Shape memory alloy with enhanced properties

Abstract

The invention provides a shape memory alloy (SMA) material with a microstructure having a both a fibre crystallographic texture and a stabiliser homogenously distributed in the whole volume of the SMA, the microstructure providing enhanced SMA properties wherein, in response to a cyclic load associated with a phase transformation in the SMA, the load comprising a force applied in or close to a [001] cubic crystallographic orientation, the microstructure is stable over many cycles with enhanced SMA properties.

Description

[0001] Title

[0002] Shape Memory Alloy with Enhanced Properties

[0003] Field

[0004] The present disclosure relates to a Shape Memory Alloy (SMA), and in particular to a Nickel Titanium (NiTi) SMA material with a novel structure and microstructure to deliver enhanced properties.

[0005] Background

[0006] SMAs are alloys which have unusual characteristics. SMAs can reversibly change their crystal structure at temperatures close to, but not limited to, room temperature. This phase transformation occurs from a symmetric crystal structure stable at higher temperature and under critical mechanical loading called austenite to a phase with lower crystal symmetry stable at lower temperature and / or under sufficient mechanical loading called martensite.

[0007] If deformed, they can return to a “remembered” shape when heated. These effects known as “shape memory” come about due to a change of their crystal structure and shape by a sufficient external force below a critical temperature but recover their original state and shape after being heated above the critical temperature. The critical temperature in this case means the temperature at which the martensitic phase starts to transform to the austenitic phase. Such SMAs materials are also known as memory metals, memory alloys, smart metals, smart alloys, and muscle wires. This characteristic is used in many applications, such as actuation and robotics.

[0008] When the material is in the high temperature phase, application of sufficient mechanical loads causes a phase transformation of the symmetric high- temperature phase austenite into a martensitic phase with lower symmetry. This phase change is accompanied by shape change (for example, the wire is significantly elongated) and the material releases heat; when the load is released, a reverse shape change occurs (for example, the wire is shortened to its original length) and the heat is absorbed during the reverse phase transformation into the austenitic phase. By heat in this case is meant the latent heat of phase transformation and this effect is known as “superelasticity”. The transduction of mechanical energy to significant heat is known as the elastocaloric effect. Here SMA can be used in thermal applications such as heating or cooling in buildings. In most cases, the basic, i.e., initial austenitic shape of SMAs materials is imprinted by its thermomechanical processing and / or industrial forming.

[0009] Nickel titanium (NiTi) family of alloys is a common SMA material used in many applications. This family of alloys contain nickel, titanium usually in a 1 :1 atom ratio or close to this and potentially one or more further elements (usually first or second row transition metals, although other elements can be included) usually at lower concentrations usually below 15%. Li et al (Materials Science & Engineering A 705 (2017) 273-281 ) describes a NiTi alloy strip with an all-round shape memory effect which is suitable for use as an actuator. Precipitates form in a few directions under an applied stress during constrained annealing, resulting in an inhomogeneous precipitate variant distribution.

[0010] Other alloy compositions, including but not limited to, copper-aluminium-nickel, copper-zinc-aluminium or iron-manganese-silicon can be used to form SMA components. Practical limitations such as small usable strain, low actuation frequency, low controllability, low accuracy, limited fatigue life, limited operating temperature range and low energy efficiency have impeded practical application of the material in some commercial products. A problem to be addressed is consistency, so that the SMA material maintains its properties when changing states over many cycles. Enhanced material properties would increase the commercial applicability of the material.

[0011] In recent years SMA materials have been proposed to be used in a wide variety of industries and applications, for example heat pump applications, energy recovery devices, HVAC applications, refrigeration systems etc.

[0012] SMA materials have attractive properties as a replacement for vapour compression / refrigerants, for example, in the heating and cooling of building spaces and water, but in using those properties the material available today also presents drawbacks. These drawbacks are: a relatively large hysteresis and nonlocalised deformation in compression, which prevent a wide super-elastic window and reduce efficiency; a relatively low tensile and compressive strength which limit the functional and structural fatigue life particularly under high loads; poor functional stability requiring material training and causing gradual reduction of latent heat and adiabatic temperature span over the working life of material, which limit the range of applications to which the SMA material can be applied.

[0013] Summary

[0014] It is an object of the invention, as defined in the appended claims, to overcome at least one of the above-referenced drawbacks. It is another object of the present invention to provide a novel combination of structural and microstructural features to deliver enhanced properties over prior art.

