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Effects of Several Factors on the Ductility of the Ti‐Ni

Miyazaki, Kohiyama, Otsuka, Duerig

Martensitic Transformations II (ed.) B.C. Muddle Forum Vols. 56‐58 pp. 765‐770

1990

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EFFECTS OF SEVERAL FACTORS ON THE DUCTILITY OF THE TI-Ni ALLOY

S. Miyazaki(a), Y. Kohiyama(a), K. Otsuka(a) and r.w. Duerig(b) (a) In st ~ule oj Materials Science, Universily 01 TSUkuba, Tsukuba, lbaraki 305, Japan (b) Raychem Corporation, Menlo Pari(, CA 94025·1164, USA

ABSTRACT The effects of the following sever al factor s o n the ductility of the Ti- Ni alloy have been invest igated: Ni -concen tration, test temperature and heat-treatment. Tensile tests we re carried out for measuring elongation (the strain) , wh ich was mainly used as a meas ure of ductility. The elongation dec r eased wi th increasing Ni - concen tration in solution- treated specimens. at a temperature above 873K was effective to increase the duc tility of col d wo r ked specimens. It was also found that t he ma r tensite phase was mo r e ductile t han t he pa r e n t phase. The elongation of the solut ion-t reated specimen s howed a test­ temDerature deDe ndence. Ho wever, the elongation of t he age­ t r eated Ni - r i c h s pecimen ha rdly s howed test tempe r ature depe ndence.

1. INTRODUCTION

The Ti-Ni alloy is the most i mportant material among many shape memory alloys for t he applications of the shape memory effect or superelasticity. Th is is partly because t he alloy is ductile in a polycrystalline state (1 - 41. Since the alloy shows the perfect shape memory effect or superelasticity after heat- treatment at any temperature . t he c r itical for s lip can be raised by i n t r oducing dislocati o ns 15 ) and/or fine precipitates 15.61. The fine precipitates can be intr oduced only in Ni-rich specimens 17 J, wh ile in all T i -N i specimens. Therefore, cold work i ng is impor t a n t for introduci ng e no ugh density of dis locations in order to ach ieve the stable s hape r ecovery In all Ti-Nl alloys as well as for form t ng the alloy into any shapes.

Workability or ducti l ity of the Ti-Nl alloy depends on the tes t temperature [1.2J, heat-treatment(4) and Ni-concentration. 766 MIYAZAKI 91 " .

However. no systematic un derstanding about the ductility of t h is alloy has been attained. The purpose of this paper is to clarify the effects of all the above factors on the ductility of the TI- Nl alloy by a systematic Investl~atlon.

2. EXPERli\IENTAL

The alloys were prepared using electron beam melting method. The compositions of t he alloys range from 50.0 to 52.0at\Nl. The ingots of t hese alloys were hot rolled. followed by final cold with 20% r eduction. The plates t hu s obtained were 1.0 rnm thick. Specimens were spar k cut, the gage portion being of 1.0 mm x 1.5 mm x 15 mm size. They were t he n subjected to t wo types of heat-treatments respectively; i.e. ( 1 ) a nnealin g at /'II temperature between 573K and l273K after (annealing) and (2) solution- treatment at l 273K for 3.Sks followed by Quenching into iced water , and then age-treatment at a temperature between 373K and 973K (age-treatment) . All speci mens were el ectro-pol ished aft er the above heat-treatment and then tensile t ested. Tensile tests were carried out with an lnstr on type tensile mac hi ne, Shimadzu Autogr a ph 055-101' -$ type. Test temperatures were controlled by keeping a s pecime n i n ethanol which were heated by a heater o r cooled by pou r i ng liquid ni t r ogen.

3 . RESULTS AND DISCUSSION Basic characteristics of the mechanical properties such as ductility can be examined using solution-treated specimens , because such specimens do not have special inte~ n al structure such as dislocations and precipitates. figures 1 and 2 show the stress-strain curves of the sol ution -treated Ti-SO.5at%Ni and 1'i- 50.9at%Ni alloys respectively, which were tensile tested to the final fracture at various temperatures between 77K and 473K. The elongat10ns of t he Ti-SO.5at%Ni and Ti-SO.9at%Ni alloys are shown as a function of test temperatur e i n figures 3 and 4, respectively. The elongation obtai ned at Ms is t he l a r ge~t for both alloys; the elongation dec r eases with incr easIng or decreasIng temperature. The r efor e. deforming at f.1s after

