Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982434
`
`JOURNAL DE PHYSIQUE
`CoZloque C4, suppZ6ment au n o 12, Tome 43, de'eembre 1982
`
`page C4-255
`
`CHARACTERISTICS OF DEFORMATION AND TRANSFORMATION PSEUDOELASTICITY I N
`Ti-Ni ALLOYS
`
`S. ~ i ~ a z a k i * , Y. ohmi* , K. ~ t s u k a * and Y. suzukiE*
`
`* ~ n s t i t u t e o f Materials Science, University of Tsukuba, Sakura-mura,
`Ibaraki-ken 305, Japan
`Y E Central Research Laboratory, Fumkawa E l e c t r i c Company, 2-9-25 Futaba-eho,
`Shinagawa-ku, Tokyo 141, Japan
`
`(Accepted 9 August 1982)
`
`Abstract. - Although the Ti-Ni alloy i s the most important material for
`applications, there has been no systematic work on i t s deformation behavior.
`In the present report, the effects of ageing, Ni-content and annealing right
`after cold work on the shape memory effect and on the pseudoelasticity are
`systematically investigated by tensile tests at various temperatures. Based
`on the obtained results the differences in deformation and transformation
`pseudoelasticity characteristics between Ti-Ni alloys and other B-phase
`alloys are discussed.
`Introduction. - The Ti-Ni alloy i s the most important material among many shape
`memory alloys for the applications of both the shape memory effect and the pseudo-
`elasticity, because only this one deforms more than 50% strain prior to fracture
`and the strain as large as 8% recovers by heating above Af o r unloading in a poly-
`crystalline state (1). However no systematic work has been done from fundamental
`view point on the characteristics of deformation and transformation pseudoelasticity
`for this alloy, except for a few pioneering work (21% ( 5 ) . In fact, there a r e many
`factors affecting the deformation behavior of this alloy, i.e. ageing effect, Ni-content,
`In the present report the effects of
`annealing right after cold work, and so on.
`these factors are examined systematically by tensile tests a t various temperatures
`( 77K~373K ) .
`It i s well known that the characteristics of deformation and transformation
`pseudoelasticity in Ti-Ni alloys are remarkably different from those in other B-phase
`alloys. For example, the formers are very ductile (1,6) . while the latters are not
`and shows the intergranular fracture ( 6 , 7 ) . The transformation pseudoelasticity
`is always observed in the latters at temperatures above Af (81% (101, while i t is
`not necessarily so in the former, i.e. a Ti-50at%Ni alloy usually does not exhibit
`the pseudoelasticity even above Af, while a Ti-5latsai alloy does so. This distinct
`difference will be explained by the fundamental deformation characteristics of Ti-Ni
`alloys, which are consistent with the above mentioned various effects.
`
`Specimens and Experimental Procedures.- The alloy preparation was described in
`the previous paper (1). The compositions of the alloys used were determined by
`chemical analysis to be Ti-49.8at%Ni, Ti-5O.lat%Ni, Ti-50.6atWi and Ti-51.6atWi
`( t o . latSki) ( the nominal compositions were Ti-50. Oatski, Ti-50.7at%i?i,Ti-51.Oat%Ni
`and Ti-51.5at%Ni, respectively ) . The ingots of these alloys were hot rolled and
`then cold drawn at R .T. to wire specimen with diameter of 1.0 mm. The specimens
`were then heat-treated for 3.6ks a t various temperatures in a range from 473K to
`1273K in a vacuum of
`Pa, and then rapidly quenched into ice water in order
`to investigate the effect of annealing temperature ( from here the heat-treatment
`is abbreviated as 1273K IQ for example ) .
