WO2002064847A1

WO2002064847A1 - A new ferromagnetic shape memory alloy system

Info

Publication number: WO2002064847A1
Application number: PCT/US2002/004239
Authority: WO
Original assignee: CRACIUNESCU CORNELIU M; WUTTIG MANFRED R; University of Maryland Baltimore; University of Maryland College Park
Current assignee: CRACIUNESCU CORNELIU M; WUTTIG MANFRED R; University of Maryland Baltimore; University of Maryland College Park
Priority date: 2001-02-13
Filing date: 2002-02-13
Publication date: 2002-08-22
Anticipated expiration: 2003-08-13

Abstract

A ferromagnetic shape memory alloy having the composition: Co2Ni1-xGa1+x wherein x is a value such that the alloy exhibits an austenitic crystal structure above a characteristic phase transformation temperature and which exhibits a martensitic twinned crystal struture below the phase transformation temperature, and further being characterized by a magnetocrystalline anisotropy energy that is sufficient for enabling motion of twin boundaries of the martensitic twinned crystal structure in response to application of a magnetic field to the martensitic twinned crystal structure and an actuating element comprising the alloy.

Description

ΓN THE UNITED STATES PATENT & TRADEMARK OFFICE

PATENT APPLICATION

TITLE

A NEW FERROMAGNETIC SHAPE MEMORY ALLOY SYSTEM

This study was supported by the Office of Naval Research contract Nos. N000 149910837 and N000 140010849.

A NEW FERROMAGNETIC SHAPE MEMORY ALLOY SYSTEM

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to novel ferromagnetic shape memory alloys and actuator materials constructed therefrom; and more particularly relates to materials that can demonstrate an actuation response to an applied external stimulus such as an applied field stimulus.

Description of the Prior Art

The ability to effectively employ actuator materials for producing motion and force in response to an applied stimulus is becoming increasingly important for advanced transportation and aeronautics applications, advanced automation and manufacturing processes, and a wide range of other fields. Of particular interest is the development of actuation materials having large strains, appreciable force generation, and rapid time of response to an external stimulus. Popular classes of actuation materials include piezoelectric, magnetostrictive, and shape memory actuation materials; each of these three classes has been found to exhibit both performance advantages as well as limitations in actuation capabilities.

Piezoelectric materials are typically ceramic materials, e.g., lead-zirconate-titanate, and are characterized by an ability to mechanically deform, i.e., expand and contract, in response to an applied electric field, in a demonstration of the inverse piezoelectric effect. Piezoelectric ceramic actuation members, conventionally employed in series in a stack form, exhibit an acceptable output energy density as well as a very high bandwidth, i.e., a relatively fast actuation stroke. A piezoelectric stack structure is generally limited, however, to only a relatively small stroke, and can typically produce only a limited output force, largely due to the characteristic brittleness of piezoelectric materials. As a result, stroke and force amplification mechanisms are often required of an actuator incorporating a piezoelectric actuation material; but for many applications, the limited piezoelectric actuation force cannot be rendered sufficient for the application as a practical matter.

Magnetostrictive actuation materials typically are characterized as being capable of producing an actuation force and an actuation stroke that are greater than that of piezoelectric materials. Application of a magnetic field to a magnetostrictive material causes the material to be strained as the domain magnetization vectors of the material rotate to align with the direction of the applied magnetic field. The unit cells of the material are strained by the magnetization rotation but their orientation is not changed.

While magnetostrictive actuation elements do exhibit a relatively high-frequency actuation response, they are fundamentally limited by their electrical conductivity, which precludes operation at very high actuation frequencies due to the formation of eddy currents in the material in response to a changing applied magnetic field, unless at least one of the material dimensions of the elements perpendicular to the field is small. An additional limiting constraint of magnetostrictive materials is that they typically are characterized by an actuation stroke that, like that of piezoelectric actuation elements, is limited in its extent; here due to the domain elongation inherent in the actuation mechanism.

The class of actuator materials known as shape memory alloys is characterized in that, when plastically deformed at one temperature or stress condition in a phase known as the martensitic phase, the alloy can recover its original shape when subjected to an alloy-specific martensitic-austenitic transformation temperature or stress condition that reverts the material to a corresponding parent, austenitic phase. This effect is based on the restoration of twin variants of the martensite phase of the material to their austenitic shape. Such materials are capable of reversing a large stress-induced martensitic deformation when transformed back to the austenitic phase, and thus can enable a large actuation stroke mechanism. Furthermore, the recoverable strain accommodated by a shape memory alloy is generally considered to be the largest achievable for any actuation material, and can be as large as about 20%, for, e.g., the Cu-Al-Ni alloy.

