Engineering Aspects of Shape Memory Alloys

491 Pages·1990·20.31 MB·English

by  T W Duerig

Engineering Aspects of
Shape Memory Alloys
T W Duerig, BS, ME, PhD
Technical Director, Raychem Corporation, Menlo Park, California, USA
K N Melton, BA, PhD
Director of Research and Development, Raychem Corporation, Swindon, UK
D StÖCkel, Dip Ing, PhD
Technical Marketing Manager, Raychem Corporation, Menlo Park, California, USA
C M Wayman, BS, MS, PhD
Professor of Materials Science and Engineering, University of Illinois, Urbana,
Illinois, USA
Butterworth-Heinemann
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First published 1990
© Butterworth-Heinemann Ltd, 1990
British Library Cataloguing in Publication Data
Engineering aspects of shape memory alloys.
1. Alloys. Shape memory effects
I. Duering,T. W.
669.94
ISBN 0-750-61009-3
Library of Congress Cataloging-in-Publication Data
Engineering aspects of shape memory alloys / edited by T.W. Duerig.
p. cm.
Includes bibliographical references and index.
ISBN 0-750-61009-3
1. Alloys-Thermomechanical properties. 2. Shape memory effect.
I.Duerig.T.W.
TN6980.E546 1990
669'.9-dc20 90-2104
CIP
Printed and bound in Great Britain by Courier International Ltd, Tiptree, Essex
Introduction
It has now been over 50 years since the first observations of shape memory, and over
20 years since people first began to find applications for the effect. Certainly many
people believe that practical application has progressed much slower than expected:
when inventive people first observe the effect they immediately begin to conjure ideas
for its application, amazed that it has been known for so long by the scientific
community and is yet nearly unknown to design engineers. Shape memory has even
become famous as "a solution looking for a problem".
But this reputation is unfair if one considers that the entire technology is new. These
are not simply new alloys of steel or titanium, but represent an entirely new philosophy
of engineering and design. The most fundamental property descriptors are different:
yield strength, modulus, and ductility are replaced by stress rate, recovery stress and
M. Moreover, product designs using shape memory are generally not evolutionary,
s
but revolutionary in nature. One can hardly expect large industries to immediately
convert basic product designs to shape memory. In fact progress has been
impressive. At the time of writing, it is estimated that the worldwide business in shape
memory exceeds 30 million US dollars, and is growing at over 25% per year. Product
diversity is also most impressive, including fine medical wires, electrical switches,
eyeglass frames, appliance controllers, pipe couplings and electronic connectors.
Still, it is clear that the technology lags behind the science. The origin and mechanism
of shape memory are now well understood, but many of the engineering aspects are
not. The purpose of this book is to extend our understanding of shape memory by
defining terms, properties and applications. It includes tutorials, overviews, and
specific design examples - all written with the intention of minimizing the science and
maximizing the engineering aspects. Although the individual chapters have been
written by many different authors, each one of the best in their fields, the overall tone
and intent of the book is not that of a proceedings, but that of a textbook. There has
been a concerted editing effort to unify terms, avoid duplication, and fill gaps.
Shape memory applications can generally be divided into four categories: free
recovery, constrained recovery, work production (actuators) and superelasticity.
These groupings are made according to the primary function of the memory element,
but are useful in defining common product screening criteria, pitfalls and engineering
design parameters. We define the groups as follows:
v
vi Introduction
1. Free recovery includes applications in which the sole function of the memory
element is to cause motion or strain. For example, one could cool a wire into the
martensitic regime, bend it to a new shape, then heat to recover the original
shape.
2. Constrained recovery includes applications in which the memory element is
prevented from changing shape and thereby generates a stress. The ideal
example would be the recovery of a ring onto a solid, rigid rod. In this case there
may be some free recovery before contact is made, but the primary function of the
memory element is to generate a stress.
3. Actuator, or work production applications are those in which there is motion
against a stress and thus work is being done. The ideal case would be a wire or
spring which lifts a weight when heated (and perhaps drops the weight again
when cooled). More often, the memory element works against a biasing spring.
Actuators are generally of two types: thermal and electrical. Electrical actuators
are actuated via direct current and are generally in competition with servo-motors
or solenoid. Thermal actuators are driven by changes in ambient temperature and
generally compete with thermostatic bimetals.
4. Superelastic or pseudoelastic applications are isothermal in nature and
involve the storage of potential energy. Although memory elements can operate
as "supersprings", the temperature range over which the effect is found is rather
narrow (usually only 80°C).
The book consists of five parts. Part I deals with the mechanism of shape memory and
the alloys that exhibit the effect. It also defines many essential terms that will be used
in later parts. Part II deals primarily with constrained recovery, but to some extent with
free recovery. There is an introductory paper which defines terms and principles, then
several specific examples of products based on constrained recovery. Parts III and
IV deal with actuators, with part III introducing engineering principles and part four
several of the specific examples. Finally part V deals with superelasticity, again with
an introductory paper and then several specific examples of product engineering.