[0015] In accordance with an aspect of the invention there is provided a shape memory alloy (SMA) material with a microstructure, having both a fibre crystallographic texture and a stabiliser which is homogenously distributed in the whole volume of the SMA, wherein in response to a cyclic load associated with a phase transformation in the SMA, the load comprising a force applied in or close to a

[0001] cubic crystallographic orientation, the microstructure is stable over many, or a plurality of, cycles with said enhanced SMA properties.

[0016] The SMA material of the present invention comprises enhanced properties compared to known polycrystalline SMA materials. The enhanced properties provide higher stable latent heat output, narrower hysteresis, ultra-low degradation and stable behaviour during thermomechanical cycling and larger superelastic window than known materials..

[0017] This delivers higher performance with increased power density and higher efficiency, long fatigue life (more than three million complete superelastic cycles), stable performance output over life with no requirement for material training, and larger superelastic window potentially negating the need for cascaded systems in many applications. The SMA material of the present invention solves the problem of an SMA material maintaining its properties when changing phase states over many cycles in response to a load applied to the material. This is highly desirable. Enhanced material properties are achieved which allows the SMA material to be used reliably in heat pump applications, energy recovery devices, HVAC applications, refrigeration systems etc.

[0018] In one embodiment the microstructure comprises ordered columnar grains, the columnar grains are orientated substantially parallel with adjacent columnar grains, and the columnar grains are axially orientated substantially in a <001 > cubic crystallographic orientation.

[0019] In one embodiment over 50% of the columnar grains are substantially aligned in a crystallographic orientation with angle dispersion defined by a solid angle of 0.03 sr to provide a strong fibre crystallographic texture.

[0020] In one embodiment the columnar grains are arranged substantially in parallel and / or series with respect to each other.

[0021] In one embodiment over 50% of the columnar grains are >1 mm in height.

[0022] In one embodiment the homogenous distribution of the stabiliser across the whole volume mitigates the formation and propagation of microstructural faults.

[0023] In one embodiment the stabiliser comprises precipitates.

[0024] In one embodiment the stabiliser comprises coherent precipitates.

[0025] In one embodiment wherein the stabiliser is orientated substantially in {111} cubic crystallographic planes.

[0026] In one embodiment the stabiliser comprises one or a combination of the following: Ni4Ti3; Ni3Ti2; or Ni3Ti. In one embodiment the enhanced properties provide a highly localised compressive stress-strain curve response.

[0027] In one embodiment the enhanced properties can exhibit the Luders-like deformation response in compression with a localised transformation strain of 4% or greater.

[0028] In one embodiment the enhanced properties provide a narrower hysteresis response relative to the known response of a known SMA, when the SMA material changes state during loading at constant temperature or when undergoing a temperature change at constant loading.

[0029] In one embodiment a stable structural and / or functional response in compression relative to the known response of a conventional SMA is achieved from a first transformation cycle up to 1 million cycles and beyond. In other words, many cycles can be achieved with out degrading the properties of the SMA.

[0030] In one embodiment a stable functional cyclic response in compression relative to the known response of a conventional SMA is achieved with uniaxial loading stresses up to 2 GPa in absolute value.

[0031] In one embodiment the SMA material may be selected from the following alloys: Nickel-titanium, copper-aluminium-nickel, copper-zinc-aluminium or iron- manganese-silicon.

[0032] In one embodiment there is provided a shape memory alloy (SMA) material with a microstructure which comprises ordered columns, wherein the columns are orientated substantially in one direction and the columns are axially orientated substantially in a <001 > cubic crystallographic orientation. In accordance with a second aspect of the invention there is provided an elastocaloric device which comprises an SMA material as defined with reference to any of the appended claims.

