U.S. patent application number 09/894403 was filed with the patent office on 2002-01-24 for semifinished product made from a shape memory alloy having a two-way effect and method for manufacturing the same.
Invention is credited to Schuster, Andreas, Voggenreiter, Heinz.
Application Number | 20020007884 09/894403 |
Document ID | / |
Family ID | 7646653 |
Filed Date | 2002-01-24 |
United States Patent
Application |
20020007884 |
Kind Code |
A1 |
Schuster, Andreas ; et
al. |
January 24, 2002 |
Semifinished product made from a shape memory alloy having a
two-way effect and method for manufacturing the same
Abstract
The present invention relates to a semifinished product made
from a shape memory alloy having a two-way effect, and to a method
for manufacturing the same. An objective in this case is to produce
a two-way effect in the shape memory alloy in simple fashion and
using only few process steps, so that the semifinished product made
of the shape memory alloy at the austenite/martensite phase
transition, is able to pass through a large number of deformation
cycles, and it exhibits high effect amounts, without requiring a
protracted training of the shape memory alloy or externally acting
forces. In one single deformation step, a linear, superelastic
phase is additionally produced in the shape memory alloy, thereby
introducing a restoring force to the shape memory alloy, so that,
under the action of this restoring force, the shape memory alloy
passes repeatedly through the deformation cycle during the
austenite/martensite phase transition.
Inventors: |
Schuster, Andreas; (Alsfeld,
DE) ; Voggenreiter, Heinz; (Muenchen, DE) |
Correspondence
Address: |
DAVIDSON, DAVIDSON & KAPPEL, LLC
485 SEVENTH AVENUE, 14TH FLOOR
NEW YORK
NY
10018
US
|
Family ID: |
7646653 |
Appl. No.: |
09/894403 |
Filed: |
June 28, 2001 |
Current U.S.
Class: |
148/654 ;
148/402; 420/441 |
Current CPC
Class: |
C22F 1/006 20130101;
C22C 19/03 20130101 |
Class at
Publication: |
148/654 ;
148/402; 420/441 |
International
Class: |
C22C 001/00; C21D
001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 29, 2000 |
DE |
100 30 790.6-24 |
Claims
What is claimed is:
1. A semifinished product comprising: a shape memory alloy, the
shape memory alloy including an active martensitic/austenitic phase
and a linear, superelastic phase forming a restoring force in the
shape memory alloy, the shape memory alloy capable of running
through a deformation cycle several times during an
austenite/martensite phase transition under action of the restoring
force.
2. The semifinished product as recited in claim 1, wherein the
alloy has an outer cross-sectional side and a mid-cross-sectional
area, the linear, superelastic phase being situated at the outer
cross-sectional side and the active martensitic/austenitic phase
being situated in the mid-cross-sectional area; in response to
heating, the martensitic phase entering into the austenitic phase,
under deformation of the shape memory alloy, and, in response to
cooling, returning to the martensitic phase, the shape memory alloy
returning to a shape before deformation, through the action of the
restoring force.
3. The semifinished product as recited in claim 1, wherein the
shape memory alloy has stress distributions, so that tensile and
compressive forces are produced which lead to a curvature of the
semifinished product.
4. The semifinished product as recited in claim 3, wherein the
tensile forces run on an outer curvature side and the compressive
forces on an inner curvature side of the semifinished product.
5. The semifinished product as recited in claim 1, wherein the
shape memory alloy is an alloy capable of exhibiting a two-way
effect.
6. The semifinished product as recited in claim 1, wherein the
shape memory alloy is composed of 55 wt % nickel and of 45 wt %
titanium.
7. The semifinished product as recited in claim 1, wherein, in a
cold, martensitic state, the shape memory alloy has a nearly
closed, annular shape and, in response to heating, enters into a
high-temperature austenite phase, the shape memory alloy being
shortened, so that the semifinished product opens; and, at a
transition to the low-temperature martensite phase, expands under
the action of the restoring force and returns to the nearly closed,
annular shape.
