1. Field of the Invention
The present invention relates to the magnetic shape memory (MSM) alloys. In particular, the present invention concerns a method of stabilization of mechanical and magneto-mechanical properties via stabilization of the twin variant structure of objects (i.e. elements, specimens or samples) which comprise MSM alloys. Thus, the invention concerns a method which provides magnetic shape memory alloy elements with stabile mechanical and magneto-mechanical properties. The present invention also relates to the use of such a method for example in the form of an MSM device, such as an actuator, a sensor or an energy harvester, comprising an actuating element produced using the method.
2. Description of Related Art
Magnetic shape memory (MSM) alloys are a class of materials capable of producing motion and or force or other related function. The essential feature of an MSM alloy is that it is capable of performing a martensitic transformation, wherein the higher-symmetry parental phase transforms to a lower-symmetry daughter phase, called martensite. MSM alloys differ from ordinary shape memory alloys in that the martensitic microstructure can be manipulated by exposing the alloys to a magnetic field.
MSM material consists of internal areas known as twin variants. These variants have different magnetic and crystallographic orientations. When an actuating element made of an MSM material is subjected to a magnetic field the proportions of the variants change resulting in a shape change of the element. Correspondingly, by mechanical deformation it is possible to change the magnetic state/properties of the MSM material e.g. permeability.
Early demonstrations of MSM effect on a Ni—Mn—Ga alloy revealed a strain of about 0.19% [1]. Later, MSM materials exhibiting shape changes of up to 10 percent or more and response times of less than a millisecond have been developed.
In the following, the MSM effect and importance of twinning stress will be examined more closely.
First it can be pointed out that magnetic-field-induced rearrangement of (ferro-)magnetic twinned martensite microstructure, accompanied by a large macroscopic deformation, or magnetic shape memory effect, is a phenomenon which attracts attention since its first demonstration on Ni—Mn—Ga single crystal in 1996 [2]. Fast actuation seems to be one of the potential practical exploitations of the phenomena as magnetic shape memory effect with large strain up to 10% was already demonstrated many times starting from 1999 [3-10] and possibility of actuation in kHz range was also clearly shown [11]. Applications based on reverse mechanism were also suggested including sensing [12-14], energy harvesting [15, 16], damping [17], etc. Ni—Mn—Ga alloys have been so far most studied and can be considered as a “prototype” magnetic shape memory material.
One way how to employ the MSM effect for linear reversible actuation is to apply a constant external stress σ along one principal axis and variable magnetic field H perpendicularly along another principal axis of a prismatic single crystal Ni—Mn—Ga MSM element,
An overview of other possible arrangements can be found in reference [19].
The MSM element is typically cut along {100} faces and contains simple two-variant twin variant structure as this type of microstructure seems to exhibit the best performance [20]. One twin variant (light in
The c and a axis of the two discussed twin variants point along two different principal directions of the MSM element. Thus, by growth or consumption of one or other variant, the orientation of c axis inside the element is changed from one principal direction to the other principal direction. This is the reason why magnetic shape memory effect is sometimes called magnetic-field-induced reorientation, or MIR. The exchange of c axis for a axis or vice versa along certain directions in the MSM element results in large deformation of the element. The maximum possible linear strain is given by the difference between a and c axis, which is for example in 5M Ni—Mn—Ga martensite typically about 6%.
Slow (≈1 Hz) and quasistatic actuation by magnetic shape memory effect can be easily demonstrated and investigated using a simple magneto-mechanical apparatus as the one shown and described in
The mobility of twin boundaries is a crucial factor determining the existence, quality and efficiency of the MSM effect [5, 21]. The mobility of twin boundaries is in close relation with twinning stress, which can be experimentally evaluated from stress-strain curves of the material [22]. Twinning stress, σTW, is typically determined from the stress-strain curve as the level of applied compressive (or tensile) uniaxial stress which causes the twin boundary motion. More specifically, it is often determined as position of de-twinning plateau on stress-strain curve, or it can be determined as stress level at half deformation strain [20], or, two stress levels can be recorded—one near beginning and the second near the end of the stress-strain curve [5]. Such determination may not fully cover more general consideration of twinning stress for which the stress level at which twinning initiates (i.e., twin variants nucleate) must be distinguished from the stress level at which twin variants grow [22, 23]. This is discussed more below.
Lower twinning stress indicates higher mobility of twin boundaries. From the model of MSM effect introduced by Likhachev and Ullakko [5] it follows that to obtain reversible MSM effect, the twinning stress must be less than half of the magnetic stress [24], i.e. less than about 1.5 MPa as the maximum magnetic stress (am) at room temperature is about 3 MPa (e.g. [24]). There is some general agreement that lower twinning stress result in better performance of MSM effect. How low below the above determined limit 1.5 MPa is satisfactory can be answered when considering the coupling factor, or efficiency of MSM effect determined as a ratio of obtained mechanical work and work spent on magnetizing of material during one magnetizing (actuating) cycle.
Table 1 [25] summarizes the calculated efficiencies for selected magnitudes of twinning stress using v=(σM−2σTW)/σM [14, 21] and σM=3 MPa.
Effects of a Twin Variant Structure on Mechanical and Magneto-Mechanical Properties of MSMAs
Additionally to the obstacles in the crystal, the twin variant structure also influences the twin boundary mobility (twinning stress) [27-29]. In other words, twin boundaries themselves can present obstacles for twin boundary motion [29]. Due to that the stress-strain response of the crystal depends on the twin variant structure, and, thus, attention must be paid to twin variant structure when determining the twinning stress or twin boundary mobility. We created well defined twin variant structures in the crystals using an external stress and/or magnetic field prior the testing. The morphology of the created twin variant structures is demonstrated in
Mechanical testing using compressive stress and measurement in quasistatic magnetic field under zero stress were performed for each microstructure presented in
The main feature observed on stress-strain curves determined for single variant, i.e., for detwinned crystal, is large load drop at the beginning of loading by which initial stress peak was formed on the stress-strain curve,
The height of the stress peak corresponds to the stress initiating twinning (nucleating new twin variants) in detwinned crystal (single variant) and, thus, we will refer to the height of the stress peak as initial twinning stress. The average height of the stress peak, or initial twinning stress, in our experiments was about 2 MPa.