The large stroke generally characteristic of shape memory alloys is offset by the typically very slow actuation response time of the materials when the martensitic/austenitic transformation is thermally controlled. As a result, shape memory actuation can not accommodate applications requiring even moderately high actuation frequencies. Furthermore, the shape memory transformation is generally characterized as a poor energy conversion mechanism; i.e., much of the heat supplied to the material to drive the martensitic/austenitic transformation is uncontrollably lost to the surroundings. Thermal control of the shape memory effect also limits the allowable operational temperature range of an application for which a shape memory alloy can be employed.

For many actuation applications, it is ideally preferred to combine the large actuation stroke provided by shape memory alloys with the fast actuation response time of magnetostrictive and piezoelectric materials. At the same time, the thermal constraints of shape memory, piezoelectric, and magnetostrictive materials would also preferably be eliminated. Previous attempts to arrive at such materials that embody all of these qualities have resulted in the introduction of such alloys as certain Ni-Mn-Ga, Ni-Fe-Co, Mn-Fe-Co and Ga-Si-Al alloys. None of these, however have proven entirely satisfactory.

The materials science and engineering of ferromagnetic shape memory alloys (FMSMA) is a relatively young field. One of the initial challenges facing researchers was the change of the martensite start temperature as a function of the magnetic field in ferrous alloys, not necessarily FMSMAs (Kakeshita T, Kuroiwa K, Shimizu K, Ikeda T. Yamagishi A, Date M. Mat. Trans. JIM 1993 ;34:4 15,423).

The recognition that a reversible domain reorientation in a magnetic field will cause a large magnetostrictive strain of the order of the FMSMA transformation eigenstrain led to the recent explosion of activity in the field. Of three alloy systems actively investigated: NiMnGa, FePd and FePt, the latter two have long been known to be SMAs [Dunne DP, Wayman M, Met. Trans.1973 ;4: 137; Kajiwara S, Owen W, Met. Trans. 1974;5:2047; Tadaki T, Shimizu K, Scripta Met. 1975;9:771; Oshima R, ScriptaMet. 1981;15:829; Oshima R, Suguyama M, Fujita FE, Met. Trans. 1988; 19A:803] and were re-investigated to study their magnetostriction [James RD, Wuttig M, Phil. Mag. A. 1998;77: 1273; Kakeshita T, Takeuchi T, Tsujiguchi T, M, Saburi T, Oshima R, Muto 5, Appl. Phys. Letters 2000;77: 1502]. The martensitic transformation in NiMnGa was discovered in 1984 [Webster PJ, Ziebeck KRA, Town SL, Peak MS. Phil. Mag. 1984;49B:295]. Its SMA character was recognized later [Zasimchuk IK, Kokorin W, Martynov VV, Tkachenko AV, Chernenko VA, Fiz. Met. Metalloved. 1990;6: 110]. Attention to its magnetostrictive properties has been more recent [Ulakko K, Huang J, Jantner C, O'Handley RC, Kokorin VV, Appi. Phys. Letters 1996;69: 1967; James RD, Wuttig M, Proc. SPIE 1996;2715:420; Tickle R, James RD, Shield T, Wuttig M, Kokorin VV, IEEE Trans. Magnetics 1999;35:4301; Vasil'ev AN, Bozhko AD, Khovailo W, Dikshtein IIE, Shavrov HE, Buchelnikov VD, Matsumoto M, Suzuki S, Takagi T, Tani J, Phys. Rev. 1999;B59: 1113; Murray SJ, Marioni M, Allen SM, O'Handley RC, Lograsso TA, Appi. Phys. Lett. 2000;77:886; Wang Z, Lijima T, He G, Oikawa K Wulff L, Sanada, N Furuya Y, Mat. Trans. JIM 2000;9;1 139.