`In order to investigate the ageing
`effect, the same alloy specimens were aged for 3.6ks below 873K followed by
`quenching into ice water after the heat treatment of 1273K IQ ( from here this
`heat treatment is abbreviated as 1273K IQ+XK
`IQ, where X ranges from 1073K to
`473K ) . After these heat-treatments, wire specimens with diameter of 1.0 mm
`and with gauge length of 30 mm long were made by electropolishing. Tensile tests
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`were carried out with an Instron type tensile machine. Shimadzu Autograph DSS-
`10T-S type. The details of the specimen preparation and the tensile tests have
`been reported elsewhere (1,ll) .
`
`,273~ IQ
`
`Results.- As mentioned in the previous section, there are many factors affecting
`In this section the effects of (a) ageing,
`the deformation behavior of Ti-Ni alloys.
`(b) Ni-content and (c) annealing right after cold work are described in the follw-
`ing in that order.
`(a) Effect of ageing. - Heat treatments of shape memory alloys have gener-
`ally been conducted by solution treatment at high temperature followed by quench-
`ing rapidly into water in order to obtain a high temperature single phase, which
`causes the shape memory effect to appear.
`A Ti-Ni alloy, however, is an exceptional
`case which shows the shape memory ef-
`fect regardless of the heat-treatment, i.e.
`rapid quenching into water or gradual
`cooling in a furnace. Moreover, the
`characteristics of pseudoelasticity of this
`alloy in a slowly cooled specimen a r e
`
`1 2 7 3 ~ IQ
`
`1273~ IQ /- - 1 0 7 3 ~ IQ
`-
`- 9.3.-
`better than those in a rapidly quenched ' -
`
`- 6 7 3 ~ IQ / - 1273K I0
`
`a
`
`2
`
`one ( unpublished work by present au-
`thors ) . This effect can be attributed
`to the ageing effect at low temperature
`during cooling, which produces fine pre-
`cipitates. The presence of fine precipi-
`tates raises the flow s t r e s s for slip, and
`thus makes the stress-induced transfom-
`ation pseudoelastic as shown later ( Fig.
`2 1 .
`
`5
`
`1273K I0
`
`-873KIQ > 1273K I0
`
`- 7 7 3 ~ IQ
`
`1273K IQ
`
`TI-50 6ol%Nt
`
`I
`LOO
`
`.
`
`~
`
`-573K 1Q
`
`1273~ 10
`-473~ IQ
`,
`~
`~
`
`~
`
`~
`300
`
`~
`,
`,
`~
`ZOO
`Temperature LK)
`Fig. 1 E f f e c t of a q e l n g t e m p e r a t u r e on t h e
`curve
`electrical r e s ~ s t a n c e - t ~ m ~ ~ r ~ t ~ ~ ~
`a T l - 5 0 . 6 a t % ~ 1 a l l o y .
`
`~
`
`,
`
`,
`
`~
`
`~
`
`~
`
`~
`
`,
`
`~
`
`,
`
`,
`
`,
`
`,
`
`~
`
`~
`
`
`
`It is convenient to examine the
`electrical resistence-temperature curve
`as a function of ageing temperature in
`order to assess the most effective tem-
`perature for ageing. Figure 1 shows
`the curves of Ti-50.6atWi alloy speci-
`mens, which were subjected to the heat-
`treatment : 1273K IQ +
`X K IQ, where X ranges
`from 1073K to 473K.
`By ageing at tempera-
`tures between 1073K and
`873K the curves change
`little. But, by ageing ;i 600
`at temperatures between
`
`800
`
`773K and 573K t h e shape '
`
`OM OR Heat-treatment
`A A
`
`1273K I0 - 773K I0
`-. 673K 10
`
`-573K
`3
`~i-50.6at%~1
`
`10
`
`of the curves depend
`strongly on ageing tem-
`perature, especially that
`aged at 673K shows the
`biggest change among
`them. The specimens
`aged at higher tempera-
`tures scarcely showed
`the pseudoelasticity at
`any deformation temper-
`ature. However, the
`specimens aged at low
`temperatures showed
`perfect pseudoelasticity
`above Af. The critical
`stresses for inducing
`
`400
`
`2
`
`(K)
`Temperature
`Fig. 2 E f f e c t of ageing on t h e c r i t i c a l s t r e s s e s
`£0, inducing martensites(ciM) and f o r reverse t r a n s -
`f ormation(og) i n a Ti-50.6at%Ni a l l o y .