After the twinning was initiated and some twin boundaries were clearly observed in the crystal, the deformation of the crystal progressed by motion of twin boundaries (with only occasional nucleation of additional twin domains). Stress required for this motion was much smaller (0.1-1 MPa) than initial twinning stress and we will refer to it as conventional twinning stress, or just twinning stress.
From the above it can be concluded that to achieve actuation with reasonable efficiency, some twin boundaries must be always present in the crystal. Otherwise, a high barrier (i.e., initial twinning stress) would have to be overcome by magnetic stress or by the external counterstress to achieve actuation. This was pointed out also in Ref [31], where it was stated that the homogeneous magnetic field-induced nucleation of twin-boundaries is not likely which means that twin boundary motion in magnetic field may proceed only from preexisting twin boundaries, that are not detected macroscopically. Experiments in magnetic field confirmed that single variant very often showed no MSM effect at all, or showed but with limited strain. Note also that the initial twinning stress of 2 MPa is too high to allow reversible MSM effect in arrangement with external counterstress,
The stress-strain curves varied considerably between individual experiments made on single variants. This was presumably due to different twin variant structure nucleated during each experiment. However, as demonstrated also in
The developed crystals showed tendency to nucleate only a pair or a few twin boundaries under compression and further deformation progressed by propagation of these nucleated twin boundaries [27]. The single twin boundary was formed from the nucleated pair when the pair was nucleated near end of a specimen and one boundary vanished at the end of the specimen. The remnant boundary showed high mobility either immediately or after a few compression-extension cycles performed manually in fingers while holding the crystal from its two ends.
Compression tests of the crystal with single highly mobile twin boundary are demonstrated in
The magnetic shape memory effect in the crystal with single highly mobile twin boundary in quasistatic magnetic field is demonstrated in
Additional important factor influencing the switching field was the demagnetization factor of the crystal along the magnetizing direction. In the shown case, the field was applied perpendicular to 2.5×20 mm2 face of the 2.5×20×1 mm3 specimen, thus, demagnetization factor was high. In spite of that, the observed switching field was rather low in agreement with the observed low twinning stress of the crystal. Magnetizing the crystal along direction with low demagnetization revealed that applied field as low as 0.03 T (300 Oe) as enough to achieve the MSM effect [34].
The low twinning stress determined from stress-strain curves and demonstrated high mobility of twin boundaries in magnetic field together with more detailed investigations (using tensile stress [27] and different temperatures clearly confirm that it is possible to achieve very high mobility of twin boundaries in Ni—Mn—Ga. However, the crystal with single twin boundary does not seem to be the best candidate for practical usage in a magnetic actuator with a narrow air gap. One of the problems is demonstrated in
The stochastic nature of single twin boundary motion [35] and slower response of crystals with small number of boundaries [31] may be listed as other reasons why to consider twin structure with many twin boundaries or fine twins as in
Additionally, handling the large untwinned volume of the crystal may not be trivial as in this volume any orientation of twins can appear. These can interfere with existing boundaries and hinder their motion. The above mentioned reasons initiated the effort to create fine twin structure in the material and to employ it in actuation, which is described in more detail below.
Fine twins, also called parallel lamellar twins or polysynthetic lamellar twins with density about 10-100 boundaries/mm, can be created in the crystals by bending [30],
A remarkable benefit of the “Fine twins” is that in sample having such twin variant structure the deformation cannot occur by single twin boundary motion. There is still a three to four degrees kink of the sample at each twin boundary, but since the distance between boundaries is small and the neighbor kinks are of opposite sign, they cancel out each other on macroscopic scale. The absence of one or few large parts with this kind of kink removes the geometrical problem shown in
The fine twins are annihilated completely from the crystal by compression of several MPa stress or by high magnetic field. Thus, achieving reversible MSM effect in the crystal with non-stabilized fine twins is not possible since the twins annihilate each other.
The sample with a fine twin variant structure showed a considerably higher twinning stress than the sample with a highly mobile single twin boundary, compare
The twins that are nucleated inside the central twin domains (dark) of triple-twin segments are rather thin (≈1-10 μm or less; see below) and they end at a twin boundary as shown in
By performing several compression tests with many carefully performed partial cycles we, indeed, detected a significant recoverable deformation in a specimen with fine twin domains ending at a twin boundary. The magnitude of the recoverable strain varied in different experiments. It presumably depended on the exact morphology of the twins nucleated during each experiment. Loading curves as in
The stress plateaus associated with the recoverable deformation were at the level of about 0.5 MPa as shown in
The microscopic observations we made were in agreement with the mechanism of magnetoelasticity based on the repulsive interaction of twinning dislocations described above. Under stress, the thin twins were much thinner near the perpendicular twin boundary at which they ended and much wider far from the boundary. This confirms that the twinning dislocations experienced difficulties in gliding past the twin boundary and, thus, piled up near the twin boundary instead. The typical observed thicknesses of the twins were ≈1-10 μm far from the boundary and ≈1 μm near the boundary (or much less, since some twins became invisible in an optical microscope when they were traced from the region far from the twin boundary toward the twin boundary).
The estimation based on the assumption that the recoverable strain is proportional to the volume occupied by the active region gives the estimated maximum macroscopic recoverable strain of 2.5% for active regions occupying 100% of the specimen volume. Various microstructures with an active region occupying a large volume of the specimen can be designed. One possible design based on triple-twin segments is shown in
In connection with the present technology, the existence of triple twins in Ni—Mn—Ga magnetic shape memory single crystals with modulated five-layered martensite has been experimentally demonstrated. The single-domain triple twins or triple-twin segments (see
Single-Variant Problems:
On the measurements demonstrated above we showed that it is necessary to distinguish between initial twinning stress, i.e., stress nucleating a twin boundary/boundaries or initiating twinning in material and conventional twinning stress or just twinning stress, i.e., the stress inducing motion of existing twin boundaries. As the former seems to be typically much higher than the latter, and additionally it was shown that single variant shows a hardly predictable magneto-mechanical response and sometimes no response at all, the state of specimen without twin boundaries (i.e. single variant or detwinned specimen) should be avoided completely during exploitation of the material in real applications: actuation, energy harvesting, sensing, etc.
