METHOD FOR PRODUCING A MONOCRYSTALLINE BODY FROM A MAGNETIC SHAPE MEMORY ALLOY

Abstract

A method for producing an MSM actuator element, having a crystal orientation along a first crystal axis, from a monocrystalline MSM body by introducing a molten alloying material into a molding shell and subsequently solidifying the alloying material, comprising the following steps: providing a molding shell which comprises a nucleation region (24), a selector region (26) and a crystal region (28) and is oriented along a longitudinal axis (22) at least in some sections, introducing the molten MSM alloying material, in particular NiMnGa-based alloying material, into the molding shell without providing a separate nucleation crystal, compacting the MSM alloying material by generating a solidification front moving from the nucleation region across the selector region into the crystal region along a solidification path.

Description

BACKGROUND OF THE INVENTION

The present invention relates to method for producing a monocrystalline MSM body for the production of an MSM actuator and such a monocrystalline MSM body, as is produced by the method.

MSM actuators (also designated “MSM-actuators”) are generally known from the prior art and utilize the effect that under the influence of a magnetic field, so-called magnetic shape memory materials (MSM=Magnetic Shape Memory) carry out an expansion movement which—typically lying along the expansion direction in the single-figure percent range relative to a length of a respective body—can be the basis for a drive and in this respect can be for instance an alternative to known actuators realized by means of permanent magnets and/or electromagnets.

In addition to the alloy which is used (typically an alloy on the basis of NiMnGa), the crystal orientation in which the MSM element is present is critical for the effectiveness of such an MSM actuator or respectively MSM actuator element: Methods to be assumed as being known from the prior art for the production of monocrystalline MSM material have the characteristic that a crystal orientation resulting by the introduction of a molten alloying material into a molding shell and subsequent cooling or respectively solidification of the alloying material is stochastic, with the result that an alignment of the crystal axes is not predeterminable and must then be developed by subsequent manufacturing steps of the MSM body. FIG. 5 shows such an arrangement of the prior art: An MSM monocrystal 10 which has been solidified and elongated in the previously described manner has a geometric longitudinal axis 12 determined by the molding shell. Largely in a stochastic manner during the solidification of the material, however, a crystal orientation has formed in the monocrystal 10, which is described by way of example by a first crystal axis 14 and a second crystal axis 16 orthogonal thereto (wherein the third axis is then automatically fixed orthogonally to them both). This then leads to an MSM element 18 (after prior determining of the crystal axes by measuring) being able to be cut out from the finished monocrystal, which has a maximum dimension delimited by the geometric relationships which are shown (with a large amount of waste material, accordingly). Consequently, this leads to the fact that with prevalent longitudinal dimensions of monocrystalline MSM elements in the range between 10 mm and 30 mm and with desired cross-sections of typically between 5 mm² and 30 mm² correspondingly large monocrystals 10 (FIG. 5) must be produced, in order to also be able to manufacture the desired minimum dimensions for the MSM element for the case of unfavourable crystal orientations. It is obvious that this procedure, which is to be assumed as being known, is inefficient in many respects; on the one hand, waste material occurs to a considerable extent through the necessary cutting processes (typically carried out by wire eroding), on the other hand in each case a measuring of the produced monocrystal is necessary with regard to determining the crystal orientation (typical procedure by X-ray diffractometry), in order to create the prerequisite at all for the subsequent cutting.

It can also be seen from observing by way of example the geometric relationships of FIG. 5 that the maximum achievable dimensions (e.g. a longitudinal extent of an MSM element which is to be produced) are limited.

It is known from the prior art that so-called nucleation crystals (seed crystals) can influence a crystal orientation in a monocrystal production process. For this purpose substantially a suitable monocrystal, oriented in the desired manner, is incorporated into the process at the start of the process, on which the crystal which is to be produced ideally nucleates in a monocrystalline manner. However, this procedure is also problematic in many respects; not only are suitable nucleation crystals costly and difficult to handle in particular for industrial manufacturing processes outside a laboratory environment, also such nucleation crystals require a very precise process management, in order to achieve the correct nucleation behaviour (with further possible disadvantages to the MSM effect of a produced MSM element if the nucleation crystal has material which is foreign to the alloy). Therefore, in addition to the obvious need for an increase in efficiency according to the problem described above, the need also exists for procedural simplification, with the aim of enabling processes which are simple to operate and are potentially large-scale for the production of monocrystal MSM bodies.