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`
`T I - ~ ~ . ~ O I % N I
`
`-
`
`j
`
`T,-50,6a,%N,
`
`TI-51.6al%N1
`
`martensites ( a M ) and for the completion of the reverse transformation ( a R ) are
`plotted against deformation temperature in Fig. 2 . Both the stresses were found
`to follow the Clausius-Clapeyron relationship in the temperature region Af<T<Tc.
`Here Tc represents the critical temperature where the plastic deformation by dis-
`location motion s t a r t s , and thus above Tc the critical stresses deviate from the
`linear relation with temperature a s shown in the figure. The specimens aged a t
`673K showed the perfect pseudoelasticity without residual strain until a~ reaches
`about 500MPa, which is the largest value among those of aged specimens.
`I t is
`concluded from these results that the ageing treatment at 673K is most effective
`in improving the characteristics of pseudoelasticity.
`(b)..affect of Ni-content. -
`Although the phase diagram of the
`Ti-Ni system is not well-established as
`y e t , there is a general agreement on the
`presence of some precipitate phases
`such as TiNi3 and/or Ti42Ni58 on the
`Ni-rich side of these diagrams (12,13) .
`Thus the ageing effect may also depend
`on Ni content a s well as on ageing tem-
`perature. The electrical resistance-tem-
`perature curves of specimens with vari- ;
`0 -
`'
`ous Ni-content whose heat-treatments
`are 1273K IQ and 1273K IQ + 673K I Q ,
`respectively, are shown in Fig. 3.
`The shape of curves and transformation
`temperatures a r e almost unchanged by
`the ageing treatment in a Ti-49.8atWi
`alloy. But, the change in the shape of
`curves by ageing a t 673K are more re-
`markable with increasing Ni-content .
`The critical stresses OM and OR are
`shown as a function of deformation tern-
`perature in Fig. 4. The values o~ in
`Ti-49.8atWi and Ti-SO. lat%Ni alloys are
`absent. because the ~seudoelasticitv was
`specimens. But
`not
`in
`in Ti-50. 6atWi and
`Ti-51.6at%Ni alloys the
`pseudoelasticity is ob-
`served in a wide tem-
`perature range, and
`the deviation of the
`stress OM from the
`linear relation with tem-
`perature is observed
`at about 6OOMPa in a
`Ti-51.6at%Ni alloy,
`which is a clear indica-
`tion for the onset of
`slip. From these re-
`sults i t is concluded
`that the effect of
`ageing is more promi-
`nent in a high Ni-
`content specimen than
`in a low Ni-content one.
`
`- 673K 10 /
`,----
`T'-50101%N1 / 1273K I0
`- 6 7 3 ~ 10
`/ s i i ~
`
`- 67% 10 " Krro
`
`1873K I0
`
`10
`
`D73K
`1273K I0
`
`- 673% 10
`
`100
`
`zoo
`Temperature (Kl
`Fig. 3 Effect of ~i-content on the electrical
`resistance-temperature curve in Ti-Ni alloys.
`
`300
`
`GI CR Ni-content
`V V 51.6at%
`
`0 . 50.6 :
`
`o ( 0 ) 50.1 r
`A (A) 49.8 '
`1273K IQ - 673K IQ
`
`/
`
`V
`
`,v'
`
`04$. YV
`
`-
`800
`
`-
`-
`600
`I -
`a"
`
`-
`
`V)
`
`-
`400
`Gi
`
`al
`z
`V) C
`,? 200-
`
`0 ,
`150
`
`200
`
`300
`250
`Temperature (K)
`Fig. 4 Effect of Ni-content onthe c r i t i c a l stresses
`for inducing martensites(aM) and for reverse trans-
`format ion (aR ) .