Single Twin Boundary Problems:
Even the specimen with pre-existing twin boundary or few separate twin boundaries contains several serious problems. For a single twin boundary, a sharp kink (by 3.5 degrees in 5M Ni—Mn—Ga, in other martensite structures, like 7M and NM, can be even higher) of the whole sample on the twin boundary causes geometrical problems in using the sample in a typical magnetic actuator and other space limited applications. Additional problems are related to the fact that single twin boundary can easily annihilate at the end of the sample or when meeting another twin boundary with same orientation after which the sample becomes a single variant [27]
Also, in contrast with an all-time-monitored bench-top experiment in laboratory environment, it is not easy to assure that more complex microstructures are not nucleated in the single variant sample or sample with single twin boundary during its use in real-life applications. Such unpredictable twin variant structures with an undesired twin configuration can show a higher twinning stress and need a higher magnetic field to achieve actuation and/or may easily lead to fast fatigue failure and braking of the MSM-element. As a result, the performance of an MSM-element (actuating, harvesting, sensing, mechanical, structural, etc.) is strongly degraded and/or unpredictable.
Problems of Non-Stabilized “Fine Twins”:
Our experiments demonstrate that a MSMA specimen can be turned easily, e.g. by bending, to a specimen with fine twins exhibiting moderate twinning stress and proportional MSM effect. In spite of the higher twinning stress, the fine twin variant structure exhibits several advantages. For example, there are no problems with sample geometry if the twin bands are fine enough. Additionally other undesired twin variant structures cannot appear in specimen with fine twin variant structure since the whole volume of specimen is already occupied by parallel twin boundaries and there is no large single-variant parts of volume where unwanted twin configurations could nucleate. Even if in specimen comprising of fine twins some undesired twin configurations do nucleate, they will be small in size and considerably less harmful than large unwanted twin configurations that can appear in single variant or single twin boundary specimen.
However, after few elongation-compression cycles by the action of external stress or magnetic field, the twin boundaries comprising the fine twin variant structure annihilate, which leads in a rapid decrease in the density of twin boundaries. Thus, very soon the sample contains only few twin boundaries, approaching and typically reaching the single twin boundary case, or sample goes in single variant state and in both cases behavior of the sample becomes again unstable or unpredictable as described above. Unintentional deformation of the specimen into single variant state is very typical for MSM alloys specimens and can block/prevent the MSM effect completely as discussed above.
Problems of Non-Stabilized “Dispersed Triple-Twin Segments” as Well as of Other Non-Stabilized Complex Twin Variant Structures:
The “dispersed triple-twin segment” microstructure has same benefits as fine twin variant structure except that it has shown to possess additionally some amount of pseudoelasticity (magnetoelasticity), also similar non-stability arguments apply for Dispersed triple-twin segments.
The dispersed triple-twin segment structure has been shown to posses weak stability under very limited conditions which is however far insufficient for the real industrial applications.
Based on the above, it appears that any artificially created twin variant structure introduced in MSMA-element after the crystal-growth is very likely to be not stable enough for real-life industrial applications without additional stabilization modification/treatment. It was demonstrated above that the twin variant structure of a magnetic shape memory alloy strongly influences on its mechanical and magneto-mechanical response. Conversely, by tailoring the twin variant structure, one can obtain rather different mechanical and/or magneto-mechanical responses of the material, which can be exploited in adjusting the actuating, energy harvesting, sensing and structural properties of MSM alloys.
Twin variant structures can posses inherently either instability, or unpredictable behavior accompanied with instability, in mechanical and magneto-mechanical response. The significant effect of twin variant structure on the mechanical and magneto-mechanical response of MSMAs was demonstrated by using 5M Ni—Mn—Ga single crystal elements as an example, but naturally the effect can apply on any MSM-alloy system (stoichiometric, non-stoichiometric, or with alloying additions like Fe, Co, Cr, Cu, Ti, Zr, Al, Mo, W, for example and any combination thereof) and any martensite structure (5M, 7M, NM, etc.) and any lattice structure (tetragonal, orthorhombic, monoclinic, triclinic, etc.).
For proper utilization of different twin variant structures (for example such as “Fine twins” or “Dispersed triple-twin segments”) accompanied with their different mechanical and/or magneto-mechanical properties, there is a need for finding ways of making such twin variant structures stable (to prevent their transformation to other (undesired) twin variant structures) in real out-of-laboratory application environments, such as actuator, energy-harvester, sensor applications for example. Any such technical solutions should be implementable on an industrial production scale also.
Thus, keeping the above introduced twin variant structure and resulting performance instability issues in mind, the present invention aims at providing solutions for stabilization of different twin variant structures, in particular (but not only) the tailor-made structure of “Fine twins” and of “Dispersed triple-twin segments”, which result in stable and easy to use MSMA-elements for various applications such as actuators, sensors, energy harvesters or any other MSM-device.
For stabilization or creation and stabilization of twin variant structures a novel solution involving functional twin-affecting surface modification has been provide. The present technology stands for the previously missing step for large scale production of MSMA-elements with properties selected from the group of stable, repeatable, tailored, designed, desired mechanical and magneto-mechanical properties and any combinations thereof.
It is an aim of the present invention to eliminate at least a part of the problems related to the known art and to provide a new method of modifying MSM materials toward having a predictable and repeatable mechanical and magneto-mechanical response.
It is a particular aim of the present invention to provide a method by which it is possible to create a stable fine twin variant structure in single crystal MSM-samples. It was found in the present invention that one or more of the above mentioned problems can be solved by surface modification of at least one surface of an element or object formed by the MSM material to achieve stabilization of the twin variant structure and the mechanical and magneto-mechanical properties of the object.
Accordingly, in one aspect the present technology concerns a method of producing a magnetic shape memory alloy objects comprising the step of modifying the surface layer of an MSM material by mechanical, chemical or thermal treatment (or a combination thereof) of said surface.
In another aspect, the present technology concerns an MSM element for producing motion and/or force or other specific function wherein the element comprises or consist of an MSM material produced by a method which includes modification of a surface of the MSM material.
More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1.
An object of an MSM alloy and an MSM device, e.g. the actuator, sensor or energy harvester, according to the invention, are characterized by what is stated in the characterizing part of claims 29 and 32.