8. The semifinished product as recited in claim 7, wherein, in the
cold, martensitic state, the product has a smaller radius of
curvature than in the warm, austenitic state.
9. A method for manufacturing a semifinished product from a shape
memory alloy having a two-way effect comprising the steps of:
carrying in out a deformation step in the low-temperature
martensite phase of an active martensitic/austenitic phase, and
producing a linear, superelastic phase in the shape memory alloy so
as to introduce a restoring force to the alloy, so that, under the
action of the restoring force, a deformation cycle of the shape
memory alloy is passed through several times during an
austenite/martensite phase transition.
10. The method as recited in claim 9, wherein the shape memory
alloy is deformed such that the linear, superelastic phase is
produced at an outer cross-sectional side of the semifinished
product, and, in the cold state, the martensitic phase is in a
mid-cross-sectional area of the semifinished product; in response
to heating, the martensitic phase entering into an austenitic
phase, under deformation of the shape memory alloy, and, in
response to cooling of the martensitic phase, the shape memory
alloy returning to the shape before deformation under the action of
the restoring force.
11. The method as recited in claim 9, wherein, as the result of
deformation, stress distributions are introduced to the shape
memory alloy, so that tensile and compressive forces are produced,
which lead to a curvature of the semifinished product.
12. The method as recited in claim 9, wherein the shape memory
alloy is in a bar-, band- or wire shape and further comprising
drawing the shape memory alloy in a cold martensitic state over a
mandrel.
13. The method as recited in claim 12, further comprising cutting
the shape memory alloy into individual, curved sections, without
the stress distributions introduced to the shape memory alloy being
thereby influenced; and securing the curved sections to a
substrate.
14. The method as recited in claim 9, wherein the alloy is in a
wire shape and further comprising weaving the alloy into fabric
structures, the wire-shaped shape memory alloy being drawn over
lancets.
15. A curved semifinished product made of a shape memory alloy
comprising: an outer surface including an active
martensitic/austenitic phase; and a section interior to the outer
surface including a linear, superelastic phase forming a restoring
force in the shape memory alloy, the curved product having an
opening capable of widening and narrowing under action of the
restoring force.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a semifinished product made
from a shape memory alloy having a two-way effect, and to a method
for manufacturing the same.
[0002] It is generally known that shape memory alloys (SMA) have
advantageous properties in comparison to conventional
structural-type materials. Due to their ability to remember a
specific shape in the low-temperature martensite phase and in the
high-temperature austenite phase, deformations can be achieved
within a set temperature range over a large number of cycles.
[0003] When working with the austenite/martensite phase transition
and its associated deformation, one can utilize two effects, namely
the one-way effect and the two-way effect. In the case of the
one-way effect, an element made of a shape-memory alloy, which had
been plastically deformed in the temperature range in which the
alloy is present in the martensitic phase, begins to return to the
shape before deformation when heated above the temperature at which
the transformation to the austenitic phase begins. The alloy
remembers the original shape and, in the austenitic phase, returns
the element to its undeformed state. However, when cooled to the
martensitic state, the shape of the alloy does not change again.
Thus, shape memory alloys having a one-way effect can only be used
for a one-time reshaping. Shape-memory alloys of this kind are
employed, for example, in connection, fastening, and sealing
technology, as well as for deployment processes in aerospace.
[0004] The two-way effect describes the fact that the shape-memory
alloy remembers both a specific shape in the high-temperature
austenite phase, as well as one in the low-temperature martensite
phase. This makes it possible to pass several times through the
deformation cycle. The transformation or reforming can be memorized
because of an external force (extrinsic two-way effect) or because
of repeated cycles of stressing the alloy. The latter is also
referred to as the intrinsic two-way effect.