With regard to the further prior art, reference is made to the following documents:

1. M. ZHU et al.: “Preparation of single crystal CuAlNiBe SMA and its performances”, JOURNAL OF ALLOYS AND COMPOUNDS, Vol. 478, No. 1-2, 10 Jun. 2009, page 404-410;

2. H. ESAKA et al.: “Analysis of single crystal casting process taking into account the shape of pigtail”, MATERIALS SCIENCE AND ENGINEERING A, Vol. 413-414, 15 Dec. 2005, pages 151, 155;

3. GB 2 330 099 A;

4. C. Li et al.: “Preparation of Single Crystal of TiNi Alloy and its Shape Memory Performance”, PROC. OF SPIE. Vol. 7493, 2009, pages 7493L1-74931L8;

5. K. ROLFS et al.: “Double twinning in NiMnGaCo”, ACTA MATERIALIA, Vol. 58, No. 7, 1 Apr. 2010, pages 2646-2651;

6. U.S. Pat. No. 5,062,469 A;

7. M. LANDA et al.: “Ultrasonic characterization of Cu—Al—Ni single crystals lattice stability in the vicinity of the phase transition”, ULTRASONICS, Vol. 42, No. 1-9, 1 Apr. 2004, pages 519-526; 8. F. XIONG et al.: “Fracture mechanism of a Ni—Mn—Ga ferromagnetic shape memory alloy single crystal” JOURNAL OF MAGNETISM and MAGNETIC MATERIALS, Vol. 285, No. 3, 1 Jan. 2005, pages 410-416.

It is therefore an object of the present invention to provide a method for the production of a monocrystal MSM body and a corresponding monocrystalline MSM body, which are improved with regard to utilization of material and efficiency of the associated monocrystal material, in particular to reduce waste of the monocrystal material for the production of one or more MSM elements from the monocrystalline MSM body and in addition to make the necessity of a nucleation crystal unnecessary.

SUMMARY OF THE INVENTION

The object is achieved by providing a method to provide a monocrystal MSM body, (preferably for use as an actuator or respectively actuator element), which proceeds from the fact that the divided body (and divided further into individual actuator elements) resulting from the method of the present invention is further treated with typical (and otherwise known) heat treatment steps and/or magnetomechanical training steps, in order to achieve or respectively optimize the magnetic shape memory behaviour. In accordance with a further development, it is also in particular included by the invention to subject individual or the plurality of MSM actuator elements to a heat treatment after the division, in order to stimulate the magnetic shape memory behaviour; alternatively, this heat treatment can also take place on the compacted MSM alloying material before the division into the plurality of MSM actuator elements. Provision is also made within preferred further developments of the invention to move the separated (distributed) MSM actuator elements in the manner of a training in a targeted and predetermined manner in order to stimulate the shape memory behaviour. Provision is made here in particular (and otherwise also assumed as known), that a separated element is moved in a targeted manner in the provided expansion direction, for instance by the application of tensile and/or pressure forces, in order to thus carry out the training with the aid of such mechanical strokes.

In an advantageous manner according to the invention, the production of the monocrystal MSM body—preferably on the basis of a NiMnGaX alloying material, wherein X has optionally one or more elements of the group Co, Fe and Cu—without the necessity to provide a (separated) nucleation crystal, rather solely from the introduction of the molten MSM alloying material into the especially configured molding shell according to the invention. More precisely, the latter has a longitudinal axis and is deflected in the region of the selector region from this longitudinal axis, according to the invention by a deflection which exceeds the maximum cross-sectional width in the selector region. Thereby, provision is made within the scope of the invention to realize a longitudinal sectional geometry of the solidification path deflecting according to the invention by the formation of the selector region so that this deflection is greater in the cross-sectional direction than a maximum cross-section width in the selector region, in other words, the region of the maximum deflection lies outside a projection of the cross-section in the crystal region to the entry of the selector region along the longitudinal axis.