[0033] Brief Description of the Drawings

[0034] The invention will be more clearly understood from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which:-

[0035] Figure 1 is an illustration of a polycrystalline precursor SMA material for use in creation of an example of an SMA material in accordance with the present invention;

[0036] Figure 2a is an illustration of an example of an SMA material in accordance with the present invention;

[0037] Figure 2b is an illustration of a 3D perspective view of Figure 2a and a magnified view of the SMA material;

[0038] Figure 3 is an image of an SMA material with a microstructure which shows the ordered columns and a magnified view of the SMA material in accordance with the present invention;

[0039] Figure 4 is an illustration of the inhomogeneities which form a stabiliser in accordance with at least one example of the present invention;

[0040] Figure 5a is graph which plots stress v strain for a known SMA material and figure 5b is graph which plots stress v strain for an example of an SMA material in accordance with the present invention;

[0041] Figure 6 is graph which plots stress v strain for another example of an SMA material in accordance with the present invention;

[0042] Figure 7a is the strain distribution at the surface of a NiTi SMA sample with enhanced properties loaded in compression;

[0043] Figure 7b is the thermogram of the surface of a segmented NiTi SMA sample with enhanced properties loaded in compression;

[0044] Figure 8 is the {001} pole figure obtained on the basis of EBSD data registered from the surface of a NiTi SMA with enhanced properties; Figure 9 is a flow diagram which shows a process for making a material in accordance with the present invention and subsequent testing and potential applications of the material;

[0045] Figure 10 is a schematic diagram which shows an example of a setup for isothermal compressive loading of a stack of three cylindrical samples at various temperatures;

[0046] Figure 11 is a graph which plots engineering stress Vs Strain at a test temperature of 20.2 °C. It illustrates methods used for determining stress hysteresis and the transformation tangent modulus;

[0047] Figure 12 shows an ingot made of a material in accordance with the present invention; and

[0048] Figure 13 is a graph which shows compressive stress-strain with a nonstable functional cyclic response.

[0049] Detailed Description of the Drawings

[0050] In at least one embodiment, the present invention provides a shape memory alloy (SMA) material comprising an SMA with a novel microstructure, as described herein with reference to the following description and / or drawings. In a preferred embodiment, the SMA is Nickel Titanium (NiTi) alloy.

[0051] In at least one example, the invention provides an SMA material comprising a nickel titanium alloy with a microstructure. The microstructure exhibits all of the following features: fibre crystallographic texture; a stabiliser homogenously distributed in the whole volume of the SMA material; the microstructure is stable over many cycles; enhanced properties in response to a compressive cyclic load applied in, or close to, the <001 > cubic crystallographic orientations.

[0052] The micro structure comprises ordered columnar grains, positioned such that the columnar grains are orientated substantially parallel with adjacent columnar grains.. The columnar grains are preferably axially orientated substantially in <001 > cubic crystallographic orientations. The structure of the material is stable with enhanced properties in response to a compressive load applied over many cycles to the material. Enhanced material properties are achieved which allows the SMA material to be used reliably in heat pump applications, energy recovery devices, HVAC applications, refrigeration systems etc.

[0053] The present invention comprises a material with a combination of several characteristics which provide improved SMA properties compared to previously reported SMAs, and particularly, improved elastocaloric properties such as, for example, large latent heat, narrow stress hysteresis, low transformation stress, fatigue resistance, mechanical cycle stability, moderate Clausius-Clapeyron, tuneable transformation temperature, higher thermal conductivity and corrosion resistance.

[0054] The application of force applied in or close to a <001 > cubic crystallographic orientation to the material which has <001 >CUbic fibre texture and homogeneously distributed stabilizer creates the enhanced properties of the material. In the present application, homogenously distributed means homogeneous in both volume of SMA and also in the crystallographic structure.

[0055] Without wishing to be bound by theory, it is the inventors’ understanding that the stabiliser blocks formation and movement of dislocations during cyclic loading and that transformation in columnar grains oriented in the sample thickness direction is less sensitive to lattice parameter variations.

[0056] The inventors further note that the material of the present invention may be created with fewer grain boundaries. The grain boundaries may be substantially normal to the loading direction. Loaded grain boundaries can create some plastic deformation. Generally, grain boundary volume fraction decreases transforming volume and thus, related latent heat (grain boundaries generally do not transform). Therefore, long grains of >1 mm, as provided for in examples of the present invention, are better because grain boundary volume fraction is lower compared to microstructure with shorter grains. The columnar grains may have a length which is less than sample thickness. Therefore, one column can consist of several grains which may be interlocking and parallel with grains in any other column.

[0057] The present invention combines a microstructure with <001 >CUbic fibre texture with homogenously distributed stabiliser. This combination shows enhanced properties such as a unique thermomechanical response when mechanically loaded at different temperatures in compression along the strongest <001 > crystallographic texture.