Other aspects of the present technology are disclosed in the dependent claims.
The present invention provides considerable advantages:
The mechanical or magneto-mechanical response of an MSM element can be adjusted according to the need and application of the element. The mechanical and mageneto-mechanical response can be made more repeatable that in a conventional MSM material.
Such elements show a mechanical response as shown in
The technology now presented for functional twin-affecting surface modification on MSM materials comprises two general embodiments, one involving twin-stabilizing surface modification and the other twin-activating surface modification. These will be described in detail in the following.
a) Twin variant structures investigated in this study: i) single variant, ii) single boundary iii) fine twins.
a) Response of single variant (de-twinned crystal) to compressive stress. The three displayed measurements were performed on three different single variant crystals. Observe 1-3 MPa stress peak connected to twin boundary nucleation in single variant specimen.
b) Magneto-mechanical response of the studied samples under near zero counter-stress showing magnetic-field induced strain (MFIS). Three representative measurements performed on the same specimen, starting each time from single variant state.
a) Response of crystal with single highly mobile twin boundary to compressive stress.
a) Due to twinning crystallography, a tilt of 90-2 arctan(c/a) is present at each twin boundary.
c) Creating dense fine twins into the specimen eliminates the problem.
a) Two identical single-domain triple-twin segments. Five twin domains show five different orientations and the tilt between the ends of the specimen is 4α=14°. Dark triangles are the central domains of each segment. [36].
a) Response of fine twins to compressive stress (determined on the same crystal as in
a) Schematic and optical microscope image of twin variant structure called “Dispersed triple-twin segment”. The shown twin variant structure exhibited the mechanical behavior as shown in
a) local plastic deformation (by dislocations) of C-variant and local transformation (by twinning) of A-variant into the C-variant.
a) The non-homogenous plastic deformation/residual elastic stress pattern does not disappear even if the element is forcefully elongated to single variant state.
a) The non-homogenous plastic deformation/residual elastic stress pattern does not disappear even if the element is forcefully compressed to single variant state.
a) Schematic figure of the LUFTVS twin variant structure stabilized recovers automatically even after compression to single variant state as shown in
b) Preferred nucleation site of C variant lamellas after the elements has been deformed to compressed single variant state.
a) Stress-strain curves of three different 5M Ni—Mn—Ga specimens (Same 5M Ni—Mn—Ga MSM-material and same specimens as in
b) Curve family showing 10 cycles of MFIS of the specimen with stabilized fine twins demonstrating excellent repeatability between cycles and proportional behavior. Material after same surface treatment as in
a) Effect of pressure of incoming jet of particles during bombardment process (method CSP, method modification CGSP) on the profile of stress-strain curve of the specimen.
a) Plastic deformation of the whole surface layer (by dislocations).
b) Plastic deformation in austenitic state by dislocations is permanent.
a) Demonstrate the permanent plastic deformation and possible residual elastic stress imprint in surface layer of MSM element in austenitic state.
b) The supposed/expected residual stresses originating from Twin-activating surface modification are shown schematically on microscopic level
a) Twin activated MSM element forcefully elongated to single variant state.
b) Twin activated MSM element forcefully compressed to single variant state.
For stabilization or creation or activation and stabilization of the (desired tailored, designed) or any twin variant structures various embodiments are provided which allow for the production on a large scale of MSMA-elements with properties selected from the non-limiting group of stable, repeatable, desired mechanical and magneto-mechanical properties and combinations thereof.
For the purpose of the present invention, the term “element” will be used interchangeably with “object” and “specimen” or “sample” to designate any item formed by an MSM-alloy of interest which has properties of super- or pseudoelasticity, magnetoelasticity or magnetic shape memory.
The “MSM alloy” according to the present invention generally refers to any MSM-alloy system of the above kind and others which are defined below. The term thus includes stoichiometric and non-stoichiometric alloys which is primarily formed by specific metals, such as Ni, Mn and Ga, with optional alloying additions, like Fe, Co, Cr, Cu, Ti, Zr, Al, Mo, W, for example and any combination thereof). All martensite structures (5M, 7M, NM, etc.) and lattice structures (tetragonal, orthorhombic, monoclinic, triclinic, etc.) are similarly included.
In a first embodiment, the present technology comprises surface treatment of MSMA-element(s) aiming at obtaining, stabilizing or combinations thereof of selected twin variant structure and optionally purposefully to affect mobility (twinning stress) of twin boundaries. This embodiment is also referred to as Functional twin-affecting surface modification.
In a second embodiment, surface treatment of MSMA-element is carried out on a tailored twin variant structure, i.e. an MSMA element in martensite state. This embodiment is also referred to as Twin-stabilizing surface modification.
In a third embodiment, cold (e.g. ambient temperature, generally lower than the martensite transformation temperature) Shot-Peening, CSP. Shot-peening by small particles. Shot-peening is carried out on MSM-element in martensitic state with, e.g. tailor-made, twin variant structure by particles.
In a fourth embodiment, cold (e.g. ambient temperature, generally lower than the martensite transformation temperature) Shot-Peening by Water-Jet-Soda, WJS. Shot-peening of MSM-element in martensitic state with tailor-made twin variant structure by small Sodium-bicarbonate particles mixed with water and air. This embodiment is particularly suitable for sample thickness <2 mm (smallest dimension).
In a fifth embodiment, cold (e.g. ambient temperature, generally lower than the martensite transformation temperature) Shot-Peening by Glass-beads, CGSP. Shot-peening of MSM-element in martensitic state with tailor-made twin variant structure by small spherical glass-beads. This embodiment is particularly suitable for sample thickness of >2 mm (smallest dimension).
In a sixth embodiment, surface treatment is carried out on an MSMA-element in austenite state to activate and support desired twin variant structure in martensite state. This embodiment is also referred to as Twin-activating surface modification.
In a seventh embodiment, Hot (e.g. at a temperature higher than the martensite transformation temperature) Shot-Peening, HSP. Shot-peening of MSM-element in austenitic state by (small) particles. Desired twin variant structure appears upon cooling to martensitic state.