According to a further development, this deflection has in longitudinal section the form of at least one spike, alternatively a spiral, a helix or another angle configuration.

Through this advantageous provision, the crystal structure of the solidifying or respectively then solidified MSM material then undergoes in a manner according to the invention a crystal orientation which orients itself on the longitudinal axis, more precisely runs along the direction of the longitudinal axis of the molding shell (or respectively deviates therefrom by an angle deviation which according to the invention is <10°, according to a further development is advantageously <6°, again according to a further development and advantageously is less than 3°).

It is thereby then advantageously achieved through the present invention that (with this then negligible orientation error in the practical realization) a monocrystal is produced, the crystal orientation of which no longer occurs stochastically, but rather is marked by the mechanical alignment of the molding shell along the longitudinal axis (or respectively of the course section formed in a deflected manner according to the invention for the solidification in the selector region). The advantageous consequence resulting therefrom for series production is evident: Not only is the waste which is necessary in a further treatment or respectively in the dividing of the solidified material into a plurality of MSM elements drastically reduced, also through the procedure according to the invention at least the crystal orientation is fixed with regard to the longitudinal axis through the procedure according to the invention, in other words, before a possible further treatment of the monocrystal for the realization of the MSM element(s), a laborious step of orientation measurement (for instance by means of X-ray diffractometry) would be superfluous.

If then, as provided advantageously and according to a further development the molding shell (in particular in the selector or respectively crystal region) is configured so as to be rectangular in cross-section, in addition the crystal orientation of the solidifying or respectively solidified MSM material can be influenced along a second crystal axis running orthogonally to the first crystal axis (and hence automatically to the third orthogonal axis), so that as a result in this way then also the complete three-dimensional crystal orientation of a resulting crystal is determined in the space (again without the necessity of measuring).

Within the practical realization of the invention, it is particularly favourable and preferred to provide the longitudinal axis in vertical direction, so as to provide it approximately perpendicularly to an (otherwise known) cold plate as cooling device in or at the nucleation region of the molding shell. If the molding shell is then (in an otherwise known manner) moved from a warmth or respectively heat environment, opposed to the longitudinal axis, with a drawing speed, alloying material which is introduced into this molding shell in liquid state solidifies owing to the temperature gradient then in an upward direction along a solidification path, which is able to be described through the longitudinal axis and, deviating therefrom, is deflected according to the invention in the selector region. According to a further development advantageously the solidification—or respectively cooling behaviour of the molding shell is arranged here so that in cross-section (radially) no significant temperature gradient is present from the interior outwards in the melt adjacent to the solidification front, and a temperature gradient of the melt close to the solidification front is set at values of between 0.3 K/mm and 20 K/mm, wherein a particularly preferred range of values for producing the desired crystal orientation lies in the range between 1 K/mm and 15 K/mm. Additionally or alternatively, it is favourable according to a further development to arrange the cooling rate, described by the speed of movement of the solidification front along the solidification path (or respectively a drawing speed of the molding shell relative to the temperature gradient), at a range of between 0.1 ram/min and 10 mm/min, wherein a particularly preferred range lies between 0.3 mm/min and 5 mm/min.

In this way a monocrystalline solidification behaviour is then advantageously achieved, which forms the first crystal axis of the crystal structure at least along the longitudinal axis (or respectively shows between these axes a maximum angular deviation of less than 10°, typically less than 6° or even less than 3°). For the case where according to a further development advantageously also the cross-section of selector region and/or crystal region (i.e. the plane perpendicular to the longitudinal axis) is configured so as to be rectangular, more preferably square), in addition an influencing (parameter) of the orthogonal second or respectively third crystal axis can be achieved in the direction of the rectangular longitudinal edges in cross-section, so that in the ideal case of an e.g. elongated and cross-sectionally rectangular crystal region of the molding shell, this region determines the three-dimensional orientation of a monocrystal which is solidified therein. According to a further development, it is particularly advantageous within the scope of the invention to carry out a division, following the solidification, (always) perpendicularly to the longitudinal axis (Z axis), because indeed in this respect, with the previously described maximum deviations, the crystal orientation is already fixed.

