Abstract:
Shape memory alloys containing Ni, Ti, and Pt or Pd display superelastic behavior. The superelasticity was shown in a range of temperatures around the austenite finish shape memory transformation temperature. The superelasticity range was approximately 210-280° C. for these alloys. Shape memory alloys that exhibit superelasticity at high temperatures will enable a new class of mechanical sensors and actuators.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/879,719, filed Sep. 19, 2013, which is incorporated herein by reference. 
     
    
     STATEMENT OF GOVERNMENT INTEREST 
       [0002]    This invention was made with Government support under contract no. DE-AC04-94AL85000 awarded by the U.S. Department of Energy to Sandia Corporation. The Government has certain rights in the invention. 
     
    
     FIELD OF THE INVENTION 
       [0003]    The present invention relates to shape memory alloys and, in particular, to nickel-titanium-based superelastic high-temperature shape memory alloys. 
       BACKGROUND OF THE INVENTION 
       [0004]    Shape memory alloys (SMAS) are alloys that can recover large deformations via a combination of heat and loading. Thermo-responsive SMAS can undergo deformation at low temperatures and then recover their original, pre-deformation shape upon heating above a transformation temperature. This phenomenon is termed the Shape Memory Effect (SME). Some SMAS also exhibit superelasticity in certain temperature ranges. Superelasticity (or pseudoelasticity) is the ability of an SMA to exhibit significant strains upon isothermal loading that are non-linear and fully recoverable upon unloading. This behavior in a metallic material has been used in applications such as eyeglass frames, kink-resistant antennae, and various medical and dental applications. 
         [0005]    SMAS can be deformed in a martensite phase at low temperature, assume an austenite phase at high temperature, and then spontaneously transform from the high temperature austenite to a low temperature martensite upon cooling. In general, the transformation is not completed immediately at a single temperature, but occurs gradually over a range of temperatures. Therefore, A s  and A f  are the temperatures at which the change to austenite starts and finishes, respectively, during heating. Similarly, M s  and M f  are the temperature at which the martensite transformation starts and finishes, respectively, during cooling. The transition from the austenite phase to the martensite phase is only dependent on temperature and stress, not time, as there is no diffusion involved with the phase transformations. The SME transformations display a thermal hysteresis during constant-load thermal cycling. If the shape returns to its original, preformed shape upon heating, the SMA is said to exhibit a one-way shape memory effect. Alternatively, an SMA may exhibit a two-way shape memory effect (TWSME) if under no load it remembers one shape at low temperatures and another shape at high temperatures. Both the SME and superelasticity of SMAS are the result of the reversible martensitic phase transformation mechanism. 
         [0006]    Transformation temperatures for current commercial SMAS limit practical uses to operating temperatures below about 100° C. Therefore, there is a need for high-temperature shape memory alloys (HTSMAs) that can enable higher temperature applications. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention is directed to HTSMAs of NiTi-based ternary alloys that exhibit superelasticity. The ternary alloys can contain Ni, Ti, and Pt, that are slightly Ti-rich (preferably greater than 50 and less than 51 atomic % Ti and, more preferably, about 50.5 atomic % Ti) with Pt concentrations from greater than 13 to less than 18 atomic % and, more preferably, 15.5 to 16.5 atomic %. 
         [0008]    The superelastic behavior is exhibited in a temperature range of between about 210-280° C., in the vicinity of the austenite-martensite transformation temperatures for these alloys. Alternatively, the ternary alloys can contain Ni, Ti, and Pd, with Pd concentrations from greater than 20 and to less than 30 atomic % and, more preferably, 25 to 26 atomic %. The superelastic behavior is exhibited in a temperature range of about 210-280° C. for these alloys. For both of these sets of ternary alloys, just above the martensite-to-austenite phase transformation temperatures (where the material is mainly austenite), when the sample is strained at a fixed temperature, it reaches a stress where stress-induced-martensite is produced. This transformation results in significant strain generation. When the stress is removed from the sample at the same temperature, a reverse reaction occurs and the material transforms back to austenite. This results in full recovery of the strain and in the hysteresis of the stress-strain response. The effect can be repeated with continued cycling. The alloys can be produced by arc-melting, but other techniques can also be used, such as vacuum induction melting. After melting and casting, the alloys can be subjected to a homogenization heat treatment, for example, at 1050° C. for 60 to 75 hours. The homogenization heat treatment smooths out the Pt or Pd composition within the microstructure and allows for the manifestation of the shape memory effect and superelasticity effect. 
