (99.999%). Sheet specimens were obtained byhot-rolling at 1200C and subsequent cold-rolling. Pieces ofthe specimens were solution treated at 1200C for 1 hour in avacuum followed by quenching in water. The microstructurewas observed using optical microscopy (OM) and trans-mission electron microscopy (TEM). A thin foil specimen forTEM observations was prepared by jet polishing in a solutionof 20% perchloric acid and 80% ethanol.The martensitic transformation temperatures (Ms, Mf , Asand Af ) were measured by diﬀerential scanning calorimetry(DSC) at heating and cooling rates of 10C/min, where thetransformation temperatures were deﬁned as the point ofintersection of the base line and the tangent line with themaximum or minimum inclination of transformation peaks inthe DSC curves.The SME was evaluated by bending a rectangular speci-men of 0:24 4 50mm3 a round shape to a surface strainof 1.2% at room temperature. The reason why the appliedstrain is relatively small is the limited ability of shaperecovery in Co and Co-Si alloys. The shape with a surfacestrain, "c, after unloading was recorded. The specimen wasthen heated by putting it into a furnace at a constanttemperature of 1000C in the air, and keeping it for about 5seconds. The shape was recorded immediately after takingout the specimen from the furnace and the residual surfacestrain, "h, was evaluated from the curvature. It was conﬁrmedby comparing a shape recovery in the air with that in avacuum that it is hardly inﬂuenced by thin scale formed*Graduate Student, Tohoku UniversityMaterials Transactions, Vol. 47, No. 9 (2006) pp. 2377 to 2380#2006 The Japan Institute of Metalsduring heating in the air in this SME test. The shape changedue to gravity at the high temperature was not detected inother Co-based alloy with the same dimension and no SMEability; therefore, the eﬀect is though to be negligible. Thesurface strain is deﬁned as " ¼ t=2r  100 (%), where t is thethickness of a specimen and r is the radius of curvature. Theshape recovery rate, RSME, and recovery strain, "SME, wereevaluated by RSME ¼ ð"c  "hÞ= "c  100 (%) and "SME ¼"c  "h, respectively.3. Results and Discussion3.1 Martensitic Transformation and MicrostructureAn optical micrograph taken from the Co92Si8 alloy atroom temperature is shown in Fig. 1, where the typicalmicrostructure of the " martensite is observed. Figure 2shows (a) a bright ﬁeld image (BFI) taken from the sameCo92Si8 alloy and selected area diﬀraction patterns (SADPs)from (b) the  parent and (c) the " martensite phases, wherethe incident beam directions are ½011 and ½1210", res-pectively. The " martensite phase with an hcp structure hasa high density of stacking faults due to lattice invariantdeformation accompanied by a streak in the c-axis directionin the corresponding SADP of Fig. 2(c). Although themartensitic transformation ﬁnish temperature Mf detectedby DSC was 523C in the Co92Si8 alloy as mentioned later,the  parent phase coexists even at room temperature. Theresidual parent phase has been reported in many alloys,such as Co and Fe alloys, undergoing a non-thermoelasticmartensitic transformation.7,9,10,12,13) The correlation betweenthe fraction of the  phase and the martensitic transformationtemperature in Co alloys will be presented elsewhere.Figures 2(b) and (c) indicate that the crystal orientationrelationship between the  and the " phases is of the Shoji-Nishiyama relationship, that is, ð111Þ == ð0001Þ" and½110 == ½1120". These microstructural features are similarto those observed in Co-Al SMAs.9,10) Although these  andmartensitic " phases are metastable and the  (Co2Si) and "phases are stable at low temperatures,11) no precipitates wereobserved in any as-quenched specimens.The martensitic transformation temperatures of Co100xSix(x ¼ 0{8) alloys are plotted in Fig. 3 together with someprevious experimental data.14–17) The present study indicatesthat the martensitic transformation temperatures linearlyincrease with Si content up to 4mol% and that the inclinationgradually becomes smaller in the range of Si over 4mol%.Increasing with Si content, the transformation temperaturehysteresis is over 100C in the high Si region over 4mol%in which the transformation intervals are also large. In the100 µmFig. 