[0058] Figure 1 shows a NiTi polycrystal 1 consisting of crystallographically randomly oriented microstructure grains 3. The polycrystalline material is a precursor material for an SMA material of the present invention. The raw material elements can also be used as precursors.

[0059] Figure 2a is an image of a NiTi SMA material 1 1 in accordance with the present invention. The preferred orientation of grains in a polycrystalline material is referred to as crystallographic texture. There are two broad categories of textures, commonly known as sheet and fibre textures. A fibre crystallographic texture can be defined by the definition of the crystallographic direction aligned parallel to the fibre axis. In this example, the SMA material 11 is characterized by columnar structures 13 predominantly oriented and / or aligned in one direction with a defined boundary 15 between one or more adjacent columnar grains. In addition, the columnar grains show a similar crystallographic orientation in the axial direction. In this and other examples, a columnar grain structure may be created with grains which are >1 pm in length / height and in which approximately >80% of grains are aligned in the same direction, though not limited. Advantageously, the SMA material of the present invention does not need to be significantly trained to achieve stable functional response. The columnar structures may also be aligned in series, one column arranged on top of another column to define a stable structure. Figure 2b is a 3D perspective image view of Figure 2a and a magnified view of the SMA material. The columns are axially orientated substantially with <001 > cubic crystallographic orientations as visualised by the cuboid structures in the magnified view of Fig 2b. The columns are solid columnar grains and can be viewed as ordered columns such that the columns are arranged in a substantially parallel and / or series arrangement. In reality, the columns can form interlocking structures that build on each other. The connection of the individual columns is realised by a transition region called the boundary of the column grains. Suitably, at least 50% of the columns are aligned in the same direction and / or crystallographic orientation.

[0060] A feature of the SMA material of the present invention is a homogenous distribution of the stabiliser across the whole volume which mitigates the formation and propagation of microstructural faults. The stabiliser is orientated substantially in {111 } cubic crystallographic planes. It is thought that the stabiliser blocks formation and movement of dislocations during cyclic loading and that transformation in columnar grains oriented in the sample thickness orientation is less sensitive to lattice parameter variations.

[0061] The axial orientation of the columns substantially in <001 > cubic crystallographic orientations and the orientation of the stabiliser substantially in {11 1} cubic crystallographic planes provide enhanced properties in the SMA material and enhanced performance in various applications.

[0062] A suitable example of a precipitate is Ni4Tis of lenticular shape with a central plane parallel to the {11 1} austenite planes.

[0063] Figure 3 is an image of an SMA material with a microstructure, according to a preferred embodiment of the present invention, clearly showing the ordered columns and a magnified view of the SMA material in accordance with the present invention. The sample shown is an ingot of NiTi alloy material weighing approximately 325 grams. The columnar nature of the material is clearly visible to the naked eye. Figure 3 also shows a 10mm measurement scale across the ingot which further illustrates the length of the columnar grains in figure 3 as being of comparable length to the 10mm measurement scale.

[0064] In this and other non-limiting examples of the present invention, the SMA material has a latent heat stabilised to >17 J / g. The full transformation latent heat is >17 J / g fora binary NiTi variant across a large temperature range when measured via isothermal cycles.

[0065] A maximum variation of material strain of <1 % over a 60 °C isothermal sweep with identical loading parameters is sufficient to induce a full material phase change from austenite to martensite. In addition, higher activation temperatures in a binary NiTi alloy of up to 110 °C can be achieved and an activation energy of potentially < 800 MPa for a full transformation in an isothermal condition.

[0066] The SMA material may be a known type of SMA material such as nickel-titanium, copper-aluminium-nickel, copper-zinc-aluminium or iron-manganese-silicon including NiTi X or NiTi XY where X and Y are metal elements, usually transition metal elements, with concentrations <15 at. %.