[0005] The intrinsic two-way effect requires a so-called training
in order to impress specific dislocation structures upon the alloy,
which cause the alloy, even when cooled, to revert to a desired or
trained shape. For this, the alloy is deformed in the martensitic
state beyond the martensite plateau, in order to also introduce
plastic deformations, by way of dislocations, to the alloy. When
heated, only a portion of the deformation component reverts to the
shape, because of the dislocations. When cooled, the plastic stress
fields existing around the dislocations produce martensite
variants, which transform the alloy into the desired
low-temperature shape. For this purpose, the deformation is
repeated in transformation cycles n-times, so that the internal
stresses in the shape memory alloy stabilize, and the alloy
memorizes the dislocation structures. However, this means that,
prior to its proper service application, the shape memory alloy
must first be subjected to this time-consuming training.
[0006] In the case of the extrinsic two-way effect, the action of
an external force, such as a weight, a counterspring, or even an
opposite shape memory element, initially deforms the element in the
martensitic state. When heated to the austenitic state, a return to
the shape before deformation (recovery) occurs at the
martensite/austenite phase transition. The subsequent cooling
leads, under the action of the external force, to renewed
deformation. Providing an external force to stimulate the two-way
effect can be disadvantageous in many applications, since
additional precautions must be taken to prepare and adjust the
shape memory alloy and the external force.
[0007] U.S. Pat. No. 4,411,711 describes a method for producing a
reversible two-way shape memory effect in a component made from a
material showing only a one-way shape memory effect. The component
made of a shape memory alloy, which, under normal conditions,
exhibits only a one-way effect, is specially treated, so that a
two-way effect is induced in this component. For this, the shape
memory alloy is first treated with a solution and is subsequently
quenched in water. The shape memory alloy is then either shot
peened using steel balls or work hardened.
SUMMARY OF THE INVENTION
[0008] Starting out from the related art, an object of the present
invention is to produce a semifinished product from a shape memory
alloy having a two-way effect. An additional or alternate object is
to devise a method for fabricating such a semifinished product, the
two-way effect being accomplished in the shape memory alloy in
simple fashion and using as few process steps as possible, so that,
at the austenite/martensite phase transition, the semifinished
product made of the shape memory alloy is able to pass through a
large number of deformation cycles and exhibits high effect
amounts, without the need, beforehand, for a protracted training of
the shape memory alloy or for externally acting forces.
[0009] The present invention provides a semifinished product made
from a shape memory alloy having a two-way effect, wherein, in
addition to the active martensitic/austenitic phase, the shape
memory alloy includes a linear, superelastic phase, which results
in a restoring force being produced in the shape memory alloy, so
that, under the action of this restoring force, the shape memory
alloy runs through the deformation cycle several times during the
austenite/martensite phase transition.
[0010] Advantageously, the linear, superelastic phase is situated
at the outer cross-sectional side of the semifinished product, and
the active martensitic/austenitic phase is situated in the
mid-cross-sectional area of the semifinished product. In response
to heating, the martensitic phase may enter into the austenitic
phase, under deformation of the shape memory alloy, and, in
response to cooling, returns to the martensitic phase, the shape
memory alloy returning to the shape before deformation, through the
action of the restoring force.
[0011] The shape memory alloy may have stress distributions so that
tensile and compressive forces are produced, which lead to a
curvature of the semifinished product. The tensile forces may run
on the outer curvature side and the compressive forces on the inner
curvature side of the semifinished product.
[0012] The shape memory alloy advantageously is an alloy which is
able to exhibit a two-way effect. The shape memory alloy may be
composed of 54.76 wt % nickel and of 45.23 wt % titanium.
[0013] The shape memory alloy, in the cold, martensitic state, may
have a nearly closed, annular shape and, in response to heating,
may enter into the high-temperature austenite phase, the shape
memory alloy being shortened, so that the semifinished product
opens; and, at the transition to the low-temperature martensite
phase, expands under the action of the restoring force and returns
to the nearly closed, annular shape, so that the semifinished
product close. In the cold, martensitic state, the alloy may have a
smaller radius of curvature than in the warm, austenitic state.