1 Optical micrograph of Co92Si8 quenched from 1200C.500 nm(a)(b) (c)000200111 000210100000MPFig. 2 (a) Bright ﬁeld image of Co92Si8, (b) selected area diﬀractionpatterns from  phase and (c) " phase. Incident beam directions are ½011in (b) and ½1210" in (c).020040060080010001200Si Content (mol% )Transformation Temperature, T / °C0 5 10 15Hashimoto 14)Köster 15)Krajewski 16)Köster 17)heating coolingAf As MfMsThis WorkFig. 3 Martensitic transformation temperatures of Co and Co-Si alloys,including the previous data.14–17)2378 T. Omori et al.Co86Si14 alloy, some diﬀusional transformations such asdecomposition of the  phase occurred before the martensiticand reverse transformations during cooling and heating in theDSC examination, which was not suppressed even at higherheating and cooling rates of 40C/min. Therefore, themartensitic transformation temperatures could not be deter-mined. However, it can be deduced that the reversemartensitic transformation temperatures are located at tem-peratures between 800 and 900C because the Co86Si14 sheetspecimen showed a shape recovery due to the SME in thetemperature range from 800 to 900C during rapid heating.It is known that the martensitic transformation temperaturesdepend on experimental conditions, such as purity, grain sizeand heating and cooling rates. The transformation temper-atures determined from some specimens in the samecondition showed only small scattering within 15C, andno signiﬁcant correlation between the scattering of trans-formation temperatures and the Si content was found.Comparing the data taken from the present study with thoseof previous reports, the results in this study are in goodagreement with the report by Ko¨ster,17) and the tendency isthe same as that in reports by Hashimoto14) and Ko¨ster,15)except for the characteristic temperatures at about 1000C inCo85:5Si14:5 alloy obtained by Ko¨ster,15) which are probablydue to equilibrating reactions.3.2 Shape Memory PropertiesThe change of surface strain, "h, in the Co92Si8 alloy inrelation to the elevated temperatures is shown in Fig. 4,where a surface strain, "c, of 0.84% was initially introducedby bending at room temperature. Here, appearances of thespecimen bent at room temperature and of that heated up to1000C after bending followed by cooling to room temper-ature are also shown in Fig. 4. Slightly decreasing by heatingup to 700C, the surface strain drastically changes at around800C, which corresponds to the reverse martensitic trans-formation temperatures as shown in Table 1 and Fig. 3. Thereason why the strain changes at temperatures below themartensitic reverse transformation temperatures is not yetclear, but this result means that an SME occurs at a very hightemperature in the Co-Si alloy.The initial surface strain, "SME, the surface strain inducedby the shape recovery strain, "SME, and the recovery rate,RSME, by heating to 1000C are plotted as a function of Sicontent in Fig. 5. It was found that the shape memoryproperties are hardly enhanced by the addition of Si and thatthe shape recovery strain is about 0.3% with a recovery rateof around 40% in Co and in all Co-Si alloys. It should beemphasized that although the shape memory property ofCo-Si alloys is inferior to that of the Co-Al alloys,9,10) theshape recovery temperatures are much higher than those ofthe other Co-based SMAs. From Fig. 3, the shape recoverytemperature can be expected to increase with increasing Sicontent, whereas the thermal stability decreases. As men-Surface Strain, εc and ε h (%)Temperature, T / °C0.30.40.50.60.70.80.90 200 400 600 800 1000 1200 14001.0Shape  RecoveryFig. 