Phase Transitions and Shape Memory in NiTi
`
`HUNG C. LING AND ROY KAPLOW
`
`The "premartensitic" transformation of the B2 phase in NiTi was studied using elec(cid:173)
`trical resistance measurement, X -ray diffraction, and shape-change effects. The degree
`of rhombohedral distortion of that transformation and the electrical resistance change
`show similar temperature dependence. In particular, when TR »M s' this encompasses
`a narrow temperature region of rapid increase just below TR, followed by an extended
`range of levelling off. The onset of the martensitic transformation is not necessarily
`preceeded by the rhombohedral distortion, and the latter proceeds continuously below
`TR and persists throughout the martensite transformation range. Thus three kinds of
`phase transformation can occur: B2 = R, B2 = M, R = M; B2 = R is not a precursory
`phenomenon. Observations un the two-way memory (TWM) and its correlation to re(cid:173)
`sistance changes in NiTi leads to the conclusion that when TR > M s , the TWM is com(cid:173)
`prised of two stages. On cooling, the B2 - R transformation is responsible for the initial
`stage, which is thermally reversible without hysteresis. The second stage of the TWM,
`which is due to martensite formation, is reversible in shape change, but with a thermal
`hysteresis. It is also suggested that on heating, the R - B2 transformation can contri(cid:173)
`bute to the primary shape memory effect in NiTi.
`
`I. INTRODUCTION
`It is generallyrecognized that the shape memory and
`related effects (see Fig. 1) in {3-aUoys of the noble
`metals are associated with the interaction of stress
`with the martensitic transformation.l Most of these
`alloys have a CsCl or Fe:>Al structure at high temper(cid:173)
`ature and all of them transform martensitically to a
`low temperature phase with a lower symmetry. A
`second group of alloys which exhibit the shape mem(cid:173)
`ory effect are binary alloys of transition metals,2 the
`most extensively studied of which is NiTi. While the
`high temperature phase in NiTi is also considered to
`be the CsCl (B2) type,3-4 and the martensite a mono(cid:173)
`clinic distortion of the B19 structure, 5 there is an in(cid:173)
`tervening temperature range where anomalous diffrac(cid:173)
`tion effects have been observed. These include diffuse
`scattering streaks,3,5-7 extra diffraction spots at 1/3
`positions of the B2 reciprocallattice,4,5,7 the intensi(cid:173)
`ties of which increase on cooling. 3,4,8 In addition, the
`electrical resistance often exhibits an anomalous in(cid:173)
`crease at temperatures above Ms, * on cooling from
`*Ms: The temperature at which a perceptible amount of low temperature,
`martensite phase is formed.
`the high temperature B2 phase.3,5,9 The latter effect
`is often preempted by a high M s temperature, but can
`be enhanced by factors which lower the M s' such as
`a judicious c~oice of the final annealing temperature,
`thermal cycling of the specimen in the transformation
`range,t° or compositional variations. l l While diffuse
`streaks above Ms also occur in other i3-alloys, only
`in NiTi are there diffraction and resistance phenomena
`which do not seem to be attributable to martensitic
`transformation. These effects, as a group, have come
`to be called the "premartensitic" phenomena. How(cid:173)
`ever, the structural change occurring in this temper(cid:173)
`ature range is not well understood.
`
`-~---------
`
`HUNG C. LING and ROY KAPLOW are Research Associate and
`Professor, respectively, Department of Materials Science and Engi(cid:173)
`neering, Massachusetts Institute of Technology, Cambridge, MA
`02139.
`Manuscript submitted June 15, 1979.
`
`For convenience, we shall refer to the various
`phases as follows:
`l.B2 - the high temperature phase, the equilibrium
`structure after the anneal at ~500°C;
`2.R - the phase associated with "premartensitic"
`behavior, apparently tied to structural changes, in(cid:173)
`duding a rhombohedral distortion of the B2 phase;3,4,8
`3.13 - the structure encompassing both B2 and R;
`4. martensite - the low temperature phase, resulting
`from the martensitic transformation of both B2 and
`R.
`As in the case of noble metal alloys, the shape
`memory and two-way memory in Ni-Ti have been dis(cid:173)
`cussed in the context of stress-assisted martensite
`transformation .12-16 The role of the "premartensitic"
`phase has been ignored. In this paper, we shall pre(cid:173)
`sent data aimed at clarifying the "premartensitic"
`phase transformation and its role in the two-way
`memory effect, especially.
`
`II. EXPERIMENTAL PROCEDURES
`
`The materials used in this study were drawn Wires,
`approximately 0.020 in. in diam. Two batches, num(cid:173)
`bered A383 and V 4865, * were chemically analyzed to
`* A383 wire was supplied by the Nitinol Technology Center, Naval Surface
`Weapons Center, White Oaks, MD. It had been strand annealed at 500°C in the
`as-received condition. V4865 wire was purchased from the Timet Corporation.
