Shape memory alloy and method for producing same

Abstract

A shape memory alloy based on Fe-Mn-Si contains (in wt.%): Mn: 15 - 20%, Si: 4 - 9%, Cr: 11 - 18%, Ni: 2.5 - 7%, V: 0.3 - 0.9%, C: 0.03 - 0.3%, N: 0.005 - 0.5%, where Cr + Si: 15 - 22%, as well as the optional elements Al: < 0.1%, P: < 0.1%, S: < 0.05%, Mo: < 3%, Co: < 1%, Nb: < 1%, rare earth metals in total < 0.3%, the remainder iron and unavoidable impurities.

Description

[0001] The invention relates to a shape memory alloy based on Fe-Mn-Si and a method for producing such a shape memory alloy based on Fe-Mn-Si.

[0002] Shape memory alloys are characterized by their shape memory effect (also known as SME). This effect is based on a reversible, diffusionless shearing movement of the metal lattice from γ-austenite (face-centered cubic, fcc) to metastable ε-martensite (hexagonal close-packed, hcp).

[0003] The original microstructure of an undeformed Fe-Mn-Si shape memory alloy (SMA) is γ-austenite. Deformation of the alloy can promote the transformation of γ-austenite to ε-martensite due to the stresses generated during deformation. This process is known as "martensitic transformation" or "forward martensitic phase transformation." After deformation, the stress-induced ε-martensite can revert to γ-austenite upon activation (e.g., by heating). This allows the original shape of an Fe-Mn-Si shape memory alloy to be restored to a certain extent. This process is also referred to as "austenitic transformation" or "reverse martensitic phase transformation."

[0004] In other words, SME is characterized by a transformation into a metastable ε-martensite induced by strain, as well as a reconversion to γ-austenite caused by heating. During the reconversion, a contraction occurs, which can build up stress in the shape memory alloy, provided it is mechanically fixed (whereby the mechanical fixation prevents the contraction).

[0005] In many applications of Fe-Mn-Si-based shape memory alloys, improved corrosion resistance is desirable. This is particularly true when the shape memory alloy is used, for example, as reinforcement or a structural element in construction (e.g., concrete structures). The increased corrosion resistance should not compromise the shape memory alloy's inherent properties.

[0006] Furthermore, an increase or at least no deterioration of the yield strength and thus the retransformation stress of the shape memory alloy is desired in order to achieve sufficiently high stresses in practical applications (e.g. concrete construction, bridge construction, etc.).

[0007] Another aspect – especially in applications in the construction sector – is that the shape memory alloy should have high strength and / or the lowest possible activation temperature to avoid damaging, for example, the concrete.

[0008] One of the problems addressed by the invention is to create a shape memory alloy with high corrosion resistance combined with high reverse transformation stress and strength. Furthermore, the invention aims to provide a method for producing a shape memory alloy with these properties.

[0009] The problem addressed by the invention is solved by the features of the independent claims. Exemplary embodiments and further developments are the subject of the dependent claims.

[0010] Accordingly, a shape memory alloy based on Fe-Mn-Si contains (in wt.%) Mn: 15 - 20%, Si: 4 - 9%, Cr: 11 - 18%, Ni: 2.5 - 7%, V: 0.3 - 0.9%, C: 0.03 - 0.3%, N: 0.005 - 0.5%, where Cr + Si: 15 - 22%, as well as the optional elements Al: < 0.1%, P: < 0.1%, S: < 0.05%, Mo: < 3%, Co: < 1%, Nb: < 1%, rare earth metals in total < 0.3%, the remainder being iron and unavoidable impurities.

[0011] Furthermore, a process for producing a shape memory alloy based on Fe-Mn-Si comprises the following steps: melting a metal melt containing (in wt.%) Mn: 15–20%, Si: 4–9%, Cr: 11–18%, Ni: 2.5–7%, V: 0.3–0.9%, C: 0.03–0.3%, N: 0.005–0.5%, where Cr + Si: 15–22%, as well as the optional elements Al: < 0.1%, P: < 0.1%, S: < 0.05%, Mo: < 3%, Co: < 1%, Nb: < 1%, rare earth metals in total < 0.3%, the remainder being iron and unavoidable impurities; casting the metal melt into a semi-finished product; Hot forming of the pre-product to a formed pre-product; and solution annealing of the formed pre-product at an annealing temperature between 1000°C and 1200°C for an annealing time of 0.5 to 10 hours.