As a result, therefore the present invention not only enables a drastic reduction in manufacturing steps or respectively upstream testing steps (because ideally any measuring of the crystal orientation can be dispensed with), the invention also permits MSM elements to be produced which are optimized with regard to dimension from the restricted interior of a molding shell, because in particular already in the described molding process by solidification along a solidification direction corresponding to the longitudinal axis of the molding shell and a crystal alignment effected therewith, a maximum length dimension is able to be produced. It is then to be expected in particular that MSM elements can be produced efficiently and with small manufacturing expenditure (and hence potentially on a large scale) as the basis for the production of MSM actuators (also by further dividing, e.g. sawing), which reach length dimensions of more than 20 mm, in particular more than 40 mm and/or permit a cross-sectional area of 15 mm² or more.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages, features and details of the invention will emerge from the following description of preferred example embodiments and with the aid of the figures; these show in:

FIG. 1 a geometric schematic diagram of a molding shell arrangement for carrying out the method according to a first example embodiment of the invention;

FIG. 2 an illustration analogous to FIG. 1, but with a different geometrical configuration in the form of a cross-sectionally rectangular crystal region of the molding shell;

FIG. 3 a diagrammatic illustration of a cylindrical MSM monocrystal and of a crystal orientation drawn therein diagrammatically on realization of the invention;

FIG. 4 an illustration analogous to FIG. 3, but with a monocrystal body in the shape of a rectangular block to illustrate the crystal orientation of an MSM actuator element (likewise in the shape of a rectangular block) provided diagrammically therein and

FIG. 5 a diagrammatic illustration of an MSM monocrystal realized according to a generic method from the prior art with crystal axes oriented therein stochastically, and with the limited cut possibilities resulting therefrom for an MSM actuator element.

DETAILED DESCRIPTION

FIG. 1 illustrates the principle by which the present invention can be realized according to a first example embodiment. A so-called molding shell is shown for the production of monocrystalline bodies by the so-called Bridgman method, which, extending from a cold plate 20 perpendicularly along a longitudinal axis (dot-and-dashed line 22), forms a nucleation region 24, subsequently a selector region 26 and a crystal region 28. Suitably molten alloying material is introduced into the device through an upper opening 30, and the liquid alloying material then solidifies from below upwards (arrow direction 32) with the formation of a correspondingly upward moving solidification front, the speed of movement of which is predetermined by a suitable temperature influence.

FIG. 1 (as also the analogous FIG. 2) illustrate how according to the invention the solidification path takes place not perpendicularly and linearly along the longitudinal axis 22, but rather has a linear course which is bent in longitudinal section from FIG. 1 or respectively FIG. 2; more precisely, in the selector region 26 the molding shell is configured so that its interior channel which is effective for the solidification (from the direction from below upwards) firstly is deflected by an angle □ of approximately 40° and then has a further, but oppositely deflected section, until the channel at the upper end of the selector region is again in alignment cross-sectionally with the cross-section on the base side. In accordance with the invention, advantageously this deflection, which in the illustrated example embodiment at its maximum lateral deflection transcends over the projection of the cross-section in the crystal region or respectively in the base region adjacent to the cold plate 20, advantageously provides for a longitudinal orientation of the crystal structures in vertical direction, i.e. in the direction of the axis 22. In the solidified state, this then leads in the region of the crystal region 28 to the monocrystal which is present there having an orientation which has at least a first crystal axis orientated in the direction of the longitudinal axis (wherein here according to the invention a maximum angle error of 10°, typically however of less than 6° or even less than 3° can be achieved).