It is thus apparent that FMSMAs can potentially be used as magneto-mechanically controlled actuators. Two possible FMSMA actuation mechanisms exist. The first utilizes the shift of the transformation temperature by an external magnetic field. This mechanism utilizes the redistribution of the simultaneously elastic and magnetic domains by an external magnetic field [James RD, Wuttig M, Phil. Mag. A 1998;77: 1273]. It is operative at any temperature below the transformation temperatures. Its power is limited by the magnetic energy density of the alloy. Co-based FMSMAs should thus be good candidates for use in FMSMA magnetostrictors.

It is an object of the invention to provide novel ferromagnetic shape memory alloys that are particularly useful as actuators.

SUMMARY OF THE INVENTION

The above and other objects are achieved by the present invention, one embodiment of which relates to a ferromagnetic shape memory alloy having the composition:

Co₂Niι._xGa_1+x wherein x is a value such that the alloy exhibits an austenitic crystal structure above a characteristic phase transformation temperature and which exhibits a martensitic twinned crystal structure below the phase transformation temperature, and further being characterized by a magnetocrystalline anisotropy energy that is sufficient for enabling motion of twin boundaries of the martensitic twinned crystal structure in response to application of a magnetic field to the martensitic twinned crystal structure.

A further embodiment of the invention concerns an actuating element comprising an alloy as described above and a magnetic actuation field source disposed with respect to the actuator material in an orientation that applies to the alloy a magnetic actuation field in a direction that is substantially parallel with a selected twin boundary direction of the martensitic twinned crystal structure thereof. A final embodiment of the invention concerns a method for controlling the orientation of the twin structure in a ferromagnetic shape memory alloy having the composition:

wherein x is a value such that the alloy exhibits an austenitic crystal structure above a characteristic phase transformation temperature and which exhibits a martensitic twinned crystal structure below the phase transformation temperature, and further being characterized by a magnetocrystalline anisotropy energy that is sufficient for enabling motion of twin boundaries of the martensitic twinned crystal structure in response to application of a magnetic field to the martensitic twinned crystal structure, or an actuating element comprising the alloy; the method comprising applying to the material a magnetic field which is of a direction and of a magnitude enough for reorienting the twin structure of the material, to produce thereby shape changes of the material and motion and/or force.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a diagram depicting representative alloys of the invention positioned in martensite, ferromagnetic and special lattice groups.

Fig.2 is a micrograph of an alloy of the invention.

Fig. 3 depicts temperature dependence relationships of several properties of several alloys of the invention.

Fig. 4 depicts magnetization curves of an alloy of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is predicated on the discovery that alloys of the formula:

Co₂Ni₁._xGa_1+x wherein x has certain values are ferromagnetic shape memory alloys. In the as-solidified state their martensite start temperatures vary in the range 20°C<T<60°C as the concentration parameter x decreases. The high and low temperature phases are body centered cubic and orthorhombic and/or monoclinic, respectively. The transformation hysteresis, i.e., the difference between the martensite and austenite start temperatures equals approximately 30 degrees. The saturation magnetization of the alloys resembles that of nickel while their coercive force is less than lOOmT.

Diffusionless, i.e. martensitic, transformations occur at certain critical average electron concentrations. For instance, the martensitic transformations in Cu-based alloys systems occur at <s>)1.4 [Mott NP, Jones H, The Theory of the Properties of Metals and Alloys, New York, Dover, 1958] while those in Fe-based alloys occur at <s+d>.8.5 [Wassermann EF, Kaestrier J, Acet M, Entel P, Proc Intntl Conf On Solid-Solid Phase Transformations, Koiwa M, Otsuka K, Miyazaki T, eds, Kyoto, The Japan Institute of Metals, Sendai, 1999:807]. The martensitic transformation in NiMnGa Heusler alloys occurs at an average valence electron concentration of approximately 7.3 [Schlagel DL, Wu YL, Zhang W, Lograsso TA, J Alloys and Compounds 2000;3 12:77]. Their saturation magnetization falls on the known Bethe-Slater plot only if all outer electrons are counted, i.e., <s+p+d>) 10 [Wuttig M, Liu L, Tsuchiya K, James RD, J. Appi. Phys. 2000;87:4707]. Not all martensites are SMAs, however, and those that are fulfill certain special relationships between the lattice parameters of the austenite and martensite phases [James RD, Hane K, Acta Materialia 2000;48: 197]. This is schematically indicated in Fig. 1.