[0014] The present invention also provides method for manufacturing
a semifinished product from a shape memory alloy having a two-way
effect, wherein, in a deformation step carried out in the
low-temperature martensite phase, besides the active
martensitic/austenitic phase, a linear, superelastic phase is
produced in the shape memory alloy, thereby introducing a restoring
force to the alloy, so that, under the action of this restoring
force, the deformation cycle of the shape memory alloy is passed
through several times during the austenite/martensite phase
transition.
[0015] As the result of deformation, stress distributions may be
introduced to the shape memory alloy, so that tensile and
compressive forces are produced, which lead to a curvature of the
semifinished product.
[0016] A bar-, band- or wire-shaped shape memory alloy may be drawn
in the cold martensitic state over a mandrel. After being drawn
over the mandrel, the shape memory alloy may be cut up into
individual, curved sections, without the stress distributions
introduced to the shape memory alloy being thereby influenced; and
the curved sections may be secured to a substrate.
[0017] During a process of weaving into fabric structures, a
wire-shaped shape memory alloy may be drawn over lancets.
[0018] With the present invention, in the cold, martensitic state,
besides the martensitic phase, the shape memory alloy exhibits a
deformation-dependent linear elastic phase or linear superelastic
phase, which results in a restoring force being produced in the
shape memory alloy itself, so that, at a high cycle number, a
two-way effect is ensured in the shape memory alloy because of the
restoring force. The linear, superelastic phase is introduced by a
single deformation step, which is implemented in the cold
martensitic state.
[0019] The effect of the shape-memory-alloy deformation is that the
linear, superelastic phase is produced at the outer cross-sectional
side of the semifinished product made of the shape memory alloy,
and, in the cold state, that the active martensitic phase resides
in the mid-cross-sectional area of the semifinished product. In
response to heating, the martensitic phase enters into the
austenitic phase, under deformation of the shape memory alloy. In
response to renewed cooling and transition into the martensitic
phase, the shape memory alloy returns to its previous shape under
the action of the restoring force produced by the linear,
superelastic phase.
[0020] As a result of the deformation, stress distributions are
contained in the shape memory alloy, so that tensile and
compressive forces are produced, which lead to a curvature of the
semifinished product made of the shape memory alloy. The tensile
forces run on the outer curvature side and the compressive forces
on the inner curvature side of the semifinished product.
[0021] The shape memory alloy used is an alloy which, in principle,
is able to exhibit a two-way effect. Ni--Ti alloys are used, for
example.
[0022] Due to the linear, superelastic phase, which effects a
restoring force in the shape memory alloy, the external force
required for the two-way effect, otherwise known as extrinsic
two-way effect, is already integrated in the shape memory alloy, so
that the force driving the two-way effect does not need to be
externally supplied, nor is advance training of the shape memory
alloy needed.
[0023] For this purpose, a bar-, band- or wire-shaped shape memory
alloy is drawn in the cold martensitic state, in the longitudinal
direction, over a mandrel under the action of force. This effects a
deformation of the shape memory alloy and, thus, induces stress
distributions, so that the linear, superelastic phase develops in
the alloy. Following the processing step, the shape memory alloy
assumes a curved or spiral shape. The shape memory alloy can
subsequently be cut up into individual, curved sections and
suitably secured to a substrate. In this manner, one obtains curved
alloy sections, which, in the cold, martensitic state, for example,
form a nearly closed, annular mechanical sticking [hook-like]
element. The induced stress distribution and the resultant
restoring force made available in the alloy are not influenced when
the wire is cut up. If, under the action of heat, the shape memory
alloy enters into the austenitic phase, the alloy remembers its
original shape and, under shortening action, changes back to its
original shape. The mechanical sticking element opens. When
subsequently cooled, the shape memory alloy expands again under the
action of the restoring force. The mechanical sticking element
closes. If it is then heated again, the connecting element opens.
The cycle is run through again.
[0024] In accordance with another specific embodiment, a
wire-shaped shape memory alloy, when woven into materials or
fabric, is run in such a way over lancets, that the one-time
grazing over the lancets produces the linear, superelastic phase in
the shape memory alloy, so that the woven-in alloy wire
automatically passes repeatedly through the above-described
opening/closing operation during the cyclical austenite/martensite
phase transition.