Acknowledgements
This volume is based on contributions to the Shape Memory Alloy technology
conference held at Michigan State University on 15-17 August 1988. The editors
wish to acknowledge the following firms who sponsored the conference:
Raychem Corporation
300 Constitution Drive
Menlo Park, CA 94025
Memry Inc
83 Keeler Avenue
Norwalk, CT 06854
An Introduction to Martensite and Shape Memory
CM. Wayman* and T.W. Duerigt
'Department of Materials of Science and Engineering
University of Illinois at Urbana-Champaign,
1304 West Green Street
Urbana, Illinois 61801
ÎRaychem Corporation
300 Constitution Dr.,
Menlo Park, CA 94025
Shape memory refers to the ability of certain materials to "remember" a shape, even
after rather severe deformations: once deformed at low temperatures (in their
martensitic phase), these materials will stay deformed until heated, whereupon they
will spontaneously return to their original, pre-deformation shape. The basis for the
memory effect is that the materials can easily transform to and from martensite. A
detailed description of martensitic transformations1-4 can be very complex and is
beyond the scope of this book; still, one cannot obtain an understanding of even the
most elementary engineering aspects of shape memory without first becoming familiar
with some basic principles of martensite and its formation. In this section, the key
microscopic and macroscopic aspects of martensite will be qualitatively reviewed.
Although one may be tempted to pass over some of the microscopic aspects, it must
be emphasized that these form a foundation upon which subsequent discussions of
shape memory are based. Many of the terms used later in the book will also be
defined. After an introduction to martensite, the shape memory event itself will be
introduced, along with the related phenomenon called "superelasticity".
1. Martensite: A Microscopic Perspective
Solid state transformations are of two types: diffusional and displacive. Diffusional
transformations are those in which a new phase can only be formed by moving atoms
randomly over relatively long distances. Long range diffusion is required because the
new phase is of a different chemical composition than the matrix from which it is
formed. Since atomic migration is required, the progress of this type of transformation
is dependent upon both time and temperature. In contrast, displacive transformations
do not require such long range movements; in these cases atoms are cooperatively
rearranged into a new, more stable crystal structure, but without changing the
chemical nature of the matrix. Because no atomic migration is necessary, these
displacive transformations generally progress in a time-independent fashion, with the
motion of the interface between the two phases being limited only by the speed of
sound. They are referred to as athermal transformations, since the amount of the new
phase present is usually dependent only upon temperature, and not the length of time
at temperature. Martensitic transformations are generally of this second type, and are
formed upon cooling from a higher temperature phase called the parent phase, or
austenite.
3
4 An Introduction to Martensite and Shape Memory
A precise definition of martensite has never been agreed upon. The terms
"martensite" and "austenite" were originally intended to refer only to phases of steels;
although some argue the point, the more generalized definition referring to the type of
transformation product, and not the particular material, is now widely accepted.
Martensitic transformations are first order transformations, meaning that heat is
liberated when martensite is formed, there is a hysteresis associated with the
transformation, and there is a temperature range over which austenite and martensite
co-exist. Summarizing the key features of martensite, we see that it is formed upon
cooling with the volume fraction of martensite increasing as the temperature is
reduced but with the volume fraction being independent of time, and that it inherits the
composition and atomic order of the parent phase. Most of the other debates
regarding what defines martensite are in regard to crystallographic details that need
not concern us in the present context.
Crystallographically, the transformation from austenite to martensite is often thought of
in two parts: the Bain strain and the lattice-invariant shear. Although these can be
crystallographically quite complex, a qualitative two-dimensional approach can be
quite simple and perfectly adequate in the present context. The Bain strain, or lattice
deformation, consists of all the atomic movements needed to produce the new
structure from the old; in Figure 1, the austenite structure is schematically illustrated in
IIIIJJJIÍL
1 1 1 1 11 :> —·—·—·—·—·
I I I I I I .
•
Figure 1 : The transformation from austenite to martensite is shown schematically in
two dimensions, (a) being completely austenitic and (d) being completely
martensitic. Note in (c) that as the interface advances, each layer of atoms is
displaced only a very small distance.
diagram (a), and the progression to a fully martensitic structure is schematically
illustrated by (b) through (d). Note that as the interface progresses one atomic layer,
each atom is required to move by only a very small amount (Figure 1(c)). The end
results of all these small coordinated movements is the new martensitic structure, and
Martensite: A Microscopic Perspective 5
the movements needed to produce the new structure are called the Bain strain. In real
materials, the Bain strains generally consists of several small atomic shuffles in
addition to the type of movement illustrated in Figure 1.