FIG. 2 shows a variant of the example embodiment of FIG. 1; here in the crystal region 28′, the channel extending vertically along the longitudinal axis 22 is square in cross-section, so that, in addition to a crystal axis orientation in vertical direction, additionally the two crystal axes orthogonal thereto extend parallel to the edge courses of the crystal region. FIG. 3 or respectively 4 illustrate these geometrical relationships, in this respect in accordance with the forms of realization of FIG. 1 or respectively FIG. 2: FIG. 3 shows the result of a monocrystal body solidified in a hollow cylindrical crystal region. The direction of the longitudinal axis (here: z-axis) corresponds approximately to the alignment of the crystal longitudinal axis c with the described small possible angle error. Owing to the cylinder structure (i.e. circular shape in the x-y plane in FIG. 3), the two further axes, orthogonal to one another and to the vertical axis c, are stochastic in their alignment. In contrast, the further development of FIG. 2 (geometry according to FIG. 4) offers the possibility, by provision of the square cross-sectional contour (running here parallel to the x- or respectively y-direction), to develop the second (a) or respectively third (b) crystal axis parallel accordingly, so that as a result of the carrying out of the method described in FIG. 2 or respectively FIG. 4 a monocrystal is achieved, which through its shape in the form of a rectangular block already to the greatest possible extent also describes its actual crystalline orientation and in this respect is potentially not (or only minimally) in need of further treatment. Also, the result of the production method according to FIG. 1 (FIG. 3) is already advantageous in so far as here with the crystal axis (c), running in the direction of the longitudinal extent (z) of the molding shell and of the blank which is solidified therein, a relevant alignment is fixed for instance for the expansion behaviour of an MSM body, and also such a cylindrical body is then able to be used without further (or only with minimal) further treatment, if the precise alignment of the a- or respectively b-crystal axes is not concerned.

The execution of the method is described below with the aid of a practical example:

Primary alloying material is produced as so-called master alloy by induction melting from the materials NiMnGa, in accordance with composition for an MSM alloy, by induction melting. A typical melting temperature is set at a range of between 50° and 400° above the liquefaction temperature of the respective alloy. Typically, the melting takes place under an Ar atmosphere between 100 mbar and 1200 mbar.

The liquid master alloy is poured into a ceramic molding shell which has a geometry in accordance with FIG. 1. This molding shell is moved in the Bridgman method relative to a temperature gradient from a hot zone into a cold zone, so that the solidification front runs through the molding shell from bottom to top. This speed of the movement of the solidification front typically lies at 0.3 mm/min; the temperature gradient in the melt close to the solidification front is set at a value of typically 3 K/mm. After running through the selector region, which is advantageously deflected according to the invention, the MSM material solidifies with a crystal axis aligned vertically, i.e. along the direction of the longitudinal axis 22, so that after concluding solidification and cooling, a cylinder can be removed from the crystal region 28 as an MSM body of the geometry shown in FIG. 3. This now offers the possibility of immediately realizing an MSM actuator with a movement-(expansion) direction extending axially; alternatively, from this body, by determining a crystalline transverse axis, the prerequisite can be created so that with little waste and minimized loss on the covering surface side, one or more MSM elements which are cross-sectionally rectangular or respectively in the shape of a rectangular block can be created with a defined crystal orientation also in the transverse direction. For such a separation, in particular cuts perpendicular to the Z-axis present themselves, because indeed in this respect the orientation is already developed.

To stimulate or respectively realize a complete shape memory functionality of actuators realized in the described manner, the material is heat-treated (either as a whole body before the separation, alternatively by heat treatment of the divided individual actuator elements). It is also advantageous to train these elements after dividing in their movement—or respectively expansion behaviour, wherein for this purpose, typically over some strokes, in the provided expansion—or respectively movement direction a movement is imprinted into the material by corresponding input of tensile force or respectively pressure force.

Whereas the arrangement described above and the operation thereof for realizing the method according to the invention are to be understood generically and in principle (and configured and adapted in a suitable manner by the specialist in the art), it is in particular also within the scope of the present invention to provide in the manner of a multi-armed molding shell a plurality of solidification paths along selector—and crystal regions which are respectively separated from one another but nevertheless adjacent.

The ranges of application of an MSM body which is produced by the present invention are potentially unlimited; it is advantageously to be expected that the present invention nevertheless considerably simplifies and configures more economically the large-scale production of such bodies which are clearly defined with regard to the crystal geometry, so that in future further fields of application are developed for MSM actuators.