TI-~ O.5a l "Ni 1273K Ih r to Defor m ed 01 XK g l ~O ~ X·77 > ~rooo ;;:"..~=:::::==~X .~4~7;;3~___ X:' .~2"9"6"" X· 283 • ::: ,~ !:• 500 ~ O~::~I~O--~2~O~--~3~OC---4~O~--~'~OC---~6~OC---­ Slr aln ("I

Figure 1. Stress-strain c urves of solution-tr eated Tl-S0.Sat%Nl specime ns which were deformed at various test temperatures. DUCTILITY OF n-N; 767

Ti-50.9a , %NI 1273K l!'If 10 Deformed 01 XK -0 1500 "- X· 77 :> X·47 3 X· 228 - 10 00 X·Z97 • !::• 500 '" 0 10 20 30 40 50 60 Strain (%)

Figure 2. Str ess-str a in curves of solution-treated Tl-S0.9at%Nl specimens wh ich were deformed at various test temperatures.

I 11-50.501 %Ni Ti-50.901"NI lOOt 1273K l!'If IQ 100 1273K l!'If 10 ii ~ c c o .2 -o o g- 50 ~ 50 o ;;;• ;;;

M. M. O~~~~'~~~~== 0 "-'1~0'0~2~0~0~3~0'0-4~0~0~5~0~0 100 200 300 4 00 500 Test Tempera lure I KJ Tes t Temperoture (K) Figure 3. Elongations o f Figure 4. Elongations of solution-Created Ti-50.Sat%Ni solution-tr eated Tl-S0.9at\Nl s pecimens wh ich were deformed specimens which were deformed a t various test tempe ratur es. at various test tempe ratures. solutio n-treatment Is suitable for deriving the best cold wo rka b lli ty. The duc ti11ty of · the marten s ite phase can be determ ined by deforming at Ms' because the martensite I s induced easily at a low str ess: wnile the ductility of t he parent phase can be determined by deforming at 473K, because this temperature is h igher t ha n Md' above whic h t he ma r tensite Is not stress-induced and plasti c deformation occurs i n t he pa r e nt phase. Figure;; shows the elongation at i'-ls or 473K as a function of Ni ­ concentration. The elongation at Ms is always larger t ha n that at 473K for a ny Ni -concen tration. This Is because t he critical str ess for slip at Ms is always l owe r t ha n t hat at 473K. It I s also f ound that the elongations at Ms for specimens wi th Ni­ concen trations be tween 50.0 and 51.0at\ arc more than 60%, wh ile t hose f o r the Tl-51 .5at%Nl a nd 52.0at%Ni alloys are a bou t 20\ o r less although they we r e deformed at f.1s . 768 MIYAZAKI at aI.

Ti-XOI'X,N! 100 12731< .... 10 Test IetY'4)efQlute • M.

Figure S. El ongations a t Ms o r 473K In solut i on-tr eated specimens with N1 - concent rations ra n gl n ~ f rom 50.0 °so 51 52 to 52. Oat\;Nl. Ni (01%1

I n or de r t o f o rm the a lloy by cold r olling or , intermediate a nn eal i ng Is necessary for r emoving work ha rdened structu re d ue to t he preceding wo rk i ng. In t his case, it I s important to deter mine a suitable r a nge of t he intermediate annealing tempe r atures for attainin g a h Igh ducti l ity. Figure 6 shows stress- strain curves of t he Ti-50.5at\Nl specimens which

T I-50.5 01 'X. Nj XK Ihr IQ Def ormed 01 R.T. X·673 X·773 X·S73 X.973

X·1273

0 ~--~'0C---~2~0--~3~0~--~5~0---'.~0C---7~0~--­ Strain IXI Figure 6. Stress-st rain curves at room temperature In Tl - S0.58t%Ni s pecimens which were nnnealed at various tempe ratures ranging from 573 to l273K for 3.6ks after t he preceding cold working.