[0012] The comparatively high chromium content results in high corrosion resistance for the shape memory alloy. The other alloying elements were adjusted to ensure that the shape memory alloy retains good shape memory properties. Furthermore, it has been found that the disclosed shape memory alloy can be produced with a high ferrite content, resulting in high strength.

[0013] The following describes the alloying elements and their effects. Percentage values ​​for alloying elements are to be understood as wt.%. Manganese (Mn)

[0014] Manganese (Mn) serves to stabilize γ-austenite (hereinafter referred to as austenite) in the microstructure and particularly influences the switching behavior (phase transition between austenite and ε-martensite) in shape memory alloys. Below a Mn content of 15%, the austenite-stabilizing effect is absent. A Mn content above 20% has adverse effects on martensite formation (unless otherwise specified, martensite in the following always refers to ε-martensite). Silicon (Si)

[0015] Silicon (Si) ensures the reversibility of the phase transformation from martensite to austenite. It also increases oxidation resistance. Below 4%, Si reduces the SME (synthetic mineralization). At silicon contents above 9%, embrittlement of the material can be observed.

[0016] Silicon reduces the stacking fault energy, thereby accelerating the transformation from austenite to martensite. Furthermore, silicon acts as a ferrite stabilizer. Chromium (Cr)

[0017] Cr increases corrosion resistance. In addition, Cr is a ferrite stabilizer.

[0018] To effectively increase corrosion resistance, a Cr content of at least 11%, preferably at least 12%, is required. A Cr content above 18% results in more than 50% ferrite being present in the microstructure. Since only austenite contributes to the SME (small mineral aggregation), an excessively high ferrite content is detrimental to the SME. Furthermore, Cr increases the stacking fault energy γSF (hereinafter referred to as SFE), which should be as low as possible to preferentially obtain martensite. If the SFE is too high, the SME disappears, as only twins then form in the austenite. Nickel (Ni)

[0019] Ni serves to stabilize the austenitic structure and also improves the formability of the shape memory alloy.

[0020] Ni contents above 7% have no positive effect on the material properties and are also avoided for cost reasons. At Ni contents above 7%, there is a risk that the austenite will be stabilized to such an extent that the martensitic phase transformation required for SME (small-scale metal alloying) is hindered and no longer occurs sufficiently during the necessary pre-deformation. Therefore, Ni contents of a maximum of 7% are preferred. Vanadium (V)

[0021] V has a higher affinity for C and N than Cr. V can therefore ensure that Cr remains dissolved in the microstructure, thus increasing corrosion resistance.

[0022] Another advantageous effect of V is that during aging treatment it forms fine vanadium carbonitrides V(C,N) (diameter < 10 µm) (so-called vanadium carbonitride precipitates), which are homogeneously distributed throughout the microstructure of the solid shape memory alloy. The vanadium carbonitrides V(C,N) result in an increase in strength.

[0023] This means that, in the presence of C > 0.1%, V enables the "cultivation" of V(C,N) precipitates during (optional) aging treatment. After aging, these precipitates are finely dispersed as V(C,N) particles within the microstructure of the shape memory alloy. These V(C,N) precipitates have a positive effect on the SME (seamless polymerization).

[0024] The minimum V content is therefore 0.3%. The maximum V content is 0.9%. At a V content greater than approximately 0.9%, unfavorable excretion kinetics with respect to V(C,N) excretion may occur. Carbon (C)

[0025] Carbon is an austenite stabilizer. Furthermore, carbon improves the SME (synthetic mineral properties). Therefore, a minimum carbon content of 0.03% was chosen.

[0026] In addition to its effect as an austenite stabilizer, C acts as an interstitial element and increases the strength of the alloy.