In its simplest terms, then, a search for new Co-based FMSMAs can thus start by identifying a potential Co-based Heusler alloy with an average valence electron concentration of approximately 7.3. Of those CoNiGa is similar to NiMnGa in that <s+p+d>.10. Thus, Co Nii-_xGai_+x alloys should be ferromagnetic and display SMA characteristics. The present invention identifies which of these alloys behave thusly. Four alloys of nominal composition, Co₂Ni₁_χGa_1+x, x= 0.06, 0.09, 0.12, 0.15, were prepared by arc melting. Their micro-structures were determined using a polarized light microscope equipped with a heating stage and shape recovery experiments were run on the alloys Co Nio.85Gau₅ and Co₂ io.₈8Gaπ₂. In addition, the internal friction and modulus defect, )/M M=[M(T₁)-M(T₂)]/ M(T_Ϊ), were determined in the temperature range -50°C< T<200°C using a known apparatus [Wuttig M, J Alloys and Compounds 1994;21 1/212:434.] Room temperature magnetization curves were determined as well using a standard vibrating sample magnetometer, VSM.

The as-solidified Co Nio_.88Ga ₂ and Co₂ i₀.₈₅Gau₅ polycrystals displayed full shape recovery upon heating after having been manually deformed at room temperature. During sample preparation, it was noticed that the CoNiGa FMSMAs are not nearly as brittle as their NiMnGa counterparts. Other experimental results are displayed in Figs. 2 to 4. Figure 2 shows the room temperature martensitic microstructure of as-solidified polycrystalline Co₂Ni₀.₈5Ga 5- Modulus defect and internal friction data of three as-solidified polycrystalline alloys are displayed in Fig. 3. Figure 4, finally, displays magnetic data. Preliminary XRD data indicate that monoclinic and/or orthorhombic martensite form reversibly from a bcc high temperature phase.

The microstructure displayed in Fig. 2 is typical of a SMA martensite as the domain boundaries are straight indicating only elastic distortions in the product phase. The modulus defect and internal friction data displayed in Fig. 3 point toward a SMA-type martensitic transformation as well [Colluzi B, Biscarini A, Campanella R, Trotta L, Mazzolai G, Tuissi A, Mazzolai FM, Acta Mater. 1999;47: 1965; Roytburd A, Su Q, Slutsker JS. Wuttig M, Acta Mater. 1998;46:5095; Wuttig M, Craciunescu C, Li J, Mat. Trans. JTM 2000;4 1:933]. They suggest Ms temperatures between slightly below room temperature and 60°C with a hysteresis of about 30°C. The )M/M vs. T characteristic of the alloy Co Nio_.85Gau₅ further suggests a sequence of two martensitic transformations. The internal friction data in the martensitic phase resemble those of NiTi indicating a high mobility of the boundaries. It was noted that heating to 850°C and subsequent quenching to room temperature lowered the Ms temperature, particularly in the Ga rich alloys.

The magnetization curve of Co₂Ni₀.₈₈Gau₂ is presented in Fig. 4. It can be seen that the saturation magnetization in the quenched state is comparable to that of nickel, as expected, and that the coercive force equals approximately 200 mT. Furthermore, the saturation magnetization depends on the state of the alloy. This agrees with previous observations on the ferromagnetic properties of Co₂Ni₁._xGa₁₊χ, l>x>0.3 [Booth et al, J. Magn. Matls. 1978; 7 :127. These indicated that the quenched alloys are ferro magnetic for x<0.5 and that the saturation magnetization depends on the composition as well as the state of anneal. They also showed that the alloys possess an ordered B2 structure.

In summary, all experimental evidence presented herein indicates that CoNiGa alloys form a new ferromagnetic shape memory alloy family. This is substantiated by the (1) visual observation of full shape recovery of the alloys upon heating, (2) typical SMA microstructure as shown in Fig. 2, (3) reversible change of the crystal structure upon heating and cooling, and (4) minimum of the elastic modulus occurring at the same temperature, Ms, as the sharp rise of the internal friction and (5) by the nickel-like saturation magnetization of the alloys.

