`
`etal and Ceramic
`
`iomaterials
`
`
`
`Volume II
`Strength and Surface
`
`Editors
`
`Paul Duchey-ne, Phi).
`Assmiale Professor of
`Biomedical Engineering
`Associate Professor of
`
`Orthopaedic Surgery Research
`University of Pennsylvania
`Philadelphia, Pennsylvania
`
`_
`'
`
`Garth W. Hastings, Ph.D., D.Sc., C.Chem., F.R.S.C.,
`F.R.R.I.
`Acting Head
`Biomedical Engineering Unit
`North Staffordshire Polytechnic
`Honorary Scientific Officer
`North Staffordshire Health Authority
`Medical Institute
`Hanshill, Sloke-on—Trent
`England
`
`CRC Series in Structure-Property Relationship of Biomaterials
`Series Editors-.in-Chief
`Garth W. Hastings and Paul Ducheyne
`
`CRC Pre'ss, Inc.
`Boca Raton, Florida
`
`
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`IRON BU RG EX2-020,I Paige-1
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`IRONBURG EX2020, Page 1
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`

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`Libraryr of Congress Cataloging in Publication
`Main entry under title:
`
`Metal and ceramic biomat'eriais.
`
`(CRC series in structureproperty relationship of biomatezials)
`Bibliography: p.
`Includes index.
`Contents: v. 1, Structure -——- v. 2. Strength and
`surface related behavior.
`I. Ceramics in medicine. 2. Metals in Surgery.
`I. Dueheyne. Paul.
`11. Hastings. Garth w.
`in. Series.
`R857.C4M47
`1934
`610223
`swims
`ISBN 0.3493.6251—x tv. 1]
`ISBN '0-84916262-8 (v. 2)
`
`This book represents information obtained from authentic and liighlg,l regarded sources. Reprinted material is
`quoted with permission, and sources are indicated. A wide variety of references are listed. Every reasonable effort
`has. been made to give reliable data and information. but the author and the publisher cannot. assume responsibility
`for the vaiidity of all materials or for the consequences oftheiruse.
`
`All rights reseriied. This book, or any pans thereof, may not be reproduccd in any form without written consent
`from the publisher.
`
`Direct all inquiries to CRC Press. [nc.. 2000 Corporate Bivd., N.W.. Boca Raton. Florida. 33431..
`
`0 1984 by CRC Press, Inc.
`
`International Standard Book Number 0-8493v626l-X (V. l)
`international Standard Book Number 0-8493-6262—8 (v. 2)
`
`Library of Congress Card Number 83—-l50l'8
`Printed in the United States
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`Volume II: Strength and Surface
`
`63
`
`Chapter 3
`
`SHAPE MEMORY ALLOYS
`
`R. Koushroek
`
`TABLE OF CONTENTS
`
`Introduction.............-......................................................... 64
`
`Historical Background .................................... _. ...................... 64
`
`C.
`1).
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`Shape Memory Alloys for Medical Application .................................. 79
`=
`A.
`Bioeornpatibility .......................................................... 80
`
`
`B.
`DesignConsiderations....................................................80
`
`
`
`
`C.
`Potential Applications .................................................... 81
`
`
`1.
`Dentistry and Orthodontics ........................................ Si
`
`
`2.
`Orthopedics ....................................................... 83
`-;-
`
`
`3.
`Rehabilitation ..................................................... 84
`' g.
`
`
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`Heart and Vascular Surgery ....................................... 85 4.
`V1.
`Concluding Remarks ............................................................. 87
`
`Nature and Mechanism of Shape Memory Alloys ................................ 65
`A.
`Martensitic Transformation ............................................... 65
`B.
`Martensitie Morphology ................................................. 66
`l.
`Thermally Induced Martensite .................................... 66
`2.
`Stress-Induced Martensite ......................................... 67
`3.
`Reorie'nted Mar-tensite............................................. 68
`Crystallographic Requirements For Shape Memory Alloys ............... 69 '
`Shape Memory Effect ......... . .......................................... 70
`l.
`One-Way Shape Memory Effect .. .
`. . .
`.
`. . . .; ...................... 70
`2.
`Two—Way Shape Memory Effect .................................. 72
`Pseudo-Elasticity ......................................................... 7'3
`l.