[0027] If the carbon content exceeds 1%, ductility begins to decrease significantly. Furthermore, an excessively high carbon content alters the transformation behavior and, during quenching hardening, leads to the formation of the martensitic α' phase (which, unlike the martensitic ε phase, is undesirable). Therefore, the maximum carbon content is 0.3%. Nitrogen (N)

[0028] Nitrogen (N) acts (like carbon) as an interstitial element and austenite stabilizer. Furthermore, N increases the alloy's strength. As with carbon, an excessively high N content alters the transformation behavior and, during quenching hardening, leads to the formation of the (undesirable) martensitic α'-phase. Therefore, the maximum N content is 0.5%. Molybdenum (Mo)

[0029] Mo reduces stacking fault energy and improves high-temperature oxidation resistance. At a molybdenum content of less than 0.1%, the effects are negligible. At a molybdenum content of more than 3%, the alloy's shape memory and hot-work properties deteriorate. Cobalt (Co)

[0030] Co-increases the SME (synthetic material strength) and hot-working properties of the alloy. At co-contents of less than 0.1%, the effects are negligible, while at co-contents of more than 1%, no further improvements are achieved. Niobium (Nb)

[0031] Nb (like V) has a higher affinity for carbon and nitrogen than Cr. Therefore, Nb can be added up to a maximum limit of 1%. Rare earth metals

[0032] Rare earth metals, especially Sc, Y, La or Ce, improve corrosion resistance and can be added in total up to, for example, 0.3%.

[0033] Examples and embodiments of the invention are explained in more detail below with reference to the drawings. Figure 1 The schematic representation shows an example of a process sequence (manufacturing route) for the production of a shape memory alloy. Figure 2 shows a representation of the phases of the microstructure of a shape memory alloy generated by electron backscatter diffraction (EBSD). Figure 3illustrates the dependence of martensite formation during pre-stretching (martensite transformation) on the stacking fault energy (SFE). Figure 4 shows stress-strain curves for three examples SMA_1, SMA_2, SMA_3 of shape memory alloys according to the disclosure and a reference shape memory alloy SMA_R during pre-strain (martensitic transformation). Figure 5 shows stress-temperature curves for three examples SMA_1, SMA_2, SMA_3 of shape memory alloys according to the disclosure and a reference shape memory alloy SMA_R during activation (austenite conversion) and subsequent cooling.

[0034] The following is based on Figure 1 The process steps described are examples and can be replaced or supplemented by other or similar process steps. In particular, further processes may be included between the process steps described below, which are not discussed in detail here.

[0035] The starting point for alloy production is a melting process 1, in which a molten metal is melted. This process can take place, for example, in an electric arc furnace (EAF), in an induction furnace (e.g., a vacuum induction degassing furnace VID or by VIM (vacuum induction melting)).

[0036] Reference numeral 2 denotes a secondary metallurgical treatment (secondary metallurgy). In this process, the alloy composition is adjusted within the specified ranges. For this purpose, an AOD (Argon Oxygen Decarburization) converter, a VOD (Vacuum Oxygen Decarburization) plant, and / or a ladle furnace (LF) can be used, for example.

[0037] The molten metal (shape memory alloy) is then cast into a semi-finished product at reference numeral 3. For example, ingot casting or bar casting can be used.

[0038] In a later process step (reference numeral 4), the semi-finished product undergoes hot forming to produce a formed semi-finished product. The formed semi-finished product can be, for example, a strip (hot-rolled strip), a sheet, a block, a bar, etc. The hot forming process 4 can include, for example, rough rolling 4.1 in a rough rolling mill and (optionally) further forming steps, such as multi-line rolling (at 4.2) in a multi-line rolling mill, or optionally, a forging treatment.

[0039] In the next step of the process, the formed semi-finished product is annealed at reference numeral 5. The annealing temperature can range between 1000°C and 1200°C. The annealing time can, for example, be from 0.5 hours to 10 hours.

[0040] Solution annealing (5) is performed to remove residual precipitates and pronounced textures from production. For this purpose, the formed pre-product is immersed in solution at a high temperature (e.g., equal to or greater than 1050°C) for a sufficiently long period of time (e.g., 3 hours or more). Solution annealing serves to homogenize concentration differences of the alloying elements through diffusion and to reduce microstructural inhomogeneities.

[0041] Solution annealing (5) allows for very precise control of the phase fractions, particularly the austenite-ferrite fractions. It has been shown that it is possible to achieve a (high) ferrite content, up to 45 vol.% of the microstructure. This additional ferritic phase increases the strength of the shape memory alloy.

[0042] Ferrite does not contribute to the SME. Surprisingly, however, it has been shown that even with a ferrite content of up to 45 vol%, a very good SME can still be achieved (due to the remaining austenite phase).