`
`350
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`.z
`$
`
`/ '273K la
`/ 873K IQ
`7 7 3 ~ IQ s
`
`-
`
`
`
`11-49 8of%N1
`
`473K I0
`
`100
`
`150
`
`250
`200
`Temperature (K)
`
`300
`
`350
`
`IU
`0
`0
`
`2
`-
`$
`IU -
`w
`
`0
`
`8 0 0 -
`
`-
`C?
`2 600-
`
`V)
`V)
`
`OM OR Heat-treatment
`0 (e) 1273K I Q
`
`. 773K I Q
`
`A A 673K 10
`TI-49.8at %NI
`
`A'
`
`,A'?//
`
`A'
`
`(c) Effect of annealing right after cold work. - As mentioned previously
`the effect of ageing was most prominent in a Ti-51.6atsNi alloy and was not appre-
`ciable in a Ti-49.8at%Ni alloy. This means that a specimen with low Ni-content
`does not exhibit the pseudoelasticity even above Af, while that with high Ni-con-
`tent does so. However, we found another effect which improved remarkably the
`characteristics of pseudoelasticity even in a specimen with low Ni-content. Figure
`5 shows the effect of annealin? right after cold work on the electrical resistance-
`temperature curve of a Ti-49.8at%Ni alloy specimen, which was annealed a t each
`temperature without solution treatment after cold drawn. The curve exhibits little
`change in a specimen annealed above 873K.
`However, the curves for specimens an-
`nealed below 773K show very conspicuous
`change which is very similar to that ob-
`served in the Ti-5O.6atSNi alloy aged below
`773K. The critical stresses 9, and OR in
`the specimens annealed a t 1273K. 773K and
`673K, respectively, are plotted against
`deformation temperature in Fig. 6. The
`specimen annealed a t 1273K did not show
`the pseudoelasticity a t any deformation
`temperature and, t h u s , a~ was absent.
`However, the specimens annealed at lower
`temperatures showed the pseudoelasticity,
`and especially
`that annealed a t 673K
`shcwed the most superior characteristics
`of pseudoelasticity. The stress-strain
`curves a s a function of deformation tem-
`perature for a specimen annealed a t 673K
`and
`those for a specimen annealed a t
`1273K and then annealed a t 673K are
`compared in Fig. 7. The specimens were
`loaded and unloaded a t each deformation
`temperature, and then heated to 373K ( >Af) ~
`~ ~ s ~ ~ ~ ~ ~ ~ ~ ~ ~ e ~ ; ~ ~ ~ ; ~ ~ ~ u ; ; e
`~
`e
`.
`~
`~
`e
`c
`~
`~
`f
`~
`~
`~
`
`in order to make the total strain to be
`f o r T I - 4 9 . 8 a t 8 ~ ~ a l l o y speclrnens, W ~ I C ~
`were annealed at each ternuerature w ~ t h o u t
`separated into the recoverable strain
`P r i o r Solution treatment a f t e r cold work.
`the residual strain,
`a s shown by the
`dotted curve.
`Permanent residual
`strain more than
`one percent took
`place as shown by
`curve (a) in the
`spec~men annealed
`a t 1273K followed by
`annealing at 673K.
`4 0 0 -
`Thls residual strain 2
`caused the specimen 2
`to show the two way
`shape memory effect
`a s shown by curve
`( b ) when it was
`deformed after the
`preceding deform-
`ation. On the con-
`t r a r y , in the speci-
`
`.
`,d/
`fi
`/,// LI
`A' A'y
`m // /: l'
`/A ,,p=.=
`y
`/ .?
`w 3 " / ' / A .ko-
`I
`2 0 0
`2 5 0
`3 0 0
`(K)
`Deformation Temperature
`Fig. 6 Effect of annealing temperature on the critical
`stresses for inducing martensites(oM) and for reverse
`transformation(uR) in Ti-49.8atZNi alloy specimens,
`which were annealed at each temperature without prior
`solution treatment after cold work.