The alloys can be prepared according to methods other than those described above, such methods being well known to those skilled in the art. Moreover, the alloys of the invention may be employed as single crystals that would need to be grown in a manner similar to Ni-alloy turbine blades. For example, single crystals can be produced according to the methods described in U.S. patents nos. 5,154,884 and 5,413,648, the entire contents and disclosures of which are incorporated herein by reference. Alternatively, they can be used as polycrystals (regular metals) in which case they need to be hot and/or cold rolled to achieve a texture. The alloys of the invention are particularly suitable for use as the latter, contrary to NiMnAl, for example.

Claims

1. A ferromagnetic shape memory alloy having the composition:

wherein x is a value such that said alloy exhibits an austenitic crystal structure above a characteristic phase transformation temperature and which exhibits a martensitic twinned crystal structure below the phase transformation temperature, and further being characterized by a magnetocrystalline anisotropy energy that is sufficient for enabling motion of twin boundaries of the martensitic twinned crystal structure in response to application of a magnetic field to the martensitic twinned crystal structure.

2. The alloy claim 1 having the composition:

3. The alloy claim 1 having the composition:

Co₂Nio.₈₈Gau₂.

4. The alloy claim 1 having the composition:

5. An actuating element comprising an alloy of claim 1 ; and a magnetic actuation field source disposed with respect to the actuator material in an orientation that applies to the actuator material a magnetic actuation field in a direction that is substantially parallel with a selected twin boundary direction of the martensitic twinned crystal structure of the actuator material.

6. The actuating element of claim 5 wherein the alloy has the composition:

7. The actuating element of claim 5 wherein the alloy has the composition:

8. The actuating element of claim 5 wherein the alloy has the composition:

9. The actuating element of claim 5 wherein the actuator material is characterized by a shape having a longest dimension, and further comprising a magnetic bias field source disposed with respect to the actuator material in an orientation that applies a magnetic bias field to the actuator material along a material dimension other than the longest dimension, the magnetic actuation field source being further disposed with respect to the actuator material in an orientation that applies the magnetic actuation field along the material longest dimension, for producing an actuation stroke along the material longest dimension.

10. The actuating element of claim 9 wherein the actuator material is characterized by a pre-actuation condition in which a substantial portion of the martensitic twin boundaries are aligned with the static magnetic bias field.

11. The actuating element of claim 5 wherein the actuator material exhibits a martensitic twinned crystal structure characterized by a number, t, of distinct twin variants, that is less than 5.

12. The actuating element of claim 11 wherein the number of distinct twin variants of the actuator material martensitic twinned crystal structure is less than 3.

13. The actuating element of claim 5 further comprising a magnetic bias field source disposed with respect to the actuator material in an orientation that applies a magnetic bias field to the actuator material in a direction that is substantially perpendicular to the direction of the applied magnetic actuation field.

14. A method for controlling the orientation of the twin structure in ferromagnetic shape memory alloy having the composition:

wherein x is a value such that said alloy exhibits an austenitic crystal structure above a characteristic phase transformation temperature and which exhibits a martensitic twinned crystal structure below the phase transformation temperature, and further being characterized by a magnetocrystalline anisotropy energy that is sufficient for enabling motion of twin boundaries of the martensitic twinned crystal structure in response to application of a magnetic field to the martensitic twinned crystal structure, or an actuating element comprising said alloy, said method comprising applying to the material a magnetic field which is of a direction and of a magnitude enough for reorienting the twin structure of the material, to produce thereby shape changes of the material and motion and/or force.

15. The method of claim 14 wherein said alloy has the composition:

Co₂Nio.₉Gaι.o9.

16. The method of claim 14 wherein said alloy has the composition:

17. The method of claim 14 wherein said alloy has the composition:

18. The method according to claim 14, wherein the magnetic field is applied on the material in the direction of the easy magnetization of the desired twin orientation.

19. The method according to claim 14, wherein the magnetic field is applied on the material in such a direction that it produces a desired shape change or motion of the material due to the reorientation of the twin structure.

20. The method according to claim 14, wherein the magnetic field is applied on the material in the direction differing from the direction of the easy direction of magnetization of the twin variants to produce axial strain, bending or twisting of the material.

21. The method according to claim 14, wherein the magnetic field is applied on the material in a changing direction and/or with changing magnitude as a function of time.

22. The method according to claim 14, wherein the method is applied with an actuator and wherein shape change, motion and/or force of the actuator is affected.

23. The method according to claim 14, wherein the method is applied to an actuator which is controlled, and the power of which is provided remote from the actuator.