In an eight embodiment, Hot (e.g. at a temperature higher than the martensite temperature) Shot-Peening by Glass-beads, HGSP. Shot-peening of MSM-element in austenitic state by small spherical glass-beads. Desired twin variant structure appears upon cooling to martensitic state. This embodiment is particularly suitable for sample thickness >1 mm.
During development of the present technology it was found that it is possible to control and optionally adjust and tailor the mechanical and magneto-mechanical properties or behavior, or both, of the whole volume of a MSMA-specimen. This can be achieved by surface layer modifications as mentioned above and as will be examined below.
The first embodiment (the group of Functional twin-affecting surface layer modifications) will be referred to hereinafter by the phrase “surface treatment”. For demonstrating the effect of the developed methods the exact composition of the tested alloys was Ni50.2Mn27.8Ga22.0 (±0.2 atomic percent). It should, however, be pointed out that the technology is applicable to a great variety of MSM-alloys having different compositions, martensite structures and lattice structures.
To demonstrate the principle of the embodiments falling within the concept of Twin-stabilizing surface modification we will use a method modification called “Cold shot-peening” (CSP). Characteristic to that stabilization method is that MSMA-element is bombarded by small particles or liquid, element being during the surface treatment in low temperature phase (martensitic state) with preliminary tailor-made twin variant structure. Such a surface treatment which is applied on a MSMA specimen with preliminary tailor-made twin variant structure results in the whole volume of a MSMA-specimen into stable twin variant configuration, which corresponds by morphology to the pre-existing tailor-made twin variant structure. Since the mechanical and magneto-mechanical properties of MSMAs are tightly bounded to the particular twin variant structure (twin variant configuration) [28, 47], it follows that by such a surface treatment method it is possible to control/adjust/tailor the mechanical and magneto-mechanical properties of MSMAs. The stabilization effect and possibility to affect on the mechanical and magneto-mechanical properties will be also proved by experimental results later in this document.
Other possible Twin-stabilizing surface modifications are:
Shot peening, sand or ball blasting, bombardment by small particles, Laser shock peening, water peening, ultrasonic shot peeing, hammer/needle peening, or similar methods which create plastic deformation in surface layer and or difference in residual elastic stresses between surface layer and bulk of the element.
To demonstrate the principle of the embodiments of Twin-activating surface modification a method modification called “Hot shot-peening” (HSP) will be employed. Characteristic to that stabilization method is that MSMA-element is bombarded by small particles, element being during the surface treatment in high temperature phase (austenitic state; cubic lattice, thus no twin variants and no twin boundaries). Such a surface treatment which is applied on a MSMA specimen in austenitic state (after cooling to low temperature phase) results in the whole volume of a MSMA-specimen being brought into the desired stable twin variant configuration, which resembles the twin variant configuration and properties shown in
Other possible Twin-activating surface modifications are:
Shot peening, sand or ball blasting, bombardment by small particles, Laser shock peening, water peening, ultrasonic shot peening, hammer/needle peening, rolling, inscribing, or any such method which creates plastic deformation in surface layer and or difference in residual elastic stresses between surface layer and bulk of the element.
All the above mentioned embodiments are preferably carried out by subjecting the surface of the element to surface treatment by directing the stream of the material at an angle which deviates by 10° at the most, preferably 5° at the most from the normal to the surface of the element.
For both basic working embodiments (Twin-activating surface modification and Twin-stabilizing surface modification) the following benefits apply which do not exist in MSMA-specimens with non-stabilized twin variant structure (untreated specimens of previous art):
2. Twin boundaries of the stabilized twin variant structure are readily movable by an external stress or magnetic field, which movement occurs instead of nucleation of new twins with possibly undesired orientation and formation of other undesired twin variant configurations with undesired properties.
The present invention activates/stabilizes the twin variant structure, so that a 3-dimensional object produced from an MSM material will exhibit the benefits given above, in particular the element will respond to mechanical stress and magnetic fields in a predictable, controllable and proportional way. Thus, the present invention will provide predictable, controllable and proportional desired mechanical and magnetomechanical properties for MSM-elements and for applications based on such twin-activated/stabilized MSM-elements.
The method of shot-peening of MSMA-element, in martensitic state, with tailor-made twin variant structure consists of two steps. By element is meant here the whole MSMA-sample.
In step A a desired twin variant structure is created (in martensitic state). In step B the created twin variant structure is locked/fixed by bombarding (shot-peening) the surface of the MSMA-element by bombardment media (i.e. small particles), in martensitic state. Remarkable new achievement here is that after the steps A-B the MSMA-element comprises of twin-variant structure which corresponds to the Tailored twin variant structure that was introduced into the element in step A. Surprising aspect is also that even when the treatment is done on surface, it governs the geometry and spatial distribution of appearing twin variant structure in the whole volume of a MSMA-element. From the obtained stable/repeatable twin variant structure it results that also the mechanical and magneto-mechanical properties of the material are stable/repeatable, which is clear improvement to the prior art MSMA-element manufacturing/properties [46]. Moreover, the mechanical and magneto-mechanical properties of MSMAs can beadjusted, changed, modified and tailored in the controlled way by the developed method to meet particular needs of the application.
The above introduced Twin-stabilizing surface modification method will now be described in detailed steps. Step A consists of creating a desired twin variant structure inside the MSMA-element. This kind of twin variant structure that is created by purpose is called Tailored twin variant structure. One of such tailored structures is Laminated Unidirectional Fine Twin Variant Structure LUFTVS (called also polysynthetic lamellar twins or dense two-variant laminate with parallel TBs), see
The method of making the LUFTVS was originally demonstrated in [30, 48]. The method of making LUFTVS on Ni—Mn—Ga MSMA-elements as well as mechanical and magneto-mechanical properties of that particular structure were described/reported in [27, 25].
In [49] a conclusion was made that the LUFTVS could be the most optimal twin variant structure from the point of view of real applications such as actuators, e-harvesters, etc. Benefits of the LUFTVS are repeatable and proportional stress-strain curve and MFIS-curve, long fatigue life, increased insensitivity to actuator geometry [47]. However, a problem was noticed at that point. The LUFTVS was namely demonstrated to disappear after few or even one mechanical or magneto-mechanical actuation cycles [27]. A critical question raised at that time, how to stabilize such structure to prevent it from disappearing.