`
`2 0 0 -
`
`'
`
`~
`
`A'
`-
`
`4
`//
`
`~
`
`~
`
`~
`
`~
`
`~
`
`~
`
`
`
`~~en~enrnf~$~2~~73K
`
`OL 150
`
`3 5 0
`
`memory effect was
`observed below As
`and the pseudoelas-
`ticity was observed
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`- .-
`Strain
`
`1%)
`
`Fig. 7 Effect of annealing temperature
`on the stress-strain curves as a func-
`tion of deformation temperature in a
`Ti-49.8atZNi alloy.
`(A) Heat-treatment : 1273K IQ - 673K IQ.
`(B) Heat-treatment : 673K IQ.
`
`Strain. (%I
`
`in a wide temperature range as shown in the right-hand side of Fig. 7. The im-
`provement of the characteristics of pseudoelasticity can not be attributed to the
`ageing effect due to precipitation, because both specimens were aged for 3.6ks at
`673K. Therefore, it is most likely that the effect of annealing at lower temperatures
`is caused by the internal structure of dislocations which were introduced during
`drawing at R.T. and then rearranged by annealing so as to diminish the internal
`strain energy. The presence of such rearranged dislocations again raises the
`flow stress for slip, and thus results in the appearance of pseudoelasticity at
`temperatures above Af.
`It is also noticed in the figure that there are two plateaus in the stress-
`strain curves in low temperature range. The critical stress for the second plateau
`corresponds to the stress O M , but that for the first plateau was found to be the
`stress for rearrangement of the rhombohedral phase variants, which appear prior
`to the onset of the martensitic transformation during cooling in the absence of
`stress (5,14,15). This is confirmed by the fact that the extrapolated point of the
`latter stress corresponds to the temperature at which the rhombohedral phase is
`induced thermally. The details about this will be published soon elsewhere.
`
`Discussion. - As mentioned in the introduction, a Ti-Ni alloy is very ductile and
`the transgranular fracture with many dimples on the fracture surface is observed,
`while a Cu-Al-Ni alloy is very brittle in a polycrystalline state and the typical
`intergranular fracture occurs. These differences of deformation and fracture be-
`havior in both alloys were attributed to the large difference in the elastic anisotropy
`of these alloys (6,16,17). The elastic anisotropy of a Cu-Al-Ni alloy is known to be
`about 13 ( l a ) , while that of a Ti-Ni alloy is about 2 (19) . However, there i s an-
`If
`other important factor which controls the deformation and fracture behavior.
`a deformation mode i s available at low stress levels the stress at grain boundaries
`is easily relaxed by this deformation mode.
`In a Ti-Ni alloy which is not given a
`special heat-treatment, dislocations move easily at a stress of about 100MPa a s
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`-2
`
`s h o w n in F i g . 6 . B u t t h e stress f o r i n d u c i n g s l i p in a Cu-Al-Ni alloy a m o u n t s t o
`a b o u t GOOMPa, w h i c h is the s a m e v a l u e of the f r a c t u r e stress in a s i n g l e crystal of
`t h a t alloy ( 1 6 ) .
`* M a n y i n v e s t i g a t i o n s have b e e n d o n e o n the t e n s i l e p r o p e r t i e s of Ti-Ni a l l o y s ,
`b u t the r e s u l t s c o f l i c t w i t h e a c h o t h e r in the e s t i m a t i o n of y i e l d s t r e s s f o r s l i p .
`Some a u t h o r s ( 2 0 ) % ( 2 2 ) s t a t e t h a t t h e y i e l d p o i n t is at the b e g i n n i n g o f t h e t h i r d
`s t a g e of d e f o r m a t i o n , b u t o t h e r s ( 1 ) t h a t s l i p o c c u r s d u r i n g the s e c o n d s t a g e .