[0025] The advantage of the present invention lies in that a stable
two-way effect is produced in simple fashion in the shape memory
alloy, so that the semifinished product made of this shape memory
alloy can pass repeatedly with a high effect stability through a
deformation cycle. There is no need for a protracted training
process for the shape memory alloy or for the action of external
forces. Merely one process step is necessary, which is implemented
in the cold, martensitic state.
[0026] In addition, the present invention is distinguished by
substantial variability, since the semifinished products are used
in various arrangements, such as in mechanically interlocking or
fastening elements. In addition, the method can be employed to
manufacture semifinished products of this kind in diverse ways, for
example to produce mechanically interlocking elements and loops, or
it can be used for automatic weaving into fabric structures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] The present invention will be described in the following on
the basis of the figures, in which:
[0028] FIG. 1 shows, schematically, the representation of the
method of functioning of the two-way effect in a shape-memory
alloy;
[0029] FIG. 2 shows, schematically, the representation of the
method of functioning of a shape memory alloy, which, in addition
to the active martensitic/austenitic phase, includes a linear,
superelastic phase;
[0030] FIG. 3 shows the stress-strain profile of a shape memory
alloy in the martensitic state;
[0031] FIG. 4 shows the stress-strain profile of a linear super
elastic material of an Ni--Ti alloy;
[0032] FIG. 5 shows the schematic representation of the arrangement
for introducing selective deformations into the shape memory
alloy;
[0033] FIGS. 6a, 6b shows the schematic representation of the
semifinished product made of a shape memory alloy having a two-way
effect in the cold and warm states, respectively; and
[0034] FIG. 7 shows the example of the deformation of an annular
SMA element made of an Ni--Ti wire having a 0.203 micrometer
diameter.
DETAILED DESCRIPTION
[0035] First, the principle of the two-way effect is explained with
reference to FIG. 1. In FIG. 1, reference numeral 1 denotes a bar-,
band-, or wire-shaped shape memory alloy, in the following also
described as SMA element. In the cold, martensitic state, SMA
element 1 is undeformed and, in initial state 1a, exhibits a linear
form. In the cold, martensitic state, SMA element 1 is deformed
under the action of a force, beyond the martensite plateau of the
stress-strain profile illustrated in FIG. 3, in order to introduce
plastic deformations by way of dislocations into the alloy.
Following the deformation, SMA element 1 assumes annular shape 1b.
If one heats the alloy, a phase transition into the austenite
follows, and only a portion of the reversible deformation component
returns to its previous form, because of the introduced
dislocations. Annular SMA element 1 does not pass completely over
into its initial state 1a, but rather is shortened only to a
certain extent and, therefore, opens. In this position 1c, the
radius of curvature of SMA element 1 is greater than in closed
state 1b. In response to cooling to the low-temperature martensite
phase, the plastic stress fields existing around the dislocations
produce martensite variants, which transform the alloy into the
desired low-temperature shape. SMA element 1 again assumes shape
1b. Due to the irreversible component, there is, therefore, a
transformation from the cold, closed, annular state 1b into open
shape 1c and back again into closed state 1b. The cycle can only be
run through repeatedly if the shape memory alloy had been
previously trained; i.e., if the shape memory alloy had run through
the deformation several times beforehand, so that, in response to
cooling or heating, the shape memory alloy remembers the particular
shape; or an additional force acts externally upon the shape memory
alloy.