TI-50.5 01 %Ni 100 XK lilt fa OefOfmed 01 R.T. ~

Figure 7. Elongations at room tem­ perature in Ti-SO.Sat'Ni specimens which were annealed at various tem­ peratures ranging from 573 to l273K 0· L.~500~--~~'~OOO~~~~oooto for 3.Sks after t he preceding cold AmeclJlg "~erc l ure (KI wo r k1ng. oucnLllY OF TI-NI 769

were annealed at various temperatures ra nging from 513K to 1273K after cold working with 20\ r eduction and t hen deformed at room temperature. The elongation vs . annealing temperature r e l ations hip is s hown in figure 7 , whe r e a large elongation 1s derived by annealing the specime n at a temperature above 873K. Since this critical temperature 1s a little higher than the r ecrystallization temperature . many dislocations introduced by t he preceding cold working were r emoved and the grain g r owth after t he r ecrystallization was promoted. resulting in lowering the critical str ess for slip. The critical str ess f or slip 15 a l so a ffected by the in Ni-r ich specimens as well as by the . Therefore, it i s necessar y to exclude precipitates as well as dislocations in order to attain a high cold workability in such specimens. Figures 8 and 9 s how the effects of annealing temperature and aging temperature on the str ess-strain curve of the Ti-50.9at\Ni alloy respectively whlch

TI-50 . 901 ~ Nj XK Ih r 10 Oeformed 01 R.T. X·673 X· 573 01500 X· 713 • X· a73 X·973 •- 1 000 • Xol273 • ~500

0 10 20 50 40 50 60 Slroln (Xl FIgure 8. Stress-strain curves at room temperature I n TI-50.9at%Nl specimens which were annealed at varIous temperatures ranging from 573 to l273K for 3.Sks after the preceding cold working .

TI-50.90IXNi 1273K Ihr 10"'XK Ihr 10 Deformed of R. T. •0 1500 •-'000 • • !: 500 ~ 0 ~--~'0~--~2~0---C5~0C---4~0~--·5~0-­ S Hain ,%, Fi gure 9 . Stress-strain curves at room temp erature i n Tt-50.9at%Nl speci mens which we r e age- treated at various temperatures rangIng from 573 to 973K for 3.6ks after the preceding solution- treatment . 770 MIYAZAKI at aI .

were tested at roo m temperature. It is clear t ha t both the heat­ treatments affect the stress- strain curve. The elongation Is shown as a func tion of annealing temperature o r aging temperature In figur e 10. wh e r e open ci r cles ind icate the elonga tions for the former. while closed circles those for the l atter . There Is a l so a critical annealing temperature similarly to the Tl-S0.5at%Nl alloy. annealing at a temperature above whic h being effective to increase the elongation; the critical t emp e r ature was almost the same as that In t he Tl-S0.5at\Nl alloy .

T/-50.9 01 %Ni 100 o XK IhrlQ • 1273K lh r[Q .... XKlhr IQ i! Oeformed 01 R.T. .~ g,50 < 0 Figure 10. Elongations at r oom tem­ OJ ~ (:"'0 ~ perature in Ti -50.9at%Ni specimens 0_ as a function of a nn ealing tempera­ ture after the preceding cold work­ 0 ing or a function of agIng temper­ 500 1000 1500 ature after the preceding solution­ Temperalure 1<' treatment.

Although a hlgh cold workability is obtained by anneal1ng at a temperature a bove t he c r itical temperature, t he elongation decr eases by aging in an intermediate temperature range between 513K and 813K as shown by the closed circles. This means that working with a Ni-rich specimen in the inte rmediate temperature r ange will deter iorate the workability . The elonga tion of s uch a n age-treated Nl-rich specimen was almost constant. i.e. about 20% for the Ti-50.9at%Nialloy, irrespectIve of test temperatur e.

ACKNOWLEDG~1ENT This wo rk was s upported by a Grant- in- Aid for Fundamental Scientifi c Resear c h Oppan-C, 1 988) from the Mi ni stry of Education , Sc ience a nd Culture, Japan. REFE RE NCES 1. Jackson , C. M. , Wagner, H. J . and Wa silewski , R.J. : NASA-SP 5110 , 1912. 2. Sastri , A.S. and Marci nkowski. M: J .; Trans. AIME . 1968, 242. 2393. 3. Miyazaki. S . . Otsuka. K. and Suzuki. Y.: Scripta Met., 1981 . 15. 281. 4 . Mercier. O. and Torok. E .: J. de Phys . . 1982 . 43. Suppl. 12, C4-261. 5. Miyazaki, S .. Ohmi. Y .. Otsuka, K. and Suzuki , Y.: ibid., C4- 255. 6. Saburi, T., Tatsumi, T. and Nenno, S.: ibid . . C4-261- 1. i'tHyazaki. S. and Otsuka , K.: Met. Trans. A, 1986, l1A, 53.