[0043] The phase composition in the microstructure depends on the annealing temperature and duration. The higher the annealing temperature and / or the longer the annealing duration, the more ferrite forms. Therefore, according to some embodiments, solution annealing 5 is carried out at a higher temperature and / or for a longer duration to create the desired austenite-ferrite composition of the shape memory alloy.

[0044] Since chromium is a ferrite former, ferrite can contain more chromium than austenite; that is, with prolonged solution annealing, the chromium content in ferrite should increase and the chromium content in austenite should decrease. However, it has been shown that despite solution annealing, chromium remains in both ferrite and austenite with a content > 11 wt%, and in particular > 12 wt%, to ensure the corrosion resistance of the shape memory alloy.

[0045] Solution annealing 5 can be terminated by cooling, e.g., by quenching (rapid temperature reduction). Quenching can be achieved, for example, by adding water. It is also referred to as quench hardening. Quenching prevents the formation of precipitates during the cooling process. Furthermore, quenching freezes the phase mixture present during solution annealing 5 (ferritic phase (bcc), austenitic phase (fcc), and, to a lesser extent, martensitic phase (hcp) and carbides).

[0046] Solution annealing 5 followed by cooling (e.g., quenching) can optionally be followed by an aging treatment 6 of the hot-rolled strip (or products, components, or samples manufactured from it). During this aging treatment, also known in engineering as "aging," V(C,N) can be selectively cultivated. V(C,N) precipitates have a positive effect on the SME (which only occurs in the austenitic phase). At the same time, V(C,N) has a strength-enhancing effect.

[0047] Another beneficial effect of V(C,N) is that it binds carbon and thus suppresses chromium carbide precipitates. Chromium carbide precipitates are undesirable because they bind chromium, which would then no longer be available for corrosion protection.

[0048] The aging treatment of the cooled, formed pre-product can be carried out at a temperature between 550°C and 800°C, particularly between 600°C and 700°C. Specifically, the aging treatment can also be performed at temperatures below 750°C, 700°C, 650°C, 640°C, or 630°C.

[0049] The duration of the aging treatment can range from 1 hour to 300 hours, specifically between 3, 4, 5, 8, 10, or 15 and 50 hours. The optional aging treatment ("aging") no longer alters the phase composition of the shape memory alloy.

[0050] It has been shown that during aging treatment at lower temperatures, the nucleation rate is higher, resulting in the formation of more finely dispersed precipitates (e.g., V(C,N) precipitates). However, the nucleation rate is lower, meaning more time is required to reach the ideal precipitate size. Therefore, favorable conditions for aging treatment can be created, for example, by using comparatively low temperatures and longer treatment times.

[0051] After solution annealing and cooling of the formed pre-product and (optional) aging treatment, the formed pre-product can be delivered to a customer, for example (at the dashed line in Figure 1The following process step 7 involves pre-stretching ("stretching") of the formed pre-product or a product manufactured from it. This process step 7 can be carried out at the customer's site or on-site during the installation of the shape memory alloy, e.g., in a component (e.g., a concrete part). The pre-stretching 7 causes the martensitic transformation.

[0052] The term shape memory alloy encompasses, in the following, the alloy itself as well as products made from this alloy, e.g. strips, sheets (plates), rods, beams, samples, etc.

[0053] One advantage of the in Figure 1 The illustrated manufacturing process consists in the fact that the shape memory alloy can be produced via a manufacturing route under atmosphere.

[0054] Table 1 summarizes the ranges of chemical composition of shape memory alloys according to the disclosure. In addition to the limit values, preferred and especially preferred range limits are given (oG: upper limit; Bev. oG: preferred upper limit; Sp_bev. oG: specially preferred upper limit; uG: lower limit; Bev. uG: preferred lower limit; Sp_bev. uG: specially preferred lower limit). All values ​​are given in wt.%. Table 1 (Alloy composition [wt.%]) Limit values C Si Mn Cr Ni V P S N oG 0,3 9 20 18 7 0,9 0,1 0,05 0,5 Bev. oG 0,25 8,5 19 16 6 0,7 0,1 0,05 0,4 Sp_bev. oG 0,21 8 18,5 15 5 0,5 0,1 0,05 0,2 Sp_bev. uG 0, 18 5 16 13 3,5 0,4 0,005 Bev. uG 0,07 4,5 15,5 12 3 0,35 0,005 uG 0,03 4 15 11 2,5 0,3 0,005 Table 1 (continued - alloy composition [wt.%)) Limit values Al Cr+Si Mon Co Note Rare earth metals oG 0,1 22 3 1 1 0,3 Bev. oG 0,07 21 1,5 0,5 0,5 0,2 Sp_bev. oG 0,05 20 0,2 0,2 0,2 0,05 Sp_bev. uG 17 Bev. uG 16 uG 15

