Ishida Kiyohito

Kainuma Ryosuke

Koeda Naoki

Omori Toshihiro

Sutou Yuji

MATERIALS TRANSACTIONS

English

Omori, Toshihiro

Koeda, Naoki

Sutou, Yuji

Kainuma, Ryosuke

Ishida, Kiyohito

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

Superplasticity of Cu-Al-Mn-Ni Shape MemoryAlloy著者 Omori  Toshihiro, Koeda  Naoki, Sutou  Yuji,Kainuma  Ryosuke, Ishida  Kiyohitojournal orpublication titleMaterials Transactionsvolume 48number 11page range 2914-2918year 2007URL http://hdl.handle.net/10097/52057Superplasticity of Cu-Al-Mn-Ni Shape Memory Alloy*1Toshihiro Omori1, Naoki Koeda1;*2, Yuji Sutou1, Ryosuke Kainuma2 and Kiyohito Ishida11Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan2Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, JapanThe microstructure and superplastic behavior at 450C and 500C in Cu-Al-Mn-based shape memory alloys were investigated usingoptical microscopy and a tensile test. It was found that a ﬁne  (fcc) +  (bcc) two-phase structure with a grain size of 3 mm in diameter can beobtained in Cu-Al-Mn-Ni alloy by annealing at 600C. The ﬂow stress of Cu-Al-Mn-Ni alloy depends on the strain rate and the strain ratesensitivity is over 0.3 with an elongation of over several hundred percent, which shows that the Cu-Al-Mn-Ni shape memory alloy exhibitssuperplasticity. For the test temperature at 500C and a strain rate of 5 104 s1, a maximal elongation of 1150% was obtained. The formationof cavity stringers lying parallel to the tensile axis was observed and the size of the cavity was larger as the specimen was more highlydeformed. [doi:10.2320/matertrans.D-MRA2007879](Received July 17, 2007; Accepted August 30, 2007; Published October 25, 2007)Keywords: copper-aluminum-manganese, shape memory alloy, superelasticity, superplasticity, cavity1. IntroductionCu-based shape memory alloys, including Cu-Al-Ni andCu-Zn-Al, have been extensively investigated because oftheir good shape memory properties, high electrical andthermal conductivity and low cost, although Ti-Ni is the onlyshape memory alloy which has been widely utilized inpractical applications. However, these Cu-based shapememory alloys are too brittle to be suﬃciently cold-worked,which is attributed to their high degree of order, high elasticanisotropy and coarse grain structure.1) Attempts have beenmade to improve the cold-workability of these Cu-basedalloys by grain reﬁning, but with limited success.1)The present authors have developed ductile Cu-Al-Mnshape memory alloy based on the phase diagram. Figure 1shows the vertical section diagram at 10 at% Mn in the Cu-Al-Mn system2) with the  (bcc) phase in Cu-Al binarysystem, where the  phase can exhibit martensitic trans-formation and shape memory properties. It is seen that the phase signiﬁcantly extends to the low Al content region bythe addition of Mn. The A2/B2 and B2/L21 orderingtransitions occur in the  phase, whose temperatures arestrongly dependent on the Al content, and thus the degree oforder in the  phase is lowered by decreasing the Al content.By control of the degree of order, ductile Cu-Al-Mn alloyscontaining Mn over 8 at% and 17 at% Al can be obtained.3–5)Moreover, by the combination of grain size and texturecontrol, the Cu-Al-Mn alloys exhibit a superelastic strain ofabout 7%,6–9) which is comparable to that of Ti-Ni alloys, aswell as some related functional properties such as a func-tionally graded property,9) a two-way shape memory ef-fect,10,11) a damping property12–14) and the Invar eﬀect.15)It is known that superplasticity can be obtained at asuitable temperature and strain rate in a ﬁne grain sizespecimen, and superplastic behavior has been reported inseveral Cu-Al and Cu-Zn alloys.16–20) In Cu-based shapememory alloys, superplasticity has already been reported inCu-Al-Ni-based,21) Cu-Zn-Al-based22) and Cu-Zn-Sn-based23,24) alloys with ﬁne microstructures. Recently, thepresent authors found that ductile Cu-Al-Mn shape memoryalloys containing Ni show a very ﬁne þ  two-phasemicrostructure.6) This fact means that the Cu-Al-Mn-Nishape memory alloys can also be expected to exhibitsuperplasticity. In the present study, the microstructure andsuperplasticity of Cu-Al-Mn-Ni shape memory alloys wereinvestigated.2. ExperimentalCu71:5Al17Mn11:5 and Cu71:3Al17Mn8:7Ni3 alloys wereprepared by induction melting under an argon atmosphere.The ingots were hot-rolled to a thickness of 10mm at 800Cand subsequently cold-rolled to a thickness of 1.15mm withintermediate annealing at 600C. Specimens were cut fromthe sheets, and heat treatment was conducted at 600C for30min or at 900C for 15min.10400600800Temperature, T / °C    at% Alαβα + ββ + γA2B2L21Cu-10MnCu-Al binary30252015Fig. 1 Vertical section of 10 at% Mn in Cu-Al-Mn system with  phaseof Cu-Al binary system indicated by broken line.