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.

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]    [0028]FIG. 1 shows, schematically, the representation of the method of functioning of the two-way effect in a shape-memory alloy;  
         [0029]    [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]    [0030]FIG. 3 shows the stress-strain profile of a shape memory alloy in the martensitic state;  
         [0031]    [0031]FIG. 4 shows the stress-strain profile of a linear super elastic material of an Ni—Ti alloy;  
         [0032]    [0032]FIG. 5 shows the schematic representation of the arrangement for introducing selective deformations into the shape memory alloy;  
         [0033]    [0033]FIGS. 6 a,    6   b  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]    [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  1   a,  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  1   b.  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  1   a,  but rather is shortened only to a certain extent and, therefore, opens. In this position  1   c,  the radius of curvature of SMA element  1  is greater than in closed state  1   b.  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  1   b.  Due to the irreversible component, there is, therefore, a transformation from the cold, closed, annular state  1   b  into open shape  1   c  and back again into closed state  1   b.  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]    [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  1   a,  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  1   b  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  1   a,  but rather into open position  1   c.  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  1   b.  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. 6 a  and  6   b.    
         [0040]    [0040]FIGS. 6 a  and  6   b  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. 6 a.  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]    [0041]FIG. 6 a  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. 6 a  and  6   b,  respectively, by dotted lines. The curved SMA semifinished product illustrated in FIGS. 6 a  and  6   b  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. 6 a  and  6   b  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 μm, 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° and 26°. 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.