`Pseudo—Elasticity by Transformation .............................. 73
`2.
`Pseudo-Elasticity by Reorientation .............................. .
`. 74
`3.
`Pseudo—Elasticity by Transformation and Reorientation ........... ”M
`Overview of the Coherence between Structure and Memory Effects ...... 75
`
`_
`=
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`E.
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`F.
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`Structure Related Mechanical Properties ......................................... 75
`A.
`_ Young‘s Modulus and YieldeStress ....................................... 75
`B.
`Recovery Stress .......................................................... 7‘7
`C.
`Shape Memory Fatigue ................................................... 78
`
`IV,
`
`V.
`
`
`
`Acknowledgments ....................................................................... 87
`
`References........... , .................................................................... 87
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`64
`
`Mara! and Ccramr‘c Bimnarcrv‘rrls
`
`I. INTRODUCTION
`
`In the past few years the shape memory alloys have attracted the attention of metallurgists
`and design engineers because of a number of remarkable properties which open a revolu—
`tionary way of designing on the basis of entirely new principles compared with conventional
`alloys. The most striking features of this family of ailoys are the shape memory effect,
`the
`pseudo—elasticity, and the very high damping capacity. A short definition of these effects
`will be given here for convenience, but a more extensive description of the shape memory
`effect and the pseudo-elasticity, which are important in the context of this chapter. will
`follow later.
`
`Shape memory effect m This is the phenomenon by which, after an apparent plastic
`deformation, a metal alloy upon heating starts to remember its original shape at a certain
`temperature and returns to its deformed shape upon cooling.
`Pseudo-elasticity .— The effect by which a material recovers the induced “plastic" strain
`upon unloading is known as the pseudo—elasticity. The amount of this reversible strain is
`much greater than the classical. elastic strain. [n constrast to the shape memory effect. the
`temperature remains constant.
`the damping
`High damping capacity -- As a result of a change in internal structure,
`capacity of these alloys can be varied considerably as a function of temperature, resulting
`in a very high damping capacity in certain temperature ranges.
`These three features are associated with a martensitic transformation and this association
`limits the family of alloys exhibiting these special effects. in principle, all the alloys which
`exhibit a martensitic transformation are potential shape memory alloys, but experience shows
`that the effects only appear significantly in alloys having a reversible martensitic transforr
`motion, e.g.. nickel— and copper-based alloys (c.g., Tim and Cu-Zn-Al).
`Although the first basic information concerning the shape memory alloys was already
`observed about 40 years ago, it was in the 19605 that the usefulness was recognized. Since
`that time many potential applications of the shape memory alioys have been suggested.
`In this review the potential uses of the shape memory alloys in medical applications will
`be highlighted (Section V). However, first, in order to understand fully the working principles
`of the biomedical devices, the basic metallurgical mechanisms of the shape memory alloys
`and the related effects will be discussed (Sections Ill and IV).
`
`ll. HISTORICAL BACKGROUND
`
`The first observed shape memory phenomenon is the pseudo-elasticity. In 1932' Olander
`observed this in a Au-Cd alloy and called it “rubbeplike” behavior.‘ In the 1950s this
`phenomenon was also recognized in other alloys, e.g., In-Tl, Ctr-Zn, and Cu—Al-Ni. Because
`of the great amount of reversible strain, this effect is also called ”superelasticity“. The
`maximum amount of reversible strain has been observed in a Cu—Al—Ni single crystal with
`a recoverable elastic strain of 24%.2
`
`The first steps on the discovery of the shape memory effect were made in 1938 by Greninger
`and M'ooradian,'-‘ observing the formation and disappearance of martensite with falling and
`rising temperature in a Cu—Zn alloy. However, this basic phenomenon of the memory effects,
`the tltcrmoelastic behavior of the martensite phase, was first extensively studied 10 years
`later by Kurdjumov and Khandros.“
`Since the first observation of the shape memory effect in Au-Cd in 1951,3 the effect has
`also been reported in other alloys, cg, Cu~Zn, ln~Tl, Cu—Al—Ni, Ag—Cd, Ag-Zn, CurAl,
`Fe-Pt. Nb—Ti. and NisAl, but the great breakthrough came in the early 19605, when Buehler
`et al." of the U.S. Naval Ordnance Laboratory (now called the U.S. Naval Surface Weapons