[0055] The shape memory alloy (SMA) disclosed here is also referred to as duplex SMA, since it consists (almost entirely, see Table 2) of the α-phase (ferrite) and the γ-phase (austenite). Conventional shape memory alloys often consist practically only of the austenite phase. Surprisingly, it has been found that with the shape memory alloys according to the invention, a very good SME (due to the remaining austenite phase) is still achievable despite the high ferrite content.

[0056] Table 2 summarizes the ranges of phase fractions in the microstructure of shape memory alloys according to the disclosure. In addition to the limit values, preferred and especially preferred range limits are listed with the abbreviations explained in Table 1. The data refer to a shape memory alloy after heat treatment (solution annealing 5 and cooling (optional quenching)) or after the optional aging treatment 6 (which leaves the phase fractions unchanged). The measurements were performed using the EBSD analysis method. Electron backscatter diffraction (EBSD) is a measurement technique used to analyze the crystalline structure of a material. EBSD systems are used in scanning electron microscopes. Table 2 (Microstructure [Vol.-%]) Limit values Austenite (fcc) Ferrite (bcc) Martensite (hcp) Carbide oG 85 45 15 5 Bev. oG 80 40 10 5 Sp_bev. oG 75 40 5 3 Sp_bev. uG 52 20 - - Bev. uG 45 15 - - uG 35 10 - -

[0057] A shape memory alloy according to the disclosure can further be characterized by the fact that the Cr content in both the austenitic and ferritic phases can be, for example, more than 11 wt.%, and in particular more than 12 wt.%. This results in increased corrosion resistance of the shape memory alloy. The measurement is carried out, for example, by EDS point analysis in a scanning electron microscope (EDS: Energy Dispersive X-ray Spectroscopy). In this method, several points (e.g., 30 points; 15 points in the ferrite, 15 points in the austenite) are selectively measured and evaluated in the ferrite and austenite phases.

[0058] Furthermore, it is known that the SME is strongly dependent on the stacking fault energy. A comparatively low stacking fault energy (SFE < 20 mJ / m²) is required to preferentially obtain ε-martensite during deformation (process 7) of the shape memory alloy and thus achieve the SME. If the stacking fault energy SFE is too high, the SME does not occur, as only twins then form in the austenite (see Figure 3 ) .

[0059] The stacking fault energy (SFE) was calculated using Qi-Xun's formula. The calculation of the stacking fault energy (SFE) refers to the entire shape memory alloy, i.e., all phases of this alloy (where % denotes wt.%).

[0060] The formula shows that austenite-forming elements, such as Mn, C, and N, increase the stacking fault energy (SFE). Ferrite-forming elements, such as Cr and Si, decrease the stacking fault energy (SFE) empirically determined according to the equation above.

[0061] A shape memory alloy according to the disclosure can be characterized by having a stacking fault energy SFE (based on the entire alloy) e.g. in the range -40 mJ / m 2< < SFE < 40 mJ / m 2< , in particular -35 mJ / m 2< < SFE < 30 mJ / m 2< , or -25 mJ / m 2< < SFE < 20 mJ / m 2< .