The step B contains the critical novelty part of the developed Twin-stabilizing surface modification method—stabilization (or fixing or locking) of the Tailored twin variant structure, in this example case the LUFTVS.
In this particular example the stabilization of the LUFTVS is realized by bombardment (shot-peening) of surface layer of MSMA-element by small particles, see
Such particles can be near-round, round or irregular in shape, soft or hard but preferably round and soft. The particles material can be metal, ceramic, glass, polymer, carbon, minerals, composite materials, semiconductor, refractory or organic. The bombardment media can also be a liquid (e.g. water). The particles can be wet (e.g. with a forcible stream of water) or dry (e.g. with a forcible stream of air). An exemplary dry method is a surface bombardment with glass beads, and an exemplary wet method is surface bombardment mixture of water and an inorganic or organic powder, such as sodium bicarbonate powder. According to a particular embodiment the surface bombardment is performed with mixture of air, water and sodium bicarbonate powder.
During the surface treatment (here bombardment) the MSMA-element is in martensitic state and comprises of the tailored twin variant structure, in this case LUFTVS.
During the bombardment of surface layer by small particles, following processes are thought to be involved on microscopic scale. Kinetic energy of the particles is supposed to be absorbed by the surface layer of the MSMA-element by two different mechanisms: local plastic deformation (by dislocations) of C-variant and local transformation (by twinning) of A-variant into the C-variant, see
It is well known in literature that transformation from one twin variant to another by twinning is reversible, instead the plastic deformation by dislocations is irreversible [50, 51], see
Thus, as an end result of steps A-B of Twin-stabilizing surface modification method, we have in surface layer of MSMA-element a non-homogenous plastic deformation/residual elastic stress pattern which geometrically corresponds to the LUFTVS that was tailor-made in step A and existed in the element during the surface treatment in step B (
The remarkable achievement of the “Twin-stabilizing surface modification” is, that the same LUFTVS nucleates and is active during the following actuation cycles even if the whole twin variant structure is erased/destroyed by forcefully bringing/deforming the MSMA-element to single variant state (by detwinning)
Repeatable and stable LUFTVS results on its turn in repeatable and stable mechanical and magneto-mechanical and fatigue properties of the MSMA-material, which was not possible to achieve by the prior art.
In the following part the assumed mechanisms will be discussed by which the non-homogenous plastic deformation/residual elastic stress pattern introduced in material during Twin-stabilizing surface modification nucleates and keeps active the desired Tailored twin variant structure (in this example LUFTVS).
The non-homogenous plastic deformation/residual elastic stress pattern created in steps A-B is permanent and is not destroyed even when the MSMA-element is forcefully deformed by twinning to elongated single-variant or almost single-variant state, see
The force, or more precisely stress, that brings the Ni—Mn—Ga MSMA-element after Twin-stabilizing surface modification to single variant (or almost single-variant) state can be external mechanical stress (app. 0.1-100 MPa, in particular about 1 to 100 MPa, advantageously about 10 to 100 MPa), or external magnetic field (magnetostress up to 3 MPa, for 5M Ni—Mn—Ga, and over—typically 10 MPa or less), or combinations of mechanical stress and magnetostress or any combination of thermal heat, mechanical stress, magnetostress which produces sufficient enough stress.
When the surface modified MSMA-element is released from external stresses, after being forced to elongated single-variant state, the non-homogenous plastic deformation/residual elastic stress pattern created in surface layer of MSMA-element nucleates and keeps in the volume of whole element the twin variant structure which corresponds to the twin variant structure that existed in the element during the surface treatment in step B.
According to observations with mechanical testing device installed in optical microscope [47], the A-variant lamellas shown in
The non-homogenous plastic deformation/residual elastic stress pattern created in steps A-B is permanent and is not destroyed even when the MSMA-element is forcefully deformed by twinning to compressed single-variant or almost single-variant state, see
When the surface modified MSMA-element is released from external stresses, after being forced to compressed single-variant state, the non-homogenous plastic deformation and residual elastic stress pattern created in surface layer of MSMA-element nucleates and keeps in the volume of whole element the twin variant structure which corresponds to the twin variant structure that existed in the element during the surface treatment in step B, see
According to observations with mechanical testing device installed in optical microscope, the C-variant lamellas shown in
The stress-strain curves of
In
b shows the pseudoelastic parts of the stress strain curves of the same samples as in
The method of shot-peening of MSMA-element, in austenitic state (Hot Shot-Peening, HSP), is a fast one-step-process. In HSP-process/method a plastic deformation and/or residual stresses are introduced in surface layer of a MSMA-element by bombarding (shot-peening) the surface of MSMA-element in austenite state, by small particles. Such particles can be near-round, round or irregular in shape, soft or hard but preferably round and soft. The particles material can be metal, ceramic, glass, polymer, carbon, minerals, composite materials, semiconductor or refractory, organic. The bombardment media can also be a liquid (e.g. water).
The particles can be wet (e.g. with a forcible stream of water) or dry (e.g. with a forcible stream of air). An exemplary dry method is a surface bombardment with glass beads, and an exemplary wet method is surface bombardment mixture of water and an organic or inorganic powder, such as glass, metal or ceramic balls. According to a particular embodiment the surface bombardment is performed with a forcible stream of air and glass beads.
A remarkable new achievement here is that after the HSP-treatment the MSMA-element comprises of fine twin-variant structure.
A surprising aspect is also that even when the treatment is done on surface, it governs the density (fineness) of appearing twin variant structure in the whole volume of a MSMA-element. From the obtained repeatable fine twin variant structure it results that also the mechanical and magneto-mechanical properties of the material are stable/repeatable, which is clear improvement to the prior art MSMA-element properties [27].
Moreover, the mechanical and magneto-mechanical properties of MSMAs can be modified in a controlled way by the developed method to meet particular needs of the application.
The above introduced Twin-activating surface modification method will now be described in detailed steps. The important point is that the HSP-treatment of MSMA-element is done in austenitic state. In Ni—Mn—Ga MSMAs the crystal structure of austenite (y) is cubic, and has only one possible state without any twin variants, see
In this particular example of Twin-activating surface modification method the activation of the desired twin variant structure is realized by bombardment (shot-peening) of surface layer of MSMA-element by small particles, see
During the Twin-activating surface treatment (here bombardment) the MSMA-element is in austenitic state, and thus without any twins.