`In the as-received condition, it is totally martensitic.
`be of composition 50.13 at. pct Ti, 49.96 at. pct Ni,
`and 50.12 at. pct Ti and 49.86 at. pct Ni, respectively,
`with an error of 0.1 at. pct. Specimens for electrical
`resistance measurement and X -ray diffraction were
`heat treated after encapsulation in vycor tubes evac(cid:173)
`uated to 1O~ torr and back filled with high purity ar(cid:173)
`gon. Electrical resistance was measured with the
`standard dc method using a four-point terminal con(cid:173)
`nection. At-temperature X -ray diffraction measure(cid:173)
`ments were made using a heating and cooling stage on
`a GE diffractometer. A number of straight wires were
`glued together to form the diffracting surface. CuKO'
`radiation was used. The integrated intensities of Bragg
`
`METALLURGICAL TRANSACTIONS A
`
`ISSN 0360-2133/8010111-0077$00.75/0
`© 1980 AMERICAN SOCIETY FOR METALS AND
`THE METALLURGICAL SOCIETY OF AIME
`
`VOLUME 11A, JANUARY 1980-77
`
`Lombard Exhibit 1028, p. 1
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`it must be due simply to the elastic response of the
`alloy to external deformation.
`Both a) and b) attained 100 pct shape recovery at
`32 ± ec and the shape memory did not deteriorate up
`to 20 cycles. However, there were differences in the
`detailed shape of the recovery curve as well as in the
`electrical resistance curve. In both cases, As (a)
`was above 20°C, which was the As (a'" 0) temperature.
`In a), the specimen consisted of a mixture of Rand
`SM after straightening; heating from 0 to 26°C resulted
`in a resistance decrease indicative of the R - B2
`transformation, with a gradual decrease between 0 and
`22° C and a rapid decrease from 22 to 26° C (Fig. 8 (a)).
`If As (a) is below 26°C, reversion of some SM to R
`would also have occurred. We believe that R - B2 is
`responsible for the small amount of shape recovery
`between O°C and As (a) while both the R - B2 and
`M - R transformations account for the shape change
`from As (a) to 26°C. At 26°C, the R phase has com(cid:173)
`plete ly converted to the B2 symmetry. The fact that
`the electrical resistance at 26°C was higher than that
`of an unstrained specimen indicated that some SM
`remained. From 26 to 33°C, the small electrical re(cid:173)
`sistance decrease is associated with the SM - B2
`transformation, which is responsible for the major
`portion of shape recovery, in accordance with the
`generally accepted model. AJ (a) was 33°C.*
`
`- - - - - " - - - - -
`*A specimen under bending strain has a variable stress (a) distribution in its
`cross-section.
`
`In case b), where the specimen consisted of a mix(cid:173)
`ture of R and oriented martensite variants after
`straightening, the electrical resistance followed a
`linear temperature dependence on heating from 0 to
`22°C. The gradual change in e in this range is due to
`R becoming less distorted (R - B2). Between 22°C
`[A~(a)J and 26°C (TR), the resistance increased (M
`- R) and both the M - Rand R - B2 transformation
`are responsible for the shape change. Above 26°C, the
`resistance decrease and the rapid and large shape re(cid:173)
`covery is due to the M - B2 transformation.
`In the past, SME in NiTi alloys has been discussed
`on the basis of the interaction between external stress
`and the martensitic transformation. There are several
`reasons:
`i) SME occurs in a variety of alloy systems which
`undergo the martensite transformation. In most of
`these systems, it appears that only the high tempera(cid:173)
`ture !3-phase to martensite transformation occurs,
`and hence that the only possible mechanism is stress(cid:173)
`assisted formation or reorientation of martensite,
`with the subsequent reversion from martensite to !3
`being responsible for the shape recovery.
`ii) Recent studies on the "premartensitic" phenom(cid:173)
`ena in NiTi5
`,6 and in other !3-alloys1 considered the
`cause of those experimental effects to be dynamic,
`soft phonon modes, which initiated the appropriate
`shear to transform the B2 phase to martensite. As
`such they would not be crystallographic changes which
`can give rise to a macroscopic shape change. On the
`other hand, Wang et al 21 interpreted their results as
`being due to a crystallographic transformation from a
`high temperature mixture of B2 phase and a phase of
`P3ml symmetry to martensite. The observation of a
`split (Oll)p peak on cooling but not on heating led
`them to propose a two step transformation on cooling
`
`and a one step transformation on heating. This was
`probably incorrect. While the rhombohedral distor(cid:173)
`tion of the reverted !3-phase might not be large enough
`to cause splitting, it should broaden the peak. This
`was observed in our experiments and clearly demon(cid:173)
`strated in Ti50Ni41Fe3.11
`iii) In shape memory experiments on NiTi,13,16-17,22
`the actual phase changes were not monitored during
`shape recovery. Thus it is not clear that the marten(cid:173)
`site to B2 phase transformation is the only mechanism
`contributing to SME.