`Center) discovered the shape memory effect in an equiatomie alloy of nickel and titanium,
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`65
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`Volume H .' Strength and Surface
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`since then popularized under the name Nitinol (Nickel-litanium Naval Qrdnance Labora-
`tory). With this alloy complete recovery of a maximum strain of 8% can be achieved by
`the shape memory effect, associated with a considerable force, which can perform work.M
`As Nitino] is difficult and expensive to manufacture and fabricate, the attention of metal-
`lurgists reverted to one of the first shape memory alloys Ctr-Zn (brass). With the discovery
`that addition of small amounts of aluminium to brass raised its transition temperature con-
`siderably," the shape memory effect of this new Cu-ZnnAl alloy could now be used in many
`practical applications at or near room temperature as was already possible with Nitir‘iol, The
`great advantage of this CunZn—Al alloy in comparison with N-itinol is that it is much cheaper
`and much easier to machine and fabricate. Since [969, a major part of the fundamental
`research on Cu-Zn-Ai shape memory alloys was done by Deiaey et al."‘"'
`Several applications of the Ti-Ni and Ctr-Zn.—Al shape memory alloys have been developed,
`e.g., tube fitting systems, self-ereetable structures, clamps, greenhouse window openers,
`thermostatic devices, different themtomechanical applications for automobiles, heat-engines.
`and biomedical applications. Several international symposia have been devoted exclusively
`to these alloys. in 1968- a first symposium concerning the shape memory alloy Nitinoi was
`held in the US.” followed in 1975 by the first international symposium on shape memory
`effects in alloys and applications at Toronto, Canada.” Since 1976 shape memory alloys
`and the mechanisms are an ever recurring topic at such conferences as the International
`' Conference on Martensitic Transformations {iCOMA'l‘).
`
`III. NATURE AND MECHANISM OF SHAPE MEMORY ALLOYS
`
`The striking features of shape memory alloys are all closely related to the martcnsit-ic
`transformation.
`It
`is thus of value to describe the martensitie transformation and related
`phenomena first. It should also he noted that the exact nature and mechanism governing the
`behavior of shape memory alloys is not identical for all shape memory alloys.
`
`A. Martensitie‘ Transformation
`Cohen et al.M formulated the next definition of martensitic transformation: “A martensitic
`transformation is a lattice-distortive, virtually diffusionless structural change having a dom-
`inant deviatoric component and associated shape change such that strain energy dominates
`the kinetics and morphology during the transformation." If diffusion rules the transformation.
`atoms are changing places. in the lattice in an uncoordinated way over long ranges. The
`resulting process can be fully described by the diffusion laws of Fick or by the method of
`Matano. However, if as is the ease in the martensitic transformation, a diffusionl'ess trans—
`formation oecurs, a coordinated movement of large blocks of atoms is decisive for the
`resulting structure. The most well-known martensitic transformation is the phase transition
`responsible for the hardening of carbon steel caused by quenching after annealing at high
`temperature.
`In this case the austenitic, highwtemperature fecvstructure changes into the
`martensitic, hot-structure on cooling.
`From now on, regardless of the crystal structure, we will denote the high-temperature
`phase as the austenitic phase, while the product of the transformation will be called mar-
`tensitic. 1n the present context, these terms are certainly not limited to ferrous materials.
`Concomitant with the homogeneous lattice deformation, caused by the movement of the
`of atoms, dominant deviatoric shear displacements can cause an external meas~
`large blocks
`d strain energy will exert a dominant influence on the
`urahle shape change. The associate
`formation. it is in this dominant influence of the strain
`kinetics and morphology of the trans
`that many nonfcrrous martensitic systems show the
`energy on the growth characteristics
`ferrous martensites do not, because of differences in
`shape memory effect, whereas most
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`66
`
`Metal and Ccr‘amt'c' Biomnrerials
`
`growth behavior. The effect of these differences in growth behavior on the potential of an
`alloy to exhibit the shape memory effect by transformation is related to the driving force at
`the onset of growth.‘0 It is generally agreed that the reason for the transformation is the
`difference between the free energies of the austenitic and martensitic structure, i.e., one
`kind of interatomic bonding is energetically favored at lower temperatures, below some
`equilibrium temperature, T“.