During the bombardment of surface layer of element in austenitic state by small particles, following process is thought to be involved on microscopic scale. Kinetic energy of the particles is supposed to be absorbed by the surface layer of the MSMA-element by plastic deformation (by dislocations), see
In materials science is also generally known that plastic deformation by dislocations creates additionally elastic stresses in deformed regions of material. Important result follows: (approximately)-homogenous (probably planar) plastic deformation/residual elastic stress imprint (can be done also non-homogenous by using some mask during bombardment, or by other means) is obtained in surface layer of the MSMA-element, see
In the following part the assumed mechanisms will be discussed by which the homogenous (probably planar) plastic deformation/residual elastic stress imprint introduced in material during Twin-activating surface modification activates a desired twin variant structure (in this example fine two-variant structure, FTVS).
When the Twin-activating surface modified MSMA-element is cooled down from austenite to martensite, the homogenous (probably planar) plastic deformation/residual elastic stress imprint HPD-RESI does not disappear. The HPD-RESI created in surface layer of MSMA-element nucleates and supports the “Fine two-variant structure” FTVS by apparently similar mechanism that is thought to be involved in CSP, see
Certainly, it is well known in the literature that after transformation from austenite to martensite the appearing twin variant structure is typically fine twin variant structure, but almost without exception it consists of all the possible twin variants, which can exist in that particular crystal structure which can be over 12 variants, see [50]. The remarkable achievement is that in MSMA-element after Twin-activating surface modification the twin variant structure consist prevalently (usually solely) of only two twin variants, see schematic picture of activated and stabilized FTVS (
The MSMA-elements after Twin-activating surface modification can be deformed by twinning to elongated single-variant or almost single-variant state (see
The second crucial feature of the MSMA-elements with Twin-activating surface modification is that even after deformation by twinning to compressed/elongated single-variant or almost single-variant state, after release of the external stress, the activated FTVS recovers back in the MSMA-element as in
Repeatable and preferably stable FTVS results in its turn in repeatable and preferably stable mechanical and magneto-mechanical properties of the MSMA-material, which could not be achieved in the art.
According to one embodiment, the present technology concerns a method of producing a magnetic shape memory element which method comprises a step of surface modification of the element.
According to another embodiment the treatment is carried out, preferably on a 3-dimensional object formed by the alloy, by mechanical, thermal or chemical processing or a combination thereof.
According to a fourth embodiment, the treatment is performed regularly to the surface of the alloy. According to another embodiment the surface modification is performed irregularly or randomly to the surface of the alloy. The various alternatives will be discussed below in more detail.
Examples of mechanical processes suitable for Twin-stabilizing surface modification are inscribing, impacting (e.g. shot peening, sand/ball blasting, bombardment by small particles, Laser shock peening, water peening, ultrasonic shot peeing, hammer or needle peening) and similar mechanical processes, wherein plastic deformation or residual elastic stresses are achieved in the surface layer of a MSMA-element. Typically, the extent of the mechanical processing is limited to a part of the surface of the object.
Examples of mechanical processes suitable for used in the embodiment entitled Twin-affecting surface modification are impacting (e.g. shot peening, sand or ball blasting, bombardment by small particles, Laser shock peening, water peening, ultrasonic shot peeing, hammer/needle peening), rolling, burnishing and similar mechanical processes wherein plastic deformation or residual elastic stresses are achieved in the surface layer of a MSMA-element. Typically, the extent of the mechanical processing is limited to a part of the surface of the object. Thus, in this embodiment, the mechanical processing is preferably applied to 1 to 95%, in particular 5 to 90%, advantageously to 10 to 80% of the surface.
The developed Twin-stabilizing surface modification method and Twin-activating surface modification method aim generally to plastic deformation or to introducing residual elastic stresses, or both, in the surface layer of MSMA-element. By the stress difference between the surface layer and the bulk of a MSMA-element it is possible to control the twin variant structure of the whole element and thus to control the mechanical and magneto-mechanical properties of the whole element.
Examples of thermal (twin stabilization or activation) processes comprise subjecting the surface of the object to a temperature of about 50 to 5000° C. preferably 500 to 1000° C. and possibly quenching the sample fast to cold temperature austenite state or directly to martensite state.
Examples of chemical (including metallurgical) processes include the binding (or formation) of material(s), e.g. metal, ceramic, glass, polymer, carbon, minerals, composite materials, semiconductor, refractory or organic to the surface (or surface layer) of the object by mechanical chemical and/or thermal processing.
The modification can be performed either on part of the surface, for example by masking, or on whole surface. In the first case, the modification can be performed on a limited number of sides of an element of 3-dimensional configuration. According to one embodiment the surface modification is carried out on at least one, for example on at least 2 or 3 surfaces of the object.
For elongated objects having a width, a height and a length, thus defining six surfaces, surface modification is in one embodiment carried out on opposite surfaces. Thus, in such objects two, four or six surfaces are treated.
Partial treatment of one surface can be carried out by patterning, i.e. by forming on the surface a pattern of stochastic or predetermined shape, the latter being represented by lines, strips and bands extending over the surface, or a part of the surface, of the object or element. The first alternative is being represented by impact marks or scratches scattered over the surface in an irregular fashion.
According to a particular embodiment of the partial treatment alternative, the surface modification is performed onto 1 to 100% of the surface of the alloy, preferably about 5 to 80% of the surface of the alloy.
According to one embodiment the alloy contains at least two of the metals nickel, manganese and gallium.
According to a particular embodiment the alloy is in the form of an elongated, three-dimensional object having a length, a width and a thickness, thus defining at least three surfaces, the smallest dimension of the object being greater than about 0.01 mm, for example greater than about 0.05 mm, in particular greater than 0.1 mm.
The samples can also be of round shape, they can comprise bars or they can be in the form of stripes, thin stripes, wires and spheres of various sizes, the smallest dimensions of said samples being typically the same as cited above for the elongated objects having a length a width and a thickness.
According one embodiment the present technology concerns an MSM device, selected from the group of actuators, sensor and energy harvesters, including an actuating element for producing motion and or force or some other function thereof, whereby the element consist of a magnetic shape memory material produced according to the invented technology.