[0067] Figure 4 illustrates an example of a stabiliser 21 which comprises inhomogeneities in the SMA material. The stabiliser 21 is homogenously distributed in the volume of the SMA material. The even distribution of the stabiliser across the whole volume contributes to a stable highly localised compressive stress-strain response. The stabiliser is preferably orientated substantially in {111 } cubic crystallographic planes. In this example, the stabiliser is Ni4Tis of lenticular shape with a central plane parallel to the {111} austenite planes. The formation of the precipitate causes inhomogeneities in the SMA material. As seen in figure 4, the precipitate is substantially homogeneously spread across the volume of the SMA material. Figure 5a also illustrates the progressive delocalization of the stress-strain response, leading to an increase in the transformation tangent modulus with each subsequent cycle, reaching a value of 20 GPa by the 1000th loading cycle. Figure 5a is a graph 31 which plots compressive stress 33 v strain 35 for a conventional, prior art, NiTi SMA material. Three families of curves 37, 39 and 41 are shown, representing training cycles (loading 43 and unloading 45) at 20°C: the first 100 cycles, the second 100 cycles, and the final 800 cycles respectively. Due to plastic deformation, redistribution of internal stresses in the microstructure and overall instability of the system, the functional superelastic response of the SMA material is degraded, thus usable latent heat released / absorbed during mechanical loading / unloading after the application of stabilization cycles is reduced.

[0068] The characteristics of the known NiTi SMA material after the stabilization cycles are as follows:

[0069] - This material shows low transformation strain and low latent heat output (8-12 J / g).

[0070] - Non-localized transformation increases the load required to release the available latent heat over a wider range of temperatures.

[0071] - Small superelastic window.

[0072] Figure 5b is a graph 51 which plots stress 53 v strain 55 for an example of an SMA material in accordance with the present invention. Ten curves are shown which are identified in the key 57 from the top of the graph to the bottom as being stress / strain measurements at temperatures ranging from 4.5 °C to 49.9 °C. Details are provided in the following table. The SMA material shows a stable functional response from the first cycle. No significant plastic deformation is observed. Functional degradation is small compared to the standard SMA material.

[0073] The characteristics of the above example of an SMA material with enhanced material properties in accordance with the invention after stabilization cycles are as follows:

[0074] - Highly localised transformation strain of about 4% or greater and high latent heat of 17-18 J / g for this particular example.

[0075] - Narrow isothermal hysteresis results in reduced work input requirement and high efficiency.

[0076] - Large superelastic window.

[0077] - The localized transformation enables the use of superelasticity in a large temperature range greater than 80°C whereas limiting the maximum loading stress to 1000 MPa.

[0078] - The material coefficient of performance reaches values higher than 30.

[0079] Figure 6 is a graph 61 which plots stress 63 v strain 65 for another example of an SMA in accordance with the present invention. In this example, SMA material of the present invention comprised a ministack 6mm in length with diameter 24mm weighing approximately 3.88g. The material was tested for 1 ,250,000 training cycles at 20 °C to check the stress-strain response. The narrow hysteresis curve 67 indicates a highly efficient performance after long term utilisation. As described above the enhanced properties provide a highly localised compressive stress-strain curve response. The enhanced properties provide a narrower hysteresis response relative to the known response of a conventional NiTi SMA, when the SMA material changes state during loading at constant temperature or when undergoing a temperature change at constant loading.

[0080] The NiTi SMA with enhanced properties exhibits a Luders-like deformation response in compression. When the material undergoes a phase change, compressive strain is not homogeneously distributed along the sample but rather is localised to regions with lower and higher strains separated by shear-bands.

[0081] Figure 7a is the strain distribution at the surface of a NiTi SMA sample with enhanced properties loaded in compression and Figure 7b is the thermogram of the surface of a segmented NiTi SMA sample with enhanced properties loaded in compression. The figures provide an example of Luders band deformation of NiTi SMA in compression with enhanced properties measured by DIC (Digital Image Correlation).

[0082] Figure 7a shows a NiTi sample deformed by compression loading. The upper part of the sample shows lower strain in comparison to the bottom part. The interphase between these two regions is clear and represents so called Luders band. Figure 7b 73 shows Luders band on another segmented sample loaded in compression observed by a thermal camera.

[0083] Figure 8 is the {001} pole figure obtained on the basis of EBSD data registered from the surface of a NiTi SMA with enhanced properties. Fibre texture can be observed in the {001} pole figure. Figure 8 shows the {001} pole figure where X1 , Y1 and Z1 are the Cartesian axes firmly connected, or associated, with the NiTi SMA sample with enhanced properties. The distribution of the {001} planes at the surface of the NiTi SMA sample is characterized by the low dispersion angle which indicates a strong <001 >CUbic fibre texture. The legend gives the frequency of occurrence of {001 } planes for a given direction. It will be further appreciated that the SMA NiTi material structure can be applied to any type of material that exhibits SMA properties.