[0036] FIG. 2 shows schematically the method of functioning of a
shape memory alloy, which, in addition to the active
martensitic/austenitic phase explained in conjunction with FIG. 1,
includes a linear, superelastic phase. SMA element 1, which
exhibits a linear initial state 1a, is deformed in the cold,
martensitic state. As a result of this deformation, analogously to
the case described in FIG. 1, plastic deformations are produced in
the alloy. In this deformation step, however, a linear,
superelastic phase is produced at the same time. The stress-strain
profile of a linear super elastic material of this kind is shown in
FIG. 4 for an Ni--Ti alloy. Tensile and compressive stresses are
produced in the longitudinal direction of the SMA element, so that,
as a result, a restoring force is produced within the shape memory
alloy itself. The restoring force is indicated schematically in
FIG. 2 by a dotted line 2. Following the deformation, SMA element 1
assumes annular shape 1b in the cold martensitic state. When making
the transition to the high-temperature austenite phase, the alloy
remembers its original shape, and annular SMA element 1 opens due
to contraction of the alloy. In the process, the radius of
curvature of annular SMA element 1 increases. Due to the
irreversible component, SMA element 1 does not pass over into its
initial linear position 1a, but rather into open position 1c. In
response to subsequent cooling to the low-temperature martensite
phase, the alloy expands under the action of the restoring force
contained in the alloy. Annular SMA element 1 closes and passes
over into position 1b. This closing movement is executed in
opposition to the compressive force running on the inner curvature
side of SMA element 1. At the same time, the action of the
restoring force in the martensitic state effects an expansion of
the alloy, so that the cycle can be executed once more when the
transition from martensite into austenite is made. In response to
the phase transition into the austenite, the alloy remembers its
original shape and is shortened. Annular SMA element 1 opens in
opposition to tensile forces running on the outer side of the
radius of curvature of SMA element 1. In contrast to the customary
two-way effects discussed in FIG. 1, a simplification is achieved
by the combination of active martensitic/austenitic phase described
in FIG. 2 and the linear superelastic phase, which represents a
gradient material. Due to the linear, superelastic phase introduced
into the shape memory alloy, and the restoring force produced by
it, the force required to expand the alloy during the
austenite/martensite phase transition is supplied by the alloy
itself, so that no external force or training is necessary. The
martensite/austenite phase transition can be reliably repeated for
a large number of cycles.
[0037] As described in conjunction with FIG. 2, in addition to the
martensitic phase present in the cold state of the shape memory
alloy, a linear, superelastic phase is introduced into the alloy.
This is achieved by one single deformation step, which
simultaneously produces the pseudo-plastic or plastic deformation
of the martensitic phase. Alternatively, the deformation can also
be carried out for the particular phase in separate steps as well,
which will not be discussed in detail here, however.
[0038] At this point, it will be explained in conjunction with FIG.
5, how the linear, superelastic phase is introduced to the shape
memory alloy. A bar-, band- or wire-shaped shape memory alloy 1 is
drawn in the cold martensitic state, with the aid of a conveyor
mechanism 3, over a mandrel 4, and weighted by a load 4. In the
process, SMA element 1 is conveyed with a curvature over mandrel 4.
The loading is carried out in the longitudinal direction of the
bar-, band- or wire-shaped SMA element 1, whose longitudinal
extension is substantially greater than its cross-sectional
dimension. The drawing over mandrel 4 can be accomplished manually
using muscular force, or in some other suitable manner. The set-up
shown in FIG. 5 is merely one example. One can conceive of a
multiplicity of other ways for attaining the proper elongation or
deformation of SMA element 1.
[0039] By drawing SMA element 1 once in its longitudinal direction
over mandrel 4, shape memory alloy 1 is deformed such that a
linear, superelastic phase is produced in the shape memory alloy.
The stress-strain profile of a linear, superelastic phase of this
kind is shown in FIG. 4 for an Ni--Ti alloy. Corresponding tensile
and compressive stress distributions are produced within the shape
memory alloy. This is elucidated on the basis of FIGS. 6a and
6b.
[0040] FIGS. 6a and 6b show a semifinished tool made of a shape
memory alloy exhibiting the two-way effect described in the context
of FIG. 2. The semifinished product is composed of a curved section
of a selectively deformed bar-, band-, or wire-shaped shape memory
alloy. Following the deformation step, curved regions are cut out,
resulting, in the cold state, in the nearly closed, annular shape
in FIG. 6a. The semifinished products can be integrated in
different ways in already existing fabric, to form, for example, a
connecting or mechanical interlocking element. A plurality of such
loops or hooks can also be placed separately, side-by-side, on a
suitable substrate.