`In Section lILA, we established that three kinds of
`crystallographic changes can occur on heating in a
`shape memory experiment on NiTi: thermal or strain(cid:173)
`martensite to R, thermal or strain-martensite to B2
`and R converting to the cubic symmetry of the B2
`phase. A contribution from the structural change R
`- B2 to shape memory cannot be ruled out. On the
`basis of simultaneous measurements of shape recov(cid:173)
`eryand electrical resistance, coupled with diffraction
`measurements of the R distortion at the same temper(cid:173)
`atures, we provide evidence that the small initial shape
`change below 20°C in the A383 specimens is due to R
`becoming less distorted. However, it may not be pos(cid:173)
`sible to delineate the separate contributions in NiTi
`because the different transformations overlap. A ter(cid:173)
`nary alloy in which the M - R transformation is com(cid:173)
`pleted before the R phase completely returns to B2,
`such as Ti50NixFe5o _ x, 11 may be used to test whether
`the R - B2 transformation can contribute more sub(cid:173)
`stantially to shape memory. Our results on two-way
`memory, which will be discussed in the next section,
`support this conjecture.
`
`C) Two-Way Memory (TWM)
`
`After one cycle of SME, the specimen was allowed
`to cool from the B2 phase. On cooling from 50° C, the
`eiectrical resistance in sample A383 fOllowed the
`curve of an un strained specimen. It began to increase
`at 26°C with the most rapid increase at 23°C. Simul(cid:173)
`taneously, the U -shaped specimen started to unwind
`so that the included angle e became greater than
`o deg. The change of e in this two-way memory fol(cid:173)
`lowed closely the resistance change in the tempera(cid:173)
`ture range above M s' and was similarly reversible
`without thermal hysteresis. For example, cooling to
`O°C followed by reheating to room temperature led to
`a net e value of 12 deg. Subsequent COOling to -19°c
`with rewarming to room temperature again resulted
`in a net e value of 12 deg. The TWM began to exhibit
`a hysteresis when the specimen was cooled below
`Ms (-24°C). For example, room temperature net e
`values of 14 and 23 deg were observed after cooling
`to -26 and -45°C, respectively, and warming to room
`temperature. (Samples were heated to 40°C to restore
`the perfect U -shape.) The variation of e with tempera(cid:173)
`ture for a final cooling to -69°C is shown in Fig. 9.
`An additional significant increase in e began near M s
`and e reached a saturation value only after the mar(cid:173)
`tensite transformation had stopped. On subsequent
`heating, the specimen showed very little change in
`shape until 21°C, when rapid reversion of the two-way
`memory occurred concurrently with a peaking of the
`electrical resistance. Results on V4865 were similar.
`Although the latter wire showed a rapid deterioration
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`MET ALLURGICAL TRANSACTIONS A
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`VOLUME IIA, JANUARY 1980-81
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`Lombard Exhibit 1028, p. 5
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`third, while the R - M transformation is responsible
`for the remaining two-thirds of the TWM in A383.
`From the above discussion, it is also clear that the
`term 'reversible memory', which is used synony(cid:173)
`mously with two-way memory in the literature, can
`be misleading. It gives the impression that two-way
`memory is reversible in shape change without ther(cid:173)
`mal hysteresis. Actually, only the memory resulting
`from the B2 - R transformation in NiTi is reversible
`in this sense. The memory associated with stress
`assisted martensite formation is only reversible in
`shape change but with a thermal hysteresis due to the
`martensite reversion hysteresis. So in alloy systems
`which undergo only the martensite transformation, the
`term 'two-way memory' should be used.
`
`IV. CONCLUSION
`
`In this study, we have demonstrated that the rhombo(cid:173)
`hedral distortions associated with B2 - R trans for -
`mation in NiTi can begin either prior to or subsequent
`to the martensitic transformation. Below the critical
`temperature, TR, the B2 - R transformation can pro(cid:173)
`ceed concurrently with the martensitic transformation
`and can persist down to very low temperature or until
`all the R phase has transformed into martensite. The
`B2 to R transformation is probably second-order with
`changes in the electrical resistance coinciding with
`the continuous rhombohedral distortion.