[0084] Figure 9 is a flow diagram which shows a process for making a material in accordance with the present invention and subsequent testing and potential applications of the material.

[0085] Selection of chemical composition 93

[0086] In the case of Ni-Ti alloys containing 40-60 at% Ni, the following composition is selected as an example of material preparation featuring the microstructure and stabilizer in accordance with the present invention and described in Figs. 2a, 3, 4. The composition is 51.3 at% Ni, with the remainder being Ti and a negligible amount of impurities. The elemental weights are adjusted based on the desired final weight of the ingot.

[0087] Selection of melting techniques 95

[0088] The objective is to achieve a homogeneous melt with thoroughly mixed elements, followed by solidification into an alloy with columnar grains. This may be achieved by applying a temperature gradient in melt to induce solidification of columnar grains of austenite with a B2 crystal structure, where the {001} planes are oriented along the columnar direction.

[0089] Alloy melting and solidification 97

[0090] An ingot was produced using material from step 93 and the method selected in step 95. Figure 3 shows the microstructure of the prepared ingot, where the grains are predominantly aligned in parallel. As the majority of crystals do not grow continuously from bottom to top, interconnected grains can be observed within individual columns. Due to the random crystallographic orientations in the x and y directions (lateral directions), and the consistent alignment of {001} planes along the z direction (the sample thickness direction), a <001 >CUbic fibre texture has been achieved as shown in pole figure, Fig. 8.

[0091] Processing step for forming stabilizer 99 The prepared ingot from step 97 was taken for step 99. In the case of the Ni-Ti alloy with 51.3 at% Ni, the stabilizing phase consists of Ni4Ti3precipitates. The processing step was performed using a process in which Ni4Ti3-precipitation is formed during aging of NiTi shape memory to achieve a homogeneous distribution of Ni4Ti3precipitates in a Ni 50.8 at% - Ti alloy.

[0092] A homogeneous distribution of Ni4Ti3precipitates is characterized by: (a) a uniform spatial distribution throughout the volume of the shape memory alloy, meaning that the number density of precipitates at the grain boundaries is equal to that within the grain interiors; and (b) a uniform distribution of crystallographic orientations of the precipitates within each grain.

[0093] The Ni4Ti3precipitates exhibit a lenticular shape, with their central planes lying in the {111 }B2 planes. Within a single grain, four crystallographic orientation variants of the precipitates are possible. By a homogeneous distribution of crystallographic orientations, we refer to an equal occurrence of all four variants within each grain.

[0094] Figure 4 presents a micrograph of the Ni4Ti3precipitates in our sample, demonstrating their homogeneous spatial and orientation distribution.

[0095] Sample cutting 101

[0096] The ingot is cut into hollow cylindrical samples with a length of 8 mm 143, an outer diameter of 6 mm 145, and an inner diameter of 3 mm 147 (Figure 12).

[0097] Sample testing under compression 103

[0098] Isothermal loading at various temperatures was performed as closest as possible to the recommendations of the ASTM E9 standard on a stack of from two to three cylindrical samples as prepared in step 101 .

[0099] The experimental setup for three cylinders is shown in Figure 10. Figure 10 is a schematic diagram 111 which shows an example of a setup for isothermal compressive loading of a stack of three cylindrical samples at various temperatures. This setup may also be used for fatigue test at 1 Hz.

[0100] Figure 10 shows a load cell 113. Jaws 115 are located in the ends of a tube 117. The tube 117 contains three SMA tubes 1 19 of a material in accordance with the present invention. Water flow inlet 121 is positioned at one end of the tube 1 17 , with corresponding outlet 126 at the opposite end and an extensometer 123 which measures changes in the length of the SMA tubes 119. O-rings 125 provide a seal between the tube 1 17 and jaws 115. Force 127 is applied to the first and second jaws 115.

[0101] The latent heat was measured using both a differential scanning calorimeter and an internal device capable of determining latent heat under compressive stress. The work density was calculated as the area enclosed by the hysteresis loop in the stress-strain diagram.

[0102] Figure 11 is a graph 131 which plots engineering stress 133 Vs Strain 135 at a test temperature of 20.2 °C. It illustrates methods used for determining stress hysteresis and the transformation tangent modulus.