`If the formation and disappearance of the martensitic phase is directly responsive to
`alternation in temperature, i.e., atherrnoelastic behavior, the driving force is small (asin
`the nonferrous systems}. while the stress necessary to induce growth is accordingly small.
`If the driving force at the onset of growth is large due to the absence of a thermoelastic
`equilibrium (the ferrous systems), growth of the martensite proceeds at high rates (of the
`order of the velocity of sound} which leads to a strongly disturbed, defective grain boundary
`which is subsequently essentially immobile. In this last case the shape memory effect appears
`for efficient application purposes between too broad and therefore unacceptable temperature
`boundaries (200 to 300°C}.
`
`3. Martensitic Morphology
`Martensitic transformations can be induced by changes in temperature as well as by the
`application of stress. This can he explained by the following effects: (1) the free enthalpy
`of the austenitic and martensitic phase and so their cquilibria depend not only on changes
`in temperature and composition, but also on stress; and (2] the nucleation and growth process
`are associated with shear strains and these will interact with stresses acting within, or applied
`to. the specimen. These thermodynamic and kinetic effects are strongly dependent on the
`direct-ion of stresses with respect to the lattice orientations. Thus,
`two main groups of
`martensite can be recognized: (l) thermally induced martensite and (2) st-resseinduced
`martensite.
`
`l. Thermally Induced Martensite
`Thermally induced martensite is mainly characterized by its temperature dependence. It
`forms and grows continuously as the temperature is lowered and shrinks and vanishes
`continuously as the temperature is raised. In this case the transformation proceeds essentially
`in equilibrium between the chemical driving energy of the transformation and a resistive
`energy whose dominating component is the stored elastic energy. The transformation as-
`sociated with this kind of thermally induced martensite is called a thermoelastic martensitic
`transformation and characterized by its transition temperature A“ Ar, M, and Mr (Figure
`IA).
`
`The growth rate appears to be governed solely by the rate of change in temperature.”I6
`However, if the transformation occurs spontaneously whenever the chemical driving energy
`largely exceeds the resistive energy (ie, the growth rate is independent of the rate of
`temperature change). the resulting martensitc is not called thcmloelastic anymore, but burst
`martensite.
`.
`
`The thennoelastic, as well as the burst martensitic transformation, are frequently either
`partially of fully self-accommodating. The martensite forms either in zigzag arrays (burst
`martcnsite), in packets (massive martcnsite}, in groups, or in hands.
`If the tnartcnsitic transformation is self—accommodating, the orientation of the growing
`martensite plate with respect to the orientation of its neighbor is the one that is energetically
`most stable in that particular strain field. The strain associated with one variant compensates
`the strain in the other variants. This requires the accommodation of all the Inca] distortions
`involved in the formation of the individual
`lamellae with minimum macroscopic strain
`regardless of crystal direct-ion. Because of the absence of sufficient time to permit any
`relaxation of the resulting local stresses, high densities of dislocations are observed in. self~
`aeconunodating martensite. These dislocations are formed as the result of local strain energy
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` Voltaire H: Strength and Surface
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`67
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`TEMFEFMTURE
`
`Thermoclastic martcnsitic transfermation in function of
`FIGURE 1 (A).
`lcmpcrnturc, with A,
`..—. start transformation on hearing; A, # end trans-
`formation on heating; M, = slart transformation on cooling; M, = end
`transfonnation on cooling, all at zero stress;and VmN == fractional volume
`of the martensite product.
`
`.3
`g
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`FIGURE. 1 (l3).
`Schematical transformation of bursl martenstte on cool—
`ing. The initial bursts are expected to be large. decreasing rapidly as the
`martensitic fractional volume increases.”
`as an alternative of the local breakdown of the lattice and not in response to any specific
`shear stress as is usually the case.
`Therefore these dislocations are immobile and called accommodation dislocations, nec—
`essary to actommodate the microscopic transformation strain.” The maximum number of
`possible martensite orientations within one grain depends on the crystal symmetry of the
`austenitic phase. For a cubic austenitic phase, 24 mancnsite plate variants can occur.”