`I t is n o w c l e a r , h o w e v e r , b y the p r e s e n t i n v e s t i g a t i o n t h a t these c o n f l i c t s in t h e
`p a s t d a t a a r e c a u s e d b y t h e d i f f e r e n c e in Ni-content a n d h e a t t r e a t m e n t o f e a c h
`s p e c i m e n .
`
`F r o m these f a c t s w e c a n c o n c l u d e that the h i g h d u c t i l i t y in the Ti-Ni alloy is
`d u e t o the low e l a s t i c a n i s o t r o p y a n d t o the low y i e l d s t r e s s f o r s l i p , while that
`t h e low d u c t i l i t y a n d b r i t t l e i n t e r g r a n u l a r f r a c t u r e in the Cu-Al-Ni alloy is d u e t o
`t h e o p p o s i t e p r o p e r t i e s . E v e n a m o n g 6 p h a s e a l l o y s , the Cu-Zn-A1 a l l o y s w i t h l o w e r
`A1 c o n t e n t is m u c h d u c t i l e t h a n the Cu-Al-Ni alloy. T h i s is b e c a u s e the y i e l d s t r e s s
`f o r s l i p i n t h e f o r m e r is m u c h l o w e r t h o n that in the l a t t e r . T h i s d i f f e r e n c e i n the
`y i e l d stress a m o n g the t w o a l l o y s are o s s i b l y r e l a t e d w i t h the crystal s t r u c t u r e s of
`t h e m a t r i x p h a s e of the t w o a l l o y s (237; the f o r m e r is of D o g t y p e o r d e r e d struc-
`t u r e , while the l a t t e r is of B 2 t y p e . B e c a u s e the B u r g e r s v e c t o r of the s u p e r d i s l o -
`c a t i o n of the f o r m e r h a s t w i c e l a r g e r t h a n t h a t of t h e latter.
`A c k n o w l e d g m e n t . - T h i s w o r k w a s p a r t i a l l y s u p p o r t e d b y t h e Grant-in-Aid f o r F u n d a -
`m e n t a l S c i e n t i f i c R e s e a r c h ( E n e r g y T o k u b e t s u , 1981) f r o m the M i n i s t r y o f E d u c a t i o n of
`J a p a n a n d p a r t i a l l y b y the R e s e a r c h G r a n t f r o m N i s s a n S c i e n c e F o u n d a t i o n .
`R e f e r e n c e s
`( 1 ) MIYAZAKI,S., O T S U K A , K . , a n d S U Z U K I , Y . , S c r i p t a Met. 1 5 (1981) 287.
`(2) C O O P E R , J . E . . B0WKER.D.E. a n d C R 0 S S . W . B . . P r o c . o f tG 15th National
`SAMPE S y m p o s i u m , L o s A n g e l e s , Calif (1969).
`[ 3) HONMA , T . , P r o c . J o i n t U . S . / J a p a n S e m i n a r o n Mechanical B e h a v i o r of Metals
`a n d A l l o y s , T r o y , U.S.A.
`(1979) 72.
`( 4 ) WASILEWSKI , R . J . , Met. T r a n s . 2 (1971) 2973.
`(5) KHAGHIN , V . N . , GJUNTER .V.E., SIVOKHA , V . P . a n d SAVVINOV,A.S.,
`o f ICOMAT-79 at B o s t o n (1979) 474.
`( 6 ) MIYAZARI , S . , OTSUKA , K . , SAKAMOTO , H . a n d SHIMIZU , K . , T r a n s . J a p a n
`I n s t . Metals 22 (1981) 244.
`(7) M I Y A Z A K I , ~ : ~ ~ ~ O T S U K A , K . , S c i . R e p . R e s . Inst. T o h o k u U n i v .
`S u p p l . 1 (1981) 111.
`a n d SHIMIZU,K., A c t a Met. 27 (1979) 585.
`(8) O T S U K A , K . , SAKA!dIOTO,H.
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`
`P r o c .
`
`PGR2015-00019 - Ex. 1021
`US ENDODONTICS, LLC., Petitioner