[0103] Figure 11 illustrates the measurement of stress hysteresis. Points A1 , A2, B2, and B1 represent the onsets and ends of the reverse and forward stress-induced martensitic transformations, respectively. These points were determined using the tangent method. The rectangular region k (indicated by red dashed lines) is defined such that its centre of gravity coincides with that of the quadrilateral A1A2B2B1 (point T). The dimensions of region k are specified as follows: the horizontal length is (11 +12)74, and the vertical length is chosen arbitrarily, provided that ^ encompasses both plateau segments.

[0104] Stress hysteresis is defined as the length of segment C1-C2, where C1 and C2 are the intersection points of the vertical axis passing through the centre of gravity of region k with the loading and unloading branches of the stress-strain curve. A narrow compressive stress hysteresis response as mentioned in claim 13 is defined as that measured at room temperature below 150 MPa

[0105] Figure 11 also demonstrates the localized nature of the compressive stressstrain response. The term “highly localized compressive stress-strain response” refers to the segment of the curve where the martensitic transformation occurs. This region is characterized by a low transformation tangent modulus ranging from 0 to 5 GPa. As shown in Figure 11 , the two moduli, corresponding to loading and unloading, are determined as the slopes of the regression lines fitted to the respective segments of the stress-strain curves within region k. If the values of the two moduli are in the range from 0 to 5 GPa, we say that the compressive stress - strain response is highly localized.

[0106] Cyclic compressive loading 105

[0107] Cyclic compressive loading, encompassing the complete forward and reverse stress-induced transformations, was performed for a total of 1.25 million loading cycles. The loading frequency was set to 1 Hz, with a stress range from 0 to - 1000 MPa. To maintain an approximately constant test temperature, a water flow at 20°C was applied at a rate of 300ml / min. The cyclic compressive loading applied to the samples exhibits a stable functional cyclic response in compression.

[0108] A stable functional cyclic response in compression relative to the known response of a conventional SMA may be achieved with uniaxial loading stresses up to 2 GPa in absolute value using nickel-titanium, copper-aluminium-nickel, copper- zinc-aluminium or iron- manganese-silicon.

[0109] Figure 13 is based on Figure 5a and presents an example of a non-stable functional cyclic response in a commercial Ni-50.8 at.% Ti alloy under compression. Figure 13 is a graph 151 which shows compressive stress 153 - strain 155 with a non-stable functional cyclic response. The material is a commercial Ni 50.8 at. % - Ti alloy. In figure 13, the stress strain cyclic response for the first training cycle 159 is compared with the stress strain cyclic response for the 1000thtraining cycle 157 under loading 163 and unloading 165.

[0110] After 1 ,000 loading cycles, the permanent deformation is 1 .87%, and it continues to increase with each subsequent mechanical cycle. In contrast, the example sample from Step 107 exhibits a permanent deformation of less than 0.5% after 1.25 million compressive cycles.

[0111] A stable functional cyclic response is defined as one in which the permanent deformation remains below 0.5 % after 1 million compressive loading cycles, including the complete stress-induced forward and reverse martensitic transformations. A consequence of the stable functional cyclic response is conservation of latent heat in each cycle generated / absorbed on the complete forward / reverse martensitic transformation.

[0112] Sample testing under compression 107

[0113] Isothermal loading, applied in the same manner as in Step 103, revealed a reduced stress hysteresis and an increased transformation tangent modulus (3.2 GPa). Nevertheless, according to our definition outlined in Step 103, the compressive stress-strain response remained localized even after 1.25 million loading cycles.

[0114] Possible component production 109

[0115] The production of NiTi components for industrial applications utilizing the enhanced properties of the prepared NiTi.

[0116] The following is a non-limiting list of SMA alloy materials that can be used to bring the invention into effect, for example:

[0117] The SMA material of the present invention may be used in a variety of applications, including, but not limited to:

[0118] HVAC-R systems for the heating and cooling of building spaces and water, and refrigeration;

[0119] Any other provision of heating, cooling and refrigeration;

[0120] Mechanical control devices in many industrial and consumer applications.

[0121] The field of actuation technologies. Here SMA-based actuation can replace electrical, pneumatic or hydraulic systems. SMA-based actuation can be more compact, lower cost and more energy efficient. Automotive actuators were one of the first high volume applications for this technology. Smartphone cameras are an application where there has been significant recent development.