`1n the absence of external applied stresses and when the volume Change is negligible, the
`thermally induced martensitie transformation is characterized by random martensite plate
`variants, resulting in a minimum or zero macroscopic shape deformation. However, if a
`constant external applied uniaxial stress assists the thermally induced martensitic transfer
`motion thermodynamically, only a limited number of thermoelastic martcnsite plate variants
`are expected to grow or, in the case of self-accommodating formations, certain martensite
`plate variants will become dominant in the different groups. This will lead to an external
`macroscopic shape change.
`
`“m
`T
`
`
`
`W
`
`TEMPERATURE
`
`2. Stresanduced Mortem't'fe
`The stress-induced martcnsite is a mechanical analogue to the thermally induced marten-
`
`'. E.
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`site. In this case the transformation proceeds continuously with increasing applied uniaxial
`?
`
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`68
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`Metal and Cermnic Biomatert'als
`
`
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`§
`
`.,
`
`l
`
`v
`
`which is followed by the immediate and also stress-assisted transformation of this austenite
`
`STRESS
`
`Stress—induced. then-noelastic martensitic transfomtation in
`FIGURE 2.
`function-of the applied stress. with o" ‘ M = stress at which transformation
`:Iustenite to martensite starts and e” i“ ‘ “ = stress at which the reverse
`transformation martensile to auslenite starts.
`
`stress and is reversed continuously when the stress is decreased (Figure 2), while the tem-
`perature remains constant. Stresswinduced martensite can be thermoelastic as well as burst-
`type maitensite.
`
`Again, those martensiteplates will preferentially grow, which are most favorably oriented
`with regard to the externally applied uniaxial stress. Therefore, stress-indoced mar-tensite
`will have a strongly textured microstructure. The influence of external stresses on the
`martensitic transformation can be expressed by using the temperature Mfg, defined as the
`temperature at which the transformation to mar‘tensite can take pla‘ee under an externally
`applied stress. The maximum temperature M, = [MDW depends on the stress conditions,
`specimen orientation or texture, the austenitic yield strength, and, possibly, other factors.
`it cannot be considered as an inherent characteristic of the material itself.l6
`If the deformation temperature, Ta,
`is approached on cooling and if A, Q 'T,J < Md,
`martensite formed from the. austeni't-ic phase will disappear .on the removal of the external
`stress. However, if M, < '1'd < A,, the stress-induced martensite variants remain predom—
`inantly thermodynamically stable upon unloading. This includes that undtir stressed condi-
`tions the M,—te'm_perature is higher than in unstressed conditions.
`
`3. Reort‘ented Martenstre
`
`If a fully thermally induced martensite structure is stressed at T < M, the martensite
`structure will be reoriented. This means that with regard to the applied stress certain pref-
`erential martensite variants will grow at the expense of" the less suitably oriented martensite
`variants. The stress necessary to initiate the reorientation decreases with increasing number
`of transformation cycles, probably due to sweeping out existing defects along the martensite
`plate boundaries. Several theories are in circulation about the mechanism related to the
`reorientation. One proposed meChanism is the elastic twinning and untw-inning in crystals,
`i.e., a reversible motion of existing twin boundaries resulting in a motion of the martensite
`plate boundary.l3 This implies that the thermally induced 'martensit'e has to be intEmally
`twinned.
`
`Another mechanism for the reorientation of the martensite plates has been proposed by
`Wasilewski. W93" If the martensite is stressed at T < M, a stress-induced austenite variant,
`transformed out of the martensite, should be a transient intermediate trahsfbrrnation step,
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`IRONBURG EX2020, Page 8
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`to another inartensite variant with another orientation with regard to the original martcnsite,
`i.e.,
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`Voltaire H: Strength and Surface
`
`69
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`Mgng’
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`IRONBURG EX2020, Page 9
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`This transformation is limited between the upper temperature Mr and the lower temperature
`Ad, corresponding with the minimum temperature at which the transformation M E,
`[3 can
`occur. The reverse transformation will ot:cur in the opposite direction upon unloading. The
`existence of the intermediate step~is a source of discussion. Other investigators describe a
`same kind of transfermation by reorientation M —) M' without this intermediate step?“
`Although the exact theory is not yet clear,
`in crystallographic terms the two models are
`equivalent. Only the atomic position before and after the application of the stress can be
`considered, but not the path followed by the atoms.