*1This Paper was Originally Published in Japanese in Journal of the JRICu45 (2006) 282–286.*2Graduate Student, Tohoku UniversityMaterials Transactions, Vol. 48, No. 11 (2007) pp. 2914 to 2918#2007 Journal of Japan Research Institute for Advanced Copper-Base Materials and TechnologiesThe microstructure was observed using an optical micro-scope and the volume fraction of the  phase was determinedby image analysis.Tensile tests were carried out by an Instron-type machineat 450C and 500C, where the tensile axis was parallel to therolling direction of the sheet and the strain rate was constantin the range of 1 101 s1–5 104 s1.3. Results and Discussion3.1 Microstructure of Cu-Al-Mn and Cu-Al-Mn-NialloysFrom Fig. 1, it can be expected that the ductile Cu-Al-Mnshape memory alloy with 17 at% Al shows the  single phaseat temperatures over about 700C, while the þ  two-phasestructure appears at lower temperatures. Since the  phase isvery ductile due to the disordered fcc structure, improvementof the ductility for the Cu-Al-Mn-base shape memory alloysis possible by the introduction of the  phase in the  phase.Therefore, the annealing temperature suitable for subsequentcold-working is around 600C and the ﬁnal shape memorytreatment to obtain the  single phase can be performed attemperatures over 800C.Figures 2(a) and (b) show the  single-phase micro-structure with a grain diameter of 400–500 mm annealed at900C for 15min and the þ  two-phase structure annealedat 600C for 30min in the Cu71:5Al17Mn11:5 alloy,respectively. The optical micrograph in Fig. 2(c) showsthe þ  two-phase microstructure taken from theCu71:3Al17Mn8:7Ni3 alloy annealed at 600C for 30min,which exhibits a much ﬁner two-phase microstructure with amean grain diameter of approximately 3 mm. The volumefractions of the  phase for the ternary and quaternary alloysshown in Figs. 2(b) and (c) are 42% and 64%, respectively.This result is consistent with the phase diagram shown inFig. 3 between the  and  phases at 700C,25) where thepartition of Mn between the  and  phases is almost equal. Itis seen that the þ  two-phase region is widened by theaddition of Ni and that the volume fraction of the  phaseincreases with the addition of Ni at Al content of 17 at%. Theincrease of the  phase volume improves the cold work-ability, and the maximum reduction in thickness before acrack appears in a sheet by cold-rolling increases from 60%to 80% by the addition of Ni.The Cu-Al-Mn-Ni shape memory alloy with the þ  twophase microstructure, which was obtained by annealing at600C, seems to have a microstructure suitable for obtainingsuperplasticity, i.e., the grain size is less than 10–15 mm.26)3.2 SuperplasticityThe tensile test was carried out for the Cu71:3Al17Mn8:7Ni3alloy annealed at 600C for 30min, whose microstructureFig. 2 Microstructure of (a)  single-phase and (b) þ  two-phase ofCu-Al-Mn and (c) þ  two-phase of Cu-Al-Mn-Ni alloy.0101214161820βαα + β15 at% Mnat% Alat% Ni2.01.51.00.5Fig. 3 Isothermal section of 15 at% Mn in Cu-Al-Mn-Ni system at 700C.Superplasticity of Cu-Al-Mn-Ni Shape Memory Alloy 2915is shown in Fig. 2(c). Figures 4(a) and (b) show the stress-strain curves at 450C and 500C, respectively. Figure 5shows the elongation to failure as a function of strain rate.The elongation is enhanced several hundred percent bydeformation at a lower strain rate and a higher temperature;in particular, that at 5 104 s1 and 500C reaches 1150%.Figure 6 shows the ﬂow stress against strain rate. It is seenthat the ﬂow stress increases with increasing strain rate andthat at the same strain rate, the ﬂow stress at 450C is alwayshigher than that at 500C. The strain rate sensitivity, m, at450C is about 0.37 and that at 500C is over 0.33. Thisplastic behavior with the large tensile elongation prior tofailure and the large strain rate dependence of ﬂow stress isclearly due to superplasticity. Actually, it has been reportedthat the Cu-Zn superplastic alloy with a þ  two phasemicrostructure shows m ¼ 0:1{0:2 at 400C and m ¼0:2{0:4 at 500C at strain rates of 1 103 s1–1102 s1.16) It can be concluded that the Cu-Al-Mn-Ni two-phase