[0122] Other applications including vibration damping, medical devices e.g. dental drills, implants, archwires, etc. and hermetic joints, spectacle frames, aerospace e.g. variable area fan nozzle, As for actuation, the invented material provides localised actuation response in compression.

[0123] Vibration damping can be used in compression up to very high stresses (2GPa) repeatedly.

[0124] Spectacle / eye glass frames were a common application using superelasticity.

[0125] Robotics and prosthetics would benefit from low hysteresis to give better response times and reduce energy consumption.

[0126] Heat to power benefits from high localised transformation strains and generated high stresses in compression.

[0127] The skilled reader will appreciate that the SMA material of the present invention, such as Nickel Titanium (NiTi) will display significant advantages with respect to known materials.

[0128] When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components. The invention may also broadly consist in the parts, elements, steps, examples and / or features referred to or indicated in the specification individually or collectively in any and all combinations of two or more said parts, elements, steps, examples and / or features. In particular, one or more features in any of the embodiments described herein may be combined with one or more features from any other embodiment(s) described herein.

[0129] Protection may be sought for any features disclosed in any one or more published documents referenced herein in combination with the present disclosure. Although certain example embodiments of the invention have been described, the scope of the appended claims is not intended to be limited solely to these embodiments. The claims are to be construed literally, purposively, and / or to encompass equivalents.

Claims

Claims1. A shape memory alloy (SMA) material with a microstructure having both a <001 >cubic fibre crystallographic texture and a stabiliser which is homogenously distributed in the whole volume of the SMA, the microstructure providing enhanced SMA properties wherein, in response to a cyclic load associated with a phase transformation in the SMA, the load comprising a force applied in or close to a <001 > cubic crystallographic orientation, the micro structure is stable over many cycles with said enhanced SMA properties.

2. The SMA material as claimed in claim 1 wherein the microstructure comprises ordered columnargrains, the columnargrains are orientated substantially parallel with adjacent columnar grains, and the columnar grains are axially orientated substantially in a <001 > cubic crystallographic orientation.

3. The SMA material as claimed in claim 2 wherein >50% of the columnar grains are substantially aligned in a crystallographic orientation with angle dispersion defined by a solid angle of 0.03 sr to provide a strong fibre crystallographic texture.

4. The SMA material as claimed in claim 2 or 3 wherein the columnar grains are arranged substantially in parallel and / or series with respect to each other.

5. The SMA material as claimed in any of claims 2 to 4 wherein >50% of the columnar grains are >1 mm in height.

6. The SMA material as claimed in any preceding claim wherein the homogenous distribution of the stabiliser across the whole volume mitigates against the formation and propagation of microstructural faults.

7. The SMA material as claimed in any preceding claim wherein the stabiliser comprises precipitates.

8. The SMA material as claimed in any preceding claim wherein the stabiliser comprises coherent precipitates.

9. The SMA material as claimed in any preceding claim wherein the stabiliser is orientated substantially in {111} cubic crystallographic planes.

10. The SMA material as claimed in any preceding claim wherein the stabiliser comprises Ni4Tis.

11. The SMA material as claimed in any preceding claim wherein the enhanced properties provide a highly localised compressive stressstrain curve response.

12. The SMA material as claimed in any preceding claim wherein the enhanced properties can exhibit the Luders-like deformation response in compression with a localised transformation strain of 4% or greater.

13. The SMA material as claimed in any preceding claim wherein the enhanced properties provide a narrower hysteresis response relative to the known response of a conventional SMA, when the SMA material changes state during loading at constant temperature or when undergoing a temperature change at constant loading.

14. The SMA material as claimed in any preceding claim wherein a stable structural and / or functional response in compression relative to the known response of a conventional SMA is achieved from a first transformation cycle up to 1 million cycles and beyond.

15. The SMA material as claimed in any preceding claim wherein a stable functional cyclic response in compression relative to the known response of a conventional SMA is achieved with uniaxial loading stresses up to 2 GPa in absolute value.

16. The SMA material as claimed in any preceding claim wherein, the SMA is selected from the following alloys: Nickel-titanium, copper- aluminium-nickel, copper-zinc-aluminium or iron- manganese-silicon, but not limited to the SMA list.

17. An elastocaloric device which comprises an SMA material as claimed in claims 1 to 16

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