Comparison of the magnetic shape memory materials produced according to prior art and according to present technology are shown in
An MSM alloy, comprising a rectangularly shaped element of an Ni, Mn, Ga alloy, according to the present technology was prepared by treating the surface of the prior art MSM element with mixture of air, water and sodium bicarbonate using mini sand-blaster. The whole surface on both active surfaces was treated with maximal air intensity (inlet to unit 6 atm) and 50% water intensity (inlet to unit 6 bar). The nozzle distance is 10 mm, jet spot size (diameter) on specimen was 2.5 mm and the process time was 60 s.
b shows MFIS (%) of MSM alloy specimen of the invented technology as a function of the strength of the magnetic field applied to it.
Thus, as pointed out in
Similar results are obtained by treating corresponding MSM objects, conventional in the art, with mixtures of air, water and sodium bicarbonate or fine glass beads using a mini sandblaster.
In the working examples, the nozzle distance has been at least 1 to 2 mm from the surface to allow for a spreading of the zone of impact. The stream has been directed preferably perpendicularly or with a deviation of 5 to 10 degrees at the most from the perpendicular direction. Thus an impact angle of 75 to 115 degrees, preferably about 80 to 100 degrees, in particular 85 to 95 degrees, advantageously about 90 degrees±2.5 degrees, against the treated surface, is preferred.
The area treated is considerably larger when compared to the smallest dimension of the twins in the alloy. In practice, the area is at least 2 times greater, in particular at least 5 times greater, preferably at least 1000 to 10000 times greater than the smallest dimension of the twins. The smallest dimension of the twin is usually positioned perpendicular to the impact direction of the treatment (explained above). In normal practice, when objects having smallest dimensions of about 0.1 to 10 mm are processed, the area of treatment is at least 1 mm2, preferably at least 5 mm2, in particular at least 10 mm2, advantageously at least 20 mm2, for example 20 to 1000 mm2.
Summarizing, in the present context, an object comprising a shape memory alloy, in particular a magnetic shape memory alloy, is subjected to a treatment, which affects primarily and preferably exclusively the surface layer of said object. Preferably, the surface layer or region affected by the treatment forms less than 20%, preferably less than 5%, advantageously less than 1% at the most of the total thickness of the object at the particular site of treatment.
By the treatment, plastic deformation, residual elastic stresses or combinations thereof are introduced into the surface layer or surface region. Particularly interesting objects are those exhibiting fine twins, or as a result, an object is produced which, when plotted in a stress-strain diagram will exhibit a slope of at least 0, preferably a slope greater than 0, and in particular a slope greater than 0 and monotonously increasing.
The present invention will provide objects of MSM alloys having stabile, repeatable and smooth mechanical and magneto-mechanical properties.
Preferably an object treated according to the present invention will have a graph, when plotted on a stress-strain diagram which exhibits less stochastic fluctuation in twinning stress than the corresponding non-treated object. In general, the fluctuation between repeated compression curves is less than ½, preferably less than ⅓, in particular less than ¼ of the magnitude of the twinning stress.
The following embodiments are specifically included in the present technology:
A method of producing an element of magnetic shape memory alloy comprising a microstructure of twin variants and having a surface, said method comprising the step of modifying the surface layer of the element to stabilize the microstructure of whole volume of the element and modify the magneto-mechanical properties of the whole element.
An embodiment as defined above, wherein the surface modification is carried out by mechanical, magnetic, thermal or chemical processing or a combination thereof.
An embodiment as defined above, wherein the surface modification is carried out by a method selected from the group of inscribing, radiating, impacting, local heat treatment, global heat treatment, binding additional material(s), injecting additional material(s) and combinations thereof.
An embodiment as defined above, wherein the surface modification is performed regularly, irregularly or randomly to the surface of the alloy. The modification is performed to at least a part of the whole of the surface. The surface modification is performed on stripes of the surface, on opposing surfaces or on combinations thereof.
In the above embodiments, the patterning is performed onto 1 to 100%, preferably 5 to 80%, of the surface of the alloy.
In any of the embodiments described above, the surface modification includes a surface deformation, for example a plastic deformation of the surface, or the surface modification includes the introduction of stress into the surface layer or the surface modification include a combination of both deformation and introduction of residual elastic stress.
In particular, the surface modification can be carried out by surface bombardment. The surface bombardment can be carried out by impacting a surface of the element with shot of soft or hard materials with a force sufficient to create plastic deformation and optionally using a pressurized gas. The surface bombardment can be carried out in dry state, e.g. using glass beads, or in wet state using a mixture of water and powder of an inorganic or organic material.
In relation to the definition of MSM alloy given above, it should here be noted that the alloy can be any suitable alloy. In one embodiment, the alloy is a composition of at least two of the elements included in the group of Ni, Mn, Ga, Cr, Co, Cu, Fe, Ti, Al, In, Zr, Mo, Ta, W, V, Nb, Y and Pd. Preferably the alloy contains at least two metals selected from nickel, manganese and gallium; in particular the alloys incorporate at least Ni, Mn and Ga. In another embodiment, the alloy is a composition of Ni, Mn and Ga, with an optional addition of at least one element selected from the group of Cr, Co, Cu, Fe, Ti, Al, In, Zr, Mo, Ta, W, V, Nb, Y and Pd. Typically, an additional element in addition to Ni, Mn and Ga can be present in amounts of 0.1 to 10 atom-%. Ferromagnetic martensitic alloy formed by the above constituent components are particularly interesting. As discussed, the alloy can, at least at some point of the process, be provided in austenite state.
The alloy can be in the form of an elongated, three-dimensional object having a length, a width and a thickness, thus defining at least three surfaces, the smallest dimension of the object being greater than 0.01 mm, preferably greater than 0.1 mm. For such an object, the surface treatment is carried out on at least one, for example on at least two or at least three surfaces of the object.
7. O. Heczko, A. Sozinov and K. Ullakko, IEEE Trans Magn 36 (2000) 3266.
Number | Date | Country | Kind |
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20115210 | Mar 2011 | FI | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/FI2012/050211 | 3/2/2012 | WO | 00 | 10/29/2013 |
Number | Date | Country | |
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61448213 | Mar 2011 | US |