Claims

1-12. (canceled)

13. A method for producing an MSM actuator element, having a determined crystal orientation along a first crystal axis, from a monocrystal MSM body by introducing a molten alloying material into a molding shell and subsequent solidification of the alloying material, comprising the steps of:

(a) providing a molding shell which comprises a nucleation region, a selector region and a crystal region having a first crystal axis oriented in the direction of a longitudinal axis at least in some sections;

(b) introducing a molten MSM alloying material into the molding shell without providing a separate nucleation crystal;

(c) compacting the MSM alloying material by generating a solidification front moving from the nucleation region across the selector region into the crystal region along a solidification path, wherein the solidification path in the crystal region runs along the longitudinal axis, forms a region which is deflected from the longitudinal axis in the selector region, the maximum deflection of which, relative to the longitudinal axis, is greater than a maximum cross-sectional width in the selector region, wherein the longitudinal axis has an angular deviation of less than 10° from the first crystal axis; and

(d) dividing the solidified MSM alloying material into a plurality of MSM actuator elements by cuts perpendicularly to the longitudinal axis.

14. The method according to claim 13, wherein the solidification path in the selector region forms a region which is deflected in a spike-like manner with two angled sections, the entry and exit side of which is aligned in alignment to the longitudinal axis.

15. The method according to claim 13, wherein the solidification path in the selector region forms a helix-shaped or zigzag-shaped region.

16. The method according to claim 13, wherein the longitudinal axis is aligned vertically to a flat cooling device associated with the nucleation region.

17. The method according to claim 13, wherein the crystal region extending in an elongated manner along the longitudinal axis has an effective cross-sectional area for the solidification front of >3 cm2.

18. The method according to claim 13, wherein the crystal region extending in an elongated manner along the longitudinal axis has an effective cross-sectional area for the solidification front of >7 cm2.

19. The method according to claim 13, wherein the crystal region extending in an elongated manner along the longitudinal axis has an effective cross-sectional area for the solidification front of >12 cm2.

20. The method according to claim 13, wherein the MSM alloying material for producing the solidification front is cooled so that a temperature gradient in the melt, occurring in the selector region, adjacent to the solidification front, is between 0.3 K/mm and 20 K/mm.

21. The method according to claim 13, wherein the MSM alloying material for producing the solidification front is cooled so that a temperature gradient in the melt, occurring in the selector region, adjacent to the solidification front, is between 1 K/mm and 15 K/mm.

22. The method according to claim 13, wherein the MSM alloying material for producing the solidification front is treated by bringing about a relative speed between the molding shell and the temperature gradient, so that the solidification front in the selector region moves at a speed of between 0.1 mm/min and 50 mm/min along the solidification path.

23. The method according to claim 13, wherein the MSM alloying material for producing the solidification front is treated by bringing about a relative speed between the molding shell and the temperature gradient, so that the solidification front in the selector region moves at a speed of between 0.3 mm/min and 5 mm/min along the solidification path.

24. The method according to claim 13, wherein the solidification front moves along the solidification path through an at least partially cross-sectionally rectangular crystal region.

25. The method according to claim 24, wherein a cross-sectionally rectangular inner contour of the crystal region determines a crystal orientation of the MSM alloying material, which is solidified in a monocrystalline manner, in at least a second crystal axis orthogonal to the first crystal axis.

26. The method according to claim 13, wherein the alloying material has Ni, Mn, Ga and at least Co in the composition NiaMnbGacCodFeeCuf, wherein a, b, c, d, e and f are indicated in atom-% and fulfill the conditions

44≦a≦51;

19≦b≦30;

18≦c≦24;

0.1≦d≦15;

0≦e≦14.9;

0≦f≦14.9;

d+e+f≦15;

a+b+c+d+e+f=100.

27. The method according to claim 13, wherein the compacting of the MSM alloying material takes place along a plurality of solidification paths which are adjacent to one another and separated from one another.

28. The method according to claim 13, including dividing of the MSM alloying material, which is solidified in the crystal region, into the plurality of MSM actuator elements without previous metrological determining of a crystal orientation in the solidified MSM alloying material.

29. The method according to claim 13, wherein the alloying material is NiMnGa.