[0062] For the shape memory alloys according to the disclosure, activation temperatures Tact < 300°C and, in particular, e.g., Tact < 200°C, Tact < 180°C or Tact < 170°C are achieved. Especially in applications in concrete construction, activation temperatures equal to or less than 200°C are required, and lower activation temperatures are advantageous. Examples

[0063] Table 3 shows examples of the alloy compositions of shape memory alloys SMA_1 to SMA_3 according to the disclosure and a reference alloy SMA_R. The reference alloy SMA_R is also an Fe-Mn-Si-based shape memory alloy, but the Cr content is too low for effective corrosion protection. In all cases, the residual content consists of iron and the unavoidable impurities, and optionally also of the aforementioned optional elements (whereby neither Al, Mo, Co, Nb nor rare earth metals were added in the examples). Table 3 (alloy compositions) (Examples) - SMA C Si Mn Cr Ni V P S N Al, Mo, Co, Nb SMA_1 0,11 5,97 16,82 12,16 4,09 0,46 0,005 0,0094 0,0095 - SMA_2 0,11 4,97 16,86 14, 18 4,08 0,47 <0,005 0,0081 0,0098 - SMA_3 0,11 6,00 16,81 13,07 4,20 0,46 0,005 0,0089 0,011 - SMA_R 0,18 5,00 17,00 10,00 4,00 0,65 0,005 0,0079 <0,05 - All values ​​are given in wt.%.

[0064] Figure 2An EBSD analysis of the microstructure of the shape memory alloy SMA_2 after hot forming is shown. It is evident that, in addition to the austenitic phase (fcc), a relatively high proportion of the ferritic phase (bcc) is present. The martensitic phase (hcp) is present only in very small amounts in the initial state of the shape memory alloy.

[0065] The in Figure 2 The microstructure shown is that of the shape memory alloy SMA_2 after solution annealing and quenching (process step 5).

[0066] The essentially two-phase structure ("duplex SMA") of the microstructure leads to high strength (due to the phase boundaries, which increase strength) of the alloy. Furthermore, the yield strength is increased, resulting in a higher reverse transformation stress Rr.

[0067] Table 4 shows the manufacturing parameters of alloys SMA_1, SMA_2, SMA_3, and SMA_R. The annealing treatment (solution annealing 5) was carried out for all alloys at 1070°C for a duration of 4 hours. The subsequent aging treatment following cooling (e.g., quenching) was performed at lower temperatures (equal to or below 660°C) for SMA_1, SMA_2, and SMA_3 than for SMA_R (700°C).

[0068] Furthermore, the aging treatment for SMA_1, SMA_2, SMA_3 was performed over a longer period (duration equal to or greater than 19 hours). Table 4 (Manufacturing parameters) SMA Solution annealing Aging treatment Temperature [°C] Duration [hrs] Temperature [°C] Duration [hrs] SMA_1 1070 4 600 19 SMA_2 1070 4 632 24 SMA_3 1070 4 660 24 SMA_R 1070 4 700 8

[0069] The specified manufacturing parameters for SMA_1, SMA_2, and SMA_3 are applicable to all shape memory alloys according to the disclosure. This means that the temperature of the aging treatment can generally be equal to, less than, or greater than 660°C, 632°C, or 600°C, and / or its duration can generally be equal to, less than, or greater than 19 hours or 24 hours. That is, all values ​​specified in Table 4 are also disclosed as upper or lower limits of ranges. In general, as already mentioned, the temperature of the aging treatment can, in particular, be equal to or less than, for example, 750°C, 700°C, 650°C, 640°C, or 630°C, and / or the duration of the aging treatment can, in particular, be equal to or greater than, for example, 3 hours, 4 hours, 5 hours, 8 hours, 10 hours, or 15 hours.

[0070] Table 5 summarizes the phase fractions and stacking fault energy of the alloy examples SMA_1, SMA_2, SMA_3 and the reference alloy SMA_R (after solution annealing and quenching). Table 5 (Microstructure) SMA bcc: ferritic fraction [vol.%] fcc: austenitic fraction + hcp (martensite) + carbides [Vol.-%] Stack fault energy SFE [mJ / m²<] SMA_1 24 rest -3,8 SMA_2 25 rest -0,1 SMA_3 37 rest -4,2 SMA_R 2 rest 15,9

[0071] SMA_R exists almost exclusively in the austenitic phase. SMA_1, SMA_2, and SMA_3 exhibit ferritic content between 24 and 37 vol.%. Furthermore, the stacking fault energy (SFE) (relative to the total alloy) is significantly lower for SMA_1, SMA_2, and SMA_3 than for SMA_R. When considering only the austenitic phase of SMA_1, SMA_2, and SMA_3, the stacking fault energy is similar to that of SMA_R.

[0072] In both the austenitic and ferritic phases, more than 12 wt% Cr was measured at SMA_1, SMA_2 and SMA_3 (using EDS point analysis).