[0041] FIG. 6a shows the semifinished product in a nearly closed,
annular shape in the cold, martensitic state. Due to the introduced
deformation, tensile forces are produced on the outer curvature
side of the semifinished product, and compressive forces on the
inner curvature side, as shown in FIGS. 6a and 6b, respectively, by
dotted lines. The curved SMA semifinished product illustrated in
FIGS. 6a and 6b thus exhibits, on the outer peripheral sides, a
linear, superelastic phase and, in the middle region, an active
martensitic/austenitic phase. The active martensitic/austenitic
phase means that this is the phase of the shape memory alloy which,
in response to the temperature-dependent phase transition, passes
over from the martensitic into the austenitic state and vice versa.
It is, therefore, this middle region which carries out the
deformation described in conjunction with FIG. 1. The outer region,
namely the linear, superelastic phase, provides, in this context,
the restoring force needed for the deformation from the austenitic
to the martensitic state. Thus, in response to heating, the
martensitic phase passes over to the high-temperature austenite
phase, and shortening of the alloy causes the annular SMA
semifinished product to open. The subsequent cooling produces an
expansion under the action of the restoring force. The annular SMA
semifinished product closes in response to the radius of curvature
becoming smaller. The opening/closing mechanism can be run through
repeatedly with a high effect stability.
[0042] Besides the drawing of the bar-, band- or wire-shaped shape
memory alloy over a mandrel, and subsequent cutting and positioning
of the curved SMA sections, the deformation process can also be
carried out automatically, for example, by weaving the sections
into selected structures. For this, an SMA wire to be woven in can
be run over lancets, so that, on the one hand, by grazing the wire
over the lancets, the linear, superelastic phase is introduced to
the material and, at the same time, the alloy is deformed in the
martensitic phase beyond the martensite plateau. Thus, the then
woven-in shape memory alloy exhibits the two-way effect discussed
in connection with FIG. 2, so that, in response to temperature
changes, the shape memory alloy passes over into corresponding
deformation states.
[0043] Semifinished products of this kind and the corresponding
method can be used, for example, in the manufacturing of releasable
VELCRO-type fasteners. In this connection, individual, annular SMA
elements depicted in FIGS. 6a and 6b can be worked manually into
existing fabric components of VELCRO-type fasteners, enabling the
fastener to be detached or closed under the influence of a
temperature change. The working-in can also be carried out
automatically, however, when weaving the fabric structures. In this
case, the semifinished product is the shape-memory-alloy wire that
is stiffened by the lancets.
EXAMPLE
[0044] A wire made of an Ni--Ti alloy (54.76 wt % nickel, 45.23 wt
% titanium, carbon concentration and oxygen concentration less than
500 ppm) having a diameter of 0.203 .mu.m, was drawn one time over
a mandrel having a 1 mm diameter. As a result, the wire took on a
spiral shape. The wire was subsequently cut in such a way that
closed wire loops or hooks were obtained. Annular hooks were
subsequently secured to a substrate. Two nearly closed hooks of
this kind are shown in FIG. 7. If the Ni--Ti wire is heated, the
alloy remembers its original shape and, under deformation, passes
over to the austenitic phase. In this case, the wire is shortened
in response to this phase transition, so that the radius of
curvature is enlarged, and an opening is formed between the
previously nearly closed hooks. The opening angle in the example
shown in FIG. 7 amounts to 30.8.degree. and 26.degree.. In this
instance, the wire ends are spaced apart by 2.58 mm and 2.19 mm,
respectively. In response to renewed cooling, the wire passes over
again into the low-temperature martensite phase, a closed position,
including a smaller radius of curvature, resulting because of the
linear expansion under the action of the restoring force contained
in the alloy.
* * * * *