`Whether the reoriented martensite variant remains irreversible upon unloading or not, is
`dependent on the kind of shape memory alloy. If for a certain amount of strain the reoriented
`martensite is thermodynamically stable upon unloading, one has the situation required for
`one-way shape memory, induced by reorientation (Sectiori III.D.1), while in the reverse the
`requirements are present for pseudo-elasticity by reorientation (Section ll-I.E.2_).
`
`C. Crystallographic Requirements for Shape Memory Alloys
`Originally the shape memory effect was ascribed to stress—assisted compositional changes,“-34
`but this theory did not stand firm for long. In 1966 the shape memory effect in Ti-Ni was
`for the first time related to a thermoelastic martensite transformation by Zijderveld et alimé
`This association seemed to be too limited. Because in "Pi—Ni. one has observed that 50 to
`80% of the specimen apparently transforms instantaneously. This implies that the growth
`rate is not governed by the rate of change in temperature and therefore does not meet the
`definition of thermoelastic transformation, although Ti-Ni exhibits a very striking memory
`behavior.
`I
`A more general relation should be that any material exhibiting a forward and a reverse
`martensitic transformation, thermally or stress induced, is a potential shape memory alloy.
`It is only necessary to determine the exact conditions, associated with a complete and useful
`shape recovery, under which the mar‘tensitic‘ transformation occurs.”
`Another selection rule for shape memory alloys is the existence of an ordered structure.‘8
`It has been shown that the ordered FesPt-alloy exhibits a thermoelastic martensitic trans-
`formation and a related shape memory effect, while a disordered Foam—alley of the same
`composition displays neitherdg'” This can be explained on the basis that the shape memory
`effect. as well as the pseudo-elasticity, originates from a complete crystallographic revers-
`ibility of the martensitic transformation. Otsuka and Shimizu28 have shown that a path in
`the reverse t'ransfonnatio'n in ordered alloys is unique.
`in contrast to multiple paths in
`disordered alloys. They propose that the complete crystallographic reversibility of' the mar-
`lensitic transformation is characteristic of ordered alloys and rationalize the fact that the
`shape memory effect has usually been observed in ordered alloys. Besides, in the case of
`an ordering, irreversible plastic accommodation requires the creation of superdislocations,
`possessing energies which are multiples of those of the dislocations in the disordered lattice,
`while the matrix' yield stress increases considerably as the result of the ordering (Figure 3).
`Therefore, ordered alloys with their reversible martensitic transformation and their absence
`of lattice invariant plastic accommodation at the habit plane are much more favorable for
`shape memory purposes than the disordered alloys. There exists only one exception to this
`among shape memory alloys: the disordered I-n-Tl alloy.H5 However, here it involves a fee
`‘:. fct transformation, which is reversible even in disordered alloys. This stems from the
`fact that the lattice correspondence is unique in the reverse transformation because of the
`very simple lattice change and lower symmetry of the fct phase?-S
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`70
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`Mam! and Cceramic Bt‘mnotert‘ots
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`l
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`STRESS
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`onoeaeo
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`DISORDERED
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`LEVEL or SHEAR AND VOLUME
`srnnms DURING PLATE GROWTH
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`-...-._-.
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`STRAIN
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`Hypothetical stress-strain curves for ordered and disordered
`FIGURE 3.
`martensile. In the latter case transformation strains exceed [he matrix (or
`mancnsite] elastic limit and interface coherence is lost."
`'
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`Another requirement fer obtaining shape memory properties is that the martensite should
`be internally twinned.”a If the deformed alloy has to revert to its initial state, e.g., when
`heated, the deformation process must be reversible, i.e., the shape memory alloy should
`not contain mobile dislocations. A possible mechanism for plastic defermat'ion without
`mobile dislocations is internal dot-winning of themartensitic substructure. The defort‘nalion
`can result from a selective dot-winning process according to which one of the two twin
`orientations grows at
`the expense of the other. However,
`this theory seems subject to
`reservations. For example, Cu-Zn alloys are not internally twinned, yet they exhibit the
`shape memory effect.