`From shape recovery experiments and electrical
`resistance measurements, it was observed that on
`cooling, the two-way memory began at a temperature
`where the resistance also showed an increase. Addi(cid:173)
`tional shape change occurred in the range where the
`martensitic transformation took place. Thus it is
`concluded that when lv1 s < TR, the TWM in NiTi is
`comprised of two stages: on cooling, the first stage
`involves the B2 - R transformation while the second
`stage results from the R to M transformation, in the
`presence of internal stress; on heating, for As < TR
`< At, the M - Rand R - B2 transformations are re(cid:173)
`sponsible for the initial shape change while M - B2
`accounts for the remainder. From analyzing the elec(cid:173)
`trical resistance variations and shape changes of a
`U -specimen with temperature on heating, it is sug(cid:173)
`gested that the R - B2 transformation also contri(cid:173)
`butes to the primary shape memory effect in NiTi.
`
`ACKNOWLEDGMENTS
`
`We greatly appreciate the efforts of Dr. David
`Goldstein of the Naval Surface Weapons Laboratory
`Nitinol Technology Center in regard to the supply of
`nitinol wire. We also wish to acknowledge the strong
`impetus for this work provided by the medical appli(cid:173)
`cation interest of Dr. Morris Simon of Beth Israel
`Hospital and by interactions with him and his col(cid:173)
`leagues: Dr. Edwin Salzman, Dr. David Freiman, Dr.
`Aubrey Palestrant and Mr. I. Bentov. The work has
`been supported by N.I.H., under contract ROIHL20554.
`
`I. H. Warlimont and L. Delaey: Progress in Materials Science, vol. 18, p. 113,
`Pergamon Press, 1974.
`2. C. M. Wayman and K. Shimizu: Met. Sci..!., 1972, vol. 6, p. 175.
`3. D. P. Dautovich and G. R Purdy: Can. Metall. Quart., 1965, vol. 4, p. 129.
`4. M. E. Meichle, M. B. Salamon, S. M. Shapiro, C. M. Wayman, and C. M.
`Hwang: UnpUblished research, University of Illinois, Urbana, Illinois and
`Brookhaven National Laboratory, Upton, NY, 1979.
`5. G. D. Sandrock, A. J. Perkins, and R. F. Hehemann: Met. Trans., 1971, vol. 2,
`p.2769.
`6. K. Chandra and G. R. Purdy: 1. Appl. Phys., 1968, vol. 39, p. 2176.
`7. F. E. Wang, W. J. Buehler, and S. J. Pickart: J. Appl. Phys., 1965, vol. 36,
`p.3232.
`8. K. Mukherjee, M. Chandrasckaran, and F. Milillo: Shape Memory hlfects in
`Alloys, p. 177, Plenum Press, NY, 1975.
`9. C. M. Wayman, I. Cornelis, and K. Shimizu: Scr. Met., 1972, vol. 6, pp. 115-
`22.
`10. C. M. Jackson, H. J. Wagner, and R. J. Wasilewski: NASA sp5110, 1972.
`11. M. Matsumoto and T. Honma: New Aspects of Martensitic Transformation,
`pp. 199-204, JIM Int. Symposium, The Japan Institute of Metals, Kobe,
`Japan, 1976.
`12. R J. Wasilewski: Met. Trans., 1971, vol. 2, pp. 2973-81.
`13. A. Nagasawa, K. Enami, Y. Ishino, Y. Abe, and S. Nenno: Scr. Met., 1974,
`vol. 8, pp. \055-60.
`14. H. Tas, L. Delaey, and A. Deruyttere: J. Less·Common Metals, 1972, vol. 28,
`pp. 141-50.
`15. J. Perkins: Scr. Met., 1974, vol. 8, pp. 1469-75.
`16. R J. Wasilewski: Scr. Met., 1975, vol. 9, pp. 417-21.
`17. W. B. Cross, A. H. Kariotis, and F. J. Slimier: NASA CR 1433,1969.
`18. K. H. Eckelmeyer: Scr. Met., 1976, vol. 10, pp. 667-72.
`19. M. B. Salamon, M. Meichle, C. M. Wayman, C. M. Hwang, and S. M. Shapiro:
`Unpublished research, University of Illinois, Urbana, Illinois and Brookhaven
`National Laboratory, Upton, NY, 1979.
`20. F. J. DiSalvo, Jr. and T. M. Rice: Phys. Today, 1979, vol. 32, pp. 32-38.
`21. F. E. Wang, S. J. Pickart, and H. A. Alperin: 1. Appl. Phys., 1972, vol. 43, p.
`97.
`22. W. J. Buehler, J. V. Gilfrich, and R C. Wiley: 1. Appl. Phys .• 1963, vol. 34,
`pp. 1475-77.
`23. M. Foos, C. Frantz, S. DUfUpt, and G. Gavoille: Scr. Met., 1977, vol. 11, pp.
`655-58.
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`METALLURGICAL TRANSACTIONS A
`
`VOLUME IIA, JANUARY 1980-83
`
`Lombard Exhibit 1028, p. 7
`
`