The second part of a martensitic transformation, the lattice invariant shear, is an
accommodation step: the martensitic structure produced by the Bain strain is of a
different shape, and often volume, than the surrounding austenite (compare Figures
1a and 1d). Martensite in steel involves both a volume and a shape change, whereas
shape memory alloys such as Ni-Ti undergo basically only a shape change. Either the
overall shape of the new phase, or the surrounding austenite must be altered to
accommodate the new structure. By way of comparison, one cannot change the
shape of a single brick in the center of a brick wall - either the surrounding bricks must
deform, or the new brick must accommodate its form to the space available. There are
two general mechanisms by which this can happen: slip (Figure 2a) and twinning
(Figure 2b). In both cases, each individual cell, or parallelogram, has the new
martensitic structure, but the overall shape is that of the original austenite. Slip is a
permanent process and is a common accommodation mechanism in many
martensites. Twinning is unable to accommodate volume changes (should that be
necessary) but can accommodate shape changes in a reversible way. For shape
memory to occur to any significant extent, we require that the accommodation be fully
(b) Accommodation by twinning
Figure 2: The two mechanisms of accommodating the shape change due to the
atomic shear of a martensitic transformation. In slip (above), the microstructure is
irreversibly "damaged-. In the case of twinning (below) accommodation is
reversible, but substantial volume changes cannot be allowed.
reversible, or in other words, that twinning be the dominant accommodation process.
In the two-dimensional model of Figure 2, only two directions or variants of shear are
required to restore the original, overall shape of the matrix; in three dimensions the
situation can be complicated: Cu-Zn-AI martensites, for example, require four
6 An Introduction to Martensite and Shape Memory
martensite variants for full, three dimensional accommodation, and Ni-Ti martensites
require three.
The twinning process of accommodation plays a key role in the shape memory effect
and should be reviewed in more detail. As can be seen in Figure 3, the twin boundary
is a mirror plane: when positioned on the boundary, the view in one direction is a
mirror of the other. Atoms situated on that boundary see the same number and types of
Figure 3: Shematic view of a twin boundary. An atom situated on the boundary sees
mirror views left and right of the boundary. The atoms at the boundary are
bonded very similarly to those not on the boundary, having the same number of
nearest neighbors. This makes twin boundaries very low energy interfaces, as
well as being very mobile.
bonds in both directions. Some key properties of twin boundaries are that they are of a
very low energy and that they are quite mobile; thus the relative stability of a
martensitic phase is not strongly affected by the number or location of these
boundaries. By comparing the edges of the structures shown in Figures 2(a) and 2(b),
one can see that slip accommodation requires that atomic bonds be broken, while all
bonds remain intact in the twinned structure. If a stress is applied to the structure
shown in Figure 2b, the twin boundaries will easily move, producing a shape which
better accommodates the applied stress. An example is shown in Figure 4. The result
of moving a twin boundary is thus to convert one orientation or twin variant into
another. That variant will be chosen which is most favorably oriented to the applied
stress. In the ideal case, a single variant of martensite can be produced by straining a
sufficient amount. This process (the condensation of many twin variants into a single
favored variant) is called detwinning. Thus far we have only considered the twins
within individual martensite plates, but crystallographic analysis has also shown that
the boundaries between martensite plates also behave as twin boundaries - i.e. the
individual plates of martensite themselves are twins with respect to adjoining plates.
Thus the term twin boundaries, generally refers to the boundaries between martensite
plates as well as the boundaries within plates.
In Figures 1 through 4, atom types are not distinguished, but in an alloy several
species of atoms are present. We need to consider then, how these atoms distribute
themselves on the lattice sites. In steel, the atoms are disordered, meaning that
different elements are randomly distributed on the lattice sites. In Ni-Ti, however, the
atoms are ordered, meaning that the Ni and Ti atoms are found on very specific sites
(see Figure 5). During a martensitic transformation, the martensite takes on the same
Martensite: A Microscopic Perspective 7
Figure 4: Twin boundaries in martensite can be readily moved by the application of a
shear stress. The motion of the twins causes an inbalance in accommodation,
and thus a net shape change.
ordering of the austenite. This is called inherited ordering. Note also the structures
shown in Figures 5a and 5b have a body centered symmetry. (Figure 5a is in fact a
Body Centered Cubic (BCC) structure, while Figure 5b is technically not BCC but is
called a B2 structure or CsCI structure.) Shape memory alloys are generally based
upon a BCC symmetry, some with the BCC structure, more often with the B2 structure,
and some with an even more complex ordering called D0 , still based upon a BCC
3
symmetry ( Figure 5c). It is interesting to note the progression from the BCC to B2 to
the D0 simply satisfies a need for the different atoms to stay separated from one
3
another.
Martensite normally appears as plates, resting on complex crystallographic planes
called habit planes. In many shape memory alloys, the martensite plates are large
and easily viewed through an optical microscope. One exception, however, is Ni-Ti
which exhibits very fine plates that cannot be individually resolved optically. In all
systems, but especially Ni-Ti, one has to take great care in preparing samples for
viewing since simple grinding and polishing can disturb the martensite, or even create
martensite when in fact there was none to begin with. Figure 6 shows a typical
micrograph of a martensitic material. A few of the ideas presented above can be
supported by this figure. First, notice that there is a scratch running from left to right;
this scratch was made on the surface while the material was austenitic, then the
material was cooled to form martensite. Each martensite plate changes the direction
of the scratch slightly (reflecting the shear nature of the transformation), but notice that
the neighboring plate brings the scratch direction back on course. This is a direct result