[0073] The values ​​for SMA_1, SMA_2, SMA_3 given in Table 5 are applicable to all shape memory alloys according to the disclosure, i.e. the values ​​given for the phase fractions and / or stacking fault energies are also disclosed as upper or lower limits of ranges.

[0074] Due to the significantly higher ferritic phase content in the microstructure, shape memory alloys according to the disclosure (e.g., SMA_1, SMA_2, SMA_3) exhibit magnetic properties. Magnetic shape memory alloys can be advantageous in various applications.

[0075] The thermomechanical properties of the shape memory alloys SMA_1, SMA_2, SMA_3, and SMA_R were measured by pre-stretching and activation tests. The pre-stretching corresponds to process step 7 of the Figure 1 .

[0076] Figure 4Figure 1 shows the stress-strain curves measured during pre-stressing for the aforementioned shape memory alloys. All shape memory alloys underwent a 2% strain and were then fully unloaded. The non-linear stress-strain behavior of the shape memory alloys is attributable to martensitic transformation, plastic flow, or a combination of these two effects.

[0077] Figure 4 This illustrates that the shape memory alloys SMA_1, SMA_2, and SMA_3, as disclosed, achieved a higher yield strength Rp0.1 than the reference alloy SMA_R. The yield strength Rp0.1 was determined at 0.1% plastic strain (intersection of the stress-strain curves with the straight line that intersects the x-axis at 0.1% strain). The higher yield strengths Rp0.1 of the alloys SMA_1, SMA_2, and SMA_3 (see also Table 6) result in an increase in the retransformation stress compared to the reference alloy SMA_R.

[0078] After pre-stressing (martensitic transformation), the alloys SMA_1, SMA_2, SMA_3, SMA_R were activated, see Figure 5 Activation involves heating and cooling the alloys. During this process, the alloys are mechanically clamped (fixed), meaning they can neither expand nor contract.

[0079] In the example shown here, activation begins at approximately 23°C with a tensile stress of slightly over 100 MPa. As the temperature increases, the alloy initially expands, thereby reducing the tensile stress in the clamped state. An alloy without SME would continue to expand with further temperature increases, eventually transitioning into the compressive stress range (see dotted line). The SME, with the austenite transformation that begins at K (kink), causes contraction and thus prevents a pressure build-up as the alloy heats up further.

[0080] During the subsequent cooling of the shape memory alloy, it attempts to contract, but this is prevented by the confined state of the alloy (in practice, for example, it is embedded in a concrete component). This results in an increase in tensile stress. At the end of the cooling process (i.e., top left in the Figure 5 The stresses specified in Table 6 (so-called reverse transformation stress Rr) are generated. These stresses are used to strengthen structural components (e.g., concrete components) in which the shape memory alloy is incorporated. In practice, concrete slabs, beams, bridges, silos, etc., can be strengthened against loads in this way, thereby reducing the amount of concrete required for the respective structure or component. Table 6 (Mechanical parameters, measured during pre-stretch and activation) SMA Yield strength Rp0, 1 [MPa] Activation process Reconversion voltage Rr [MPa] Activation temperature T akt [°C] SMA_1 415 315 160 SMA_2 402 324 160 SMA_3 443 323 160 SMA_R 354 342 160

[0081] The pre-stretch ( Figure 4 ) and the activation attempts ( Figure 5 The tests were performed using a Zwick Z020 tensile testing machine equipped with a climate chamber. A mini-MFA 2 strain gauge with an accuracy class of 0.2 according to EN ISO 8513 was used to measure strain. The linearity error of the strain gauge, including hysteresis, was within 0.05%. During activation (with an activation temperature Takt of 160°C), the strain was monitored directly via the strain gauge, whose thermal expansion was compensated for by the testing machine after calibration. The measurements were performed on SMA specimens cut into a dog bone shape, measuring 32 mm in length and 1.6 mm in width.

[0082] The reverse conversion voltage Rr (see Figure 5The yield strength (Rr) of the shape memory alloys according to the disclosure can generally be greater than 300 MPa, in particular 320 MPa. Further tests showed that re-transformation stresses Rr > 400 MPa can be achieved. The yield strength Rp0.1 of the shape memory alloys according to the disclosure can generally be greater than 300 MPa, in particular 400 MPa or 430 MPa.