alloy, which is similar to other superplastic Cu-basealloys, is a superplastic shape memory alloy.Figure 7 displays the specimens before and after the0501001502000Strain (%)0501001502002500Strain (%)Stress, σ / MPa1×102×105×10-1 (s-1)(a) 450 °C(b) 500 °CStress, σ / MPa-2 (s-1)-3 (s-1)-4 (s-1)1×101×102×105×10-2 (s-1)-3 (s-1)-4 (s-1)50040030020010012501000750500250Fig. 4 Stress-strain curves of Cu71:3Al17Mn8:7Ni3 alloy at indicated strainrates at (a) 450C and (b) 500C.0200400600800100012001400Elongation (%)500 C450 10010-110-210-310-4Strain Rate, ε / s-1°C°Fig. 5 Superplastic elongation as a function of strain rate inCu71:3Al17Mn8:7Ni3 alloy.1101001000Stress, σ / MPa 500 C450 10010-110-210-310-4m=0.37m=0.33m=0.97Strain Rate, ε / s-1°C°Fig. 6 Flow stress as a function of strain rate in Cu71:3Al17Mn8:7Ni3 alloy.Fig. 7 Cu71:3Al17Mn8:7Ni3 alloy before and after tensile tests at variousstrain rates at 450C and 500C.2916 T. Omori, N. Koeda, Y. Sutou, R. Kainuma and K. Ishidatensile test. While necking can be observed in the specimensdeformed at higher strain rates at the lower temperature, thespecimens at lower strain rates at the higher temperatureshow a large elongation without necking.3.3 Microstructure after tensile testThe microstructures of the specimens tested until ruptureat a strain rate of 5 104 s1 at 450C and 500C are shownin Fig. 8, where the tensile axis corresponds to the transversedirection. Figures 8(a) and (c) show the microstructure nearthe grip of the tensile test specimens, and (b) and (d) are takenfrom the regions near the fracture tip. Here, the cavities areseen lying parallel to the tensile axis and the shape of grains isapproximately equiaxed as shown in Figs. 8(b) and (d),which are common features in the superplastic alloys.27) It isalso observed in the specimen after the 1150% elongation (inFig. 8(d)) that the mean size of the cavities is larger than thatof the specimen subjected to static isothermal annealing (inFig. 8(c)) and that noticeable grain coarsening occurs. This isclear evidence of strain-enhanced grain growth due to thehigh degree of deformation, which is consistent withconventional knowledge that the magnitude of grain growthincreases with increasing strain.26)4. Conclusions(1) The addition of Ni was found to be eﬀective in reducingthe grain size of the þ  two phase structure, and aﬁne microstructure with a grain size of about 3 mm wasobtained in the Cu-Al-Mn-Ni alloy.(2) The tensile test was carried out at strain rates of 1101 s1–5 104 s1 and at 450C and 500C, resultsshowing that the ﬂow stress is strain rate dependent andthat the strain rate sensitivity is 0.33–0.97. Theelongation to fracture is over several hundred percentand 1150% elongation was obtained at 5 104 s1and 500C.(3) Cavities stringers lying parallel to the tensile axis wereseen in the specimens tested until rupture, and largercavities and grain growth was observed in the largestrain specimen.(4) The Cu-Al-Mn-Ni shape memory alloy exhibits super-plasticity at around 500C, which suggests that it hashigh potential as a high productivity of the shapememory alloy.AcknowledgmentThis work was supported by CREST, Japan Science andTechnology Agency.REFERENCES1) T. Tadaki: Shape Memory Materials, ed. by K. Otsuka and C. M.Wayman, (Cambridge University Press, Cambridge, UK, 1998) pp. 97–116.2) R. Kainuma, N. Satoh, X. J. Liu, I. Ohnuma and K. Ishida: J. AlloysComp. 266 (1998) 191–200.3) R. Kainuma, S. Takahashi and K. Ishida: J. JRICu 34 (1995) 213–219.4) R. Kainuma, S. Takahashi and K. Ishida: J. de Phys. IV 5 C8 (1995)961–966.Fig. 8 Microstructures of specimens tensile-tested at indicated temperatures at a strain rate of 5 104 s1 with the tensile axis beinghorizontal.Superplasticity of Cu-Al-Mn-Ni Shape Memory Alloy 29175) R. Kainuma, S. Takahashi and K. Ishida: Metall. Mater. Trans. A 27A(1996) 2187–2195.6) Y. Sutou, T. Omori, R. Kainuma, N. Ono and K. Ishida: Metall. Mater.Trans. A 33A (2002) 2817–2824.7) Y. Sutou, T. Omori, K. Yamauchi, N. Ono, R. Kainuma and K. Ishida:Acta Mater. 53 (2005) 4121–4133.8) Y. Sutou, T. Omori, J. J. Wang, R. Kainuma and K. Ishida: J. Phys. IV112 (2002) 511–514.9) Y. Sutou, T. Omori, J. J. Wang, R. Kainuma and K. 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Superplasticity of Cu-Al-Mn-Ni Shape Memory Alloy

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Superplasticity of Cu-Al-Mn-Ni Shape Memory Alloy

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