`Although not all
`the crystallographic requirements are conclusive, the shape memory
`effects are a reality. Fortunately, a detailed understanding of the physics of a process is not
`always necessary for successful exploitation.
`
`D, Shape Memory Effect
`When a. conventional material is plastically deformed, a permanent deformation remains
`after unloading. The material can only regain its original shape by a new plastic deformation.
`The shape memory alloys are distinguished from the “usual" alloys by the fact that,
`when these alloys are apparently plastically deformed, they remember their preceding shape
`on changing the temperature. Depending on the initial amount of deformation,
`they can
`regain their original shape (totally or partially) during heating. One can recognize two kinds
`of shape memory effects: ( l) one-way shape memory effect and (2) two-way shape memory
`effect.
`
`1. OnevWay Shape Memory Efiecr
`Figure 4 depicts schematically the one-way shape memory effect. The macroscopic de-
`formation, which may not exceed a critical strain limit, is accompanied by a martensitic
`transformation. not reversed by removing the applied stress. If the specimen is fully mar—
`tensitic at the onset of the deformation (T.1 < Mr), the existing thermally induced martensite.
`M, is transformed by reorientation, according to one of the mechanisms of Section Il‘I.B.3,
`to the martensite variant M'. The reorientation takes place at the region BC of Figure 4,
`On unloading, the material recovers elastically, but a permanent deformation AD remains
`with a thermodynamically stable martensite structure. If the material is subsequently heated,
`the recovery of this deformation starts at the temperature A,“ The martensitevariant obtained
`by reorientation reverts to the austenitic phase.
`In the ideal case, strain recovery is complete if A, is reached, For Nitinol, strains of the
`order of 6 to 8% may completely recover. If subsequent changes in temperature,
`in the
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`Voinme U: Strength and Surface
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`71
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`1‘EMPERATURE
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`STRAIN
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`0:0
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`FIGURE 4. One-way shape memory effect.
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`M
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`TEMF'ERATURE
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`STRESS m
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`“-c‘lnqgu.u
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`FIGURE 5.
`Stress-assisted transformation in Cu-Aeri.”
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`absence of external forces, do 'not change the macroscopic shape anymore, the phenomenon
`is called the one-way shape memory effect.
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`Another possibility'15 that the specimen is partially both niartensitic and austenitic. If, for
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`example, M[ < Ta < Ms (Tr1 reached on cooling}, the austenitic phaseis transformed upon
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`unloading to the irreversible stress-induced ma‘rtensitc variant, resulting in a predominantly
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`martensitic structure, concomitant with a remaining deformation.
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`The last poSsiblity is that "l"_i > A,, i.e... the specimen is fully austenitic For 01-AlNi
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`the transformation path does not simply consist of a forward and a reverse transformation
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`as usual, but consists of four steps” (Figure 5). By deformation of the austenitic structure
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`[3, stress-induced martensite {3’ will grow resulting in‘ a new 11’-martensite variant con—
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`comitant with a permanent strain upon cooling below M under constant stress (._'1-.e.. sness-
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`assisted transformation) and after subsequent unloading This strain can be annihilated by
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`subsequent heating above the A;Af range, where the 'y rmartensite variant regains its original
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`B—austcnite structure.
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`72
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`Metal and Ceramic Biortmterictfs
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`i
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`STRESS
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`TEMPERATURE
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`the number of martensite variants, formed when an alloy is repeatedly heated and cooled
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`FIGURE 6.
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`Two-way shape memory effect,
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`A remarkable effect on heating is the appearance of an external measurable force, which
`can perform work. Because of the higher symmetry of the austenitic structure compared
`with-that of the martensitic structure, the specimen strongly prefers to regain the austenitic
`state on heating. A part of the transformation energy is released as this recovery stress.
`
`2. Two-Way Shape Memory Efiecr
`In contrast with the one-way shape memory effect, subsequent cooling does indeed in-
`fluence the macroscopic shape, while no external forces are applied. As Well as the material
`remembering its undeformed shape on heating,
`it also remembers the deformed shape on
`cooling. Figure 6 depicts this phenomenon schematically. After deformation and unloading
`the material, a permanent deformation, AB, remains. Due to an initial n