Claims

1. Shape memory alloy based on Fe-Mn-Si, containing (in wt.%): Me: 15 - 20%, If: 4 - 9%, Cr: 11 - 18%, From: 2,5 - 7%, V: 0,3 - 0,9%, C: 0,03 - 0,3%, N: 0,005 - 0,5%, where Cr + Si: 15 - 22%, as well as the optional elements Al: < 0,1%, P: < 0,1%, S: < 0,05%, For: < 3%, Company: < 1%, Note: < 1%, Rare earth metals in total: < 0.3%, the remainder being iron and unavoidable impurities.

2. Shape memory alloy according to claim 1, wherein Cr: 12 - 16%, in particular 13 - 15%.

3. Shape memory alloy according to claim 1 or 2, wherein Cr + Si: 16 - 21%, in particular 17 - 20%.

4. Shape memory alloy according to any of the preceding claims, wherein V: 0.35 - 0.7%, in particular 0.4 - 0.5%.

5. Shape memory alloy according to one of the preceding claims, comprising an austenite phase (in vol.%) between 35 and 85%, in particular 45 to 80% or 52 to 75% and / or a ferrite phase (in vol.%) between 15 and 45%, in particular 15 to 40% or 20 to 40%.

6. Shape memory alloy according to one of the preceding claims, wherein (in wt.%) more than 11% Cr, in particular more than 12% Cr, is present in an austenite phase of the shape memory alloy and / or more than 11% Cr, in particular more than 12% Cr, is present in a ferrite phase of the shape memory alloy.

7. Shape memory alloy according to one of the preceding claims, having a stacking fault energy (SFE) in the range of -40 mJ / m 2 < SFE < 40 mJ / m 2 , in particular -35 mJ / m 2 < SFE < 30 mJ / m 2 , or -25 mJ / m 2 < SFE < 20 mJ / m 2 exhibits.

8. Shape memory alloy according to one of the preceding claims, having a yield strength Rp0.1 > 300 MPa, in particular Rp0.1 > 400 MPa.

9. Shape memory alloy according to one of the preceding claims, having a reverse conversion voltage Rr > 300 MPa, in particular Rr > 320 MPa.

10. Shape memory alloy according to one of the preceding claims, wherein an activation temperature T akt the shape memory alloy T akt < 200°C, especially T akt < 170°C.

11. Method for producing a shape memory alloy based on Fe-Mn-Si, comprising the following steps: Melting a metal melt containing (in wt.%): Me: 15 - 20%, If: 4 - 9%, Cr: 11 - 18%, From: 2,5 - 7%, V: 0,3 - 0,9%, C: 0,03 - 0,3%, N: 0,005 - 0,5%, where Cr + Si: 15 - 22%, as well as the optional elements Al: < 0,1%, P: < 0,1%, S: < 0,05%, For: < 3%, Company: < 1%, Note: < 1%, Rare earth metals in total: < 0.3%, the remainder iron and unavoidable impurities; casting of the molten metal into a semi-finished product; hot forming of the semi-finished product into a formed semi-finished product; and solution annealing of the formed semi-finished product at an annealing temperature between 1000°C and 1200°C for an annealing time of 0.5 to 10 hours.

12. Method according to claim 11, wherein the annealing temperature is above 1050°C, in particular above 1070°C, for an annealing duration of over 3.0 hours, in particular over 3.5 hours or 4.0 hours.

13. Method according to claim 11 or 12, wherein the solution annealing is carried out such that the shape memory alloy has an austenite phase (in vol.%) between 35 and 85%, in particular 45 to 80% or 52 to 75% and / or a ferrite phase (in vol.%) between 15 and 45%, in particular 15 to 40% or 20 to 40%.

14. Method according to any one of claims 11 to 13, further comprising: cooling, in particular quenching, the formed pre-product after solution annealing; and aging treatment of the cooled formed pre-product at a temperature between 550°C and 800°C, in particular 600°C and 700°C.

15. Method according to claim 14, wherein the aging treatment is carried out for a period of time between 1 hour and 300 hours, in particular between 3 hours and 50 hours.

16. Method according to one of claims 14 or 15, wherein C > 0.1% and the aging treatment is carried out such that V(C,N) precipitates are produced.

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