4 Shape change of Co92Si8 sheet specimen by heating, where asurface strain, "c, of 0.84% was initially introduced at room temperature.The surface strain, "h, was measured at each heating cycle, the heatingtemperature being increased at intervals of 100C from 100C to1300C.Table 1 Martensitic transformation temperatures, hystereses and intervals of Co and Co-Si alloys.Si (mol%)Martensitic Trans. Temp. Hysteresis IntervalMs Mf As Af Af -Ms As-Mf Ms-Mf Af -As0 404 372 439 461 57 67 32 224 536 508 624 649 113 116 28 258 578 523 743 785 207 220 55 4210 15Si Content (mol%)00.10.20.30.40.50.60.70.80 5Strain, εc and ε SME (%)Shape Recovery Rate, RSME (%)01020304050601.00.9Initial Surface Strain, εcShape Recovery Strain, εSMEShape Recovery Rate, RSMEFig. 5 Initial surface strain, "c, shape recovery strain, "SME, and recoveryrate, RSME, of Co and Co-Si alloys by heating to 1000C as a function of Sicontent.FCC/HCP Martensitic Transformation and High-Temperature Shape Memory Properties in Co-Si Alloys 2379tioned in the previous section, the Co86Si14 alloy exhibitsshape recovery at temperatures between 800 and 900C, butdecomposition can easily occur at temperatures over 600C.Considering their thermal stability, the Co-Si alloys withlower Si content can be used as high-temperature SMAsoperating at temperatures up to 600C.4. SummaryThe microstructures of Co-Si alloys were examined usingOM and TEM, and the typical " martensite in the  parentphase was observed at room temperature. The =" marten-sitic transformation temperatures were determined, andfound to be in good agreement with those in the previousreports. The transformation temperatures increased withincreasing Si content up to 4mol% and the slope becamesmaller and the transformation hysteresis and intervals werelarger in the range of Si content over 4mol%. The Co-Sialloys exhibited a shape recovery at an elevated temperaturecorresponding to the reverse martensitic transformationtemperature. The addition of Si hardly enhanced the shapememory properties, and a shape strain of 0.3% was obtainedby heating to 1000C with a recovery rate of 40%, which iscomparable to values in pure Co. However, the shaperecovery temperatures of Co-Si alloys were found to behigher than those of other Co-base alloys and, therefore, theCo-Si alloys have a possibility for HTSMAs operating attemperatures up to 600C.AcknowledgmentThe support from CREST, Japan Science and TechnologyAgency is acknowledged.REFERENCES1) A. O¨lander: J. Am. Chem. Soc. 54 (1932) 3819–3833.2) L. C. Chang and T. A. Read: Trans. AIME 191 (1951) 47–52.3) S. Miyazaki and K. Otsuka: ISIJ Int. 29 (1989) 353–377.4) J. Van Humbeeck: J. Eng. Mater. Tech. 121 (1999) 98–101.5) A. Nagasawa: Phys. Status Solidi A8 (1971) 531–538.6) Y. N. Koval: Mater. Sci. Forum 327–328 (2000) 271–278.7) H. C. Shin, S. H. Lee, J. H. Jun and C. S. Choi: Mater. Sci. Technol. 18(2002) 429–432.8) W. M. Zhou, Y. Liu, B. H. Jiang, Z. Qi and Y. N. Liu: Appl. Phys. Lett.82 (2003) 760–762.9) T. Omori, Y. Sutou, K. Oikawa, R. Kainuma and K. Ishida: Mater.Trans. 44 (2003) 2732–2735.10) T. Omori, Y. Sutou, K. Oikawa, R. Kainuma and K. Ishida: Scr. Mater.52 (2005) 565–569.11) K. Ishida, T. Nishizawa and M. E. Schlesinger: J. Phase Equilib. 12(1991) 578–586.12) D. P. Koistinen and R. E. Marburger: Acta Metall. 7 (1959) 59–60.13) H. Otsuka, H. Yamada, T. Maruyama, H. Tanahashi, S. Matsuda andM. Murakami: ISIJ Int. 30 (1990) 674–679.14) U. Hashimoto: J. Jpn. Inst. Met. 1 (1937) 135–143.15) W. Ko¨ster and E. Schmid: Z. Metallkd. 29 (1937) 232–233.16) W. Krajewski, J. Krunger and H. Winterhanger: Cobalt 48 (1970) 120–128.17) W. Ko¨ster, H. Warlimont and T. Godecke: Z. Metallkd. 64 (1973) 399–405.2380 T. Omori et al.

FCC/HCP Martensitic Transformation and High-Temperature Shape Memory Properties in Co-Si Alloys

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FCC/HCP Martensitic Transformation and High-Temperature Shape Memory Properties in Co-Si Alloys

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