         [0009]    The alloys can be useful for thermal safety devices, thermal switches, or actuators that are required to operate in the 150 to 250° C. range. For example, the alloy can be used as an SME flap or SME switch element. In these applications, a bent SME alloy can return to a flat shape during heating to close an aperture, or to switch to a different contact position in a thermal switch. 
         [0010]    Furthermore, due to their superelasticity, the alloy can continue to recover its flat shape at the high temperatures even when deformed to relatively large strains. These alloys show the SME effect under no-load conditions (unbiased SME) and also display useful shape memory behavior under load (biased SME). Thus, the alloys can perform useful work in actuator applications. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The detailed description will refer to the following drawings, wherein like elements are referred to by like numbers. 
           [0012]      FIG. 1(   a ) is a graph of the martensite start transformation temperature M s  as a function of Pt composition for a Ni 50-x Pt x Ti 50  ternary alloys.  FIG. 1(   b ) is a graph of the transformation temperature in the range of 150-250° C. for Pt compositions in the range of about 13 to 18 atomic %. 
           [0013]      FIG. 2  is a DSC plot of the transformation temperatures for the sample composition Ni 33.0 Pt 16.5 Ti 50.5  (Pt165). The heating ramp rate was 10° C./min. 
           [0014]      FIG. 3(   a ) is a scanning electron micrograph of a sample that was pulled to failure at room temperature, after which the grip section was imaged.  FIG. 3(   b ) shows elemental maps obtained from energy-dispersive x-ray spectroscopy of the sample. 
           [0015]      FIG. 4(   a ) through  FIG. 4(   e ) show graphs of the observed superelasticity on Pt165 at temperatures:  FIG. 4(   a ) 224° C.,  FIG. 4(   b ) 236° C.,  FIG. 4(   c ) 248° C.,  FIG. 4(   d ) 257° C., and  FIG. 4(   e ) 270° C. 
           [0016]      FIG. 5(   a ) through  FIG. 5(   f ) show graphs of the observed superelasticity on Ni 24.5 Pd 25.0 Ti 50.5  (Pd250) at temperatures:  FIG. 5(   a ) 213° C.,  FIG. 5(   b ) 219° C.,  FIG. 5(   c ) 224° C.,  FIG. 5(   d ) 229° C.,  FIG. 5(   e ) 234° C., and  FIG. 5(   f ) 241° C.  FIG. 6(   a ) through  FIG. 6(   d ) show graphs of stabilized superelastic loops at each test temperature for  FIG. 6(   a ) Pt165, FIG. 6(   c ) Pd250, and corresponding stress-induced transformation rate  FIG. 6(   b ) Pt165,  FIG. 6(   d ) Pd250. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    The properties of shape memory alloys, such as binary Ni-Ti (nitinol), are very sensitive to composition. See J. Ma et al.,  Intl. Mat. Rev.  55, 257 (2010); S. Shimizu et al.,  Mater. Lett.  34, 23 (1998); and D. Abu Judom et al., U.S. Pat. No. 5,114,504. Especially on the Ni-rich side of stoichiometry, the martensite/austenite phase transformation temperature decreases abruptly when Ni is increased only a few tenths of a weight percent. See W. Tang,  Met. Mat. Trans. A  28A, 537 (1997). In ternary HTSMAs containing Pt, Pd, or Hf, this compositional sensitivity can cause even greater shifts in transformation temperature—potentially hundreds of degrees. See Shimizu et al.,  Mater. Lett.  34, 23 (1998); and D. AbuJudom et al., U.S. Pat. No. 5,114,504. In particular, substitutional replacement of Ni with Pt and/or Pd in NiTi can increase the shape memory transformation temperature.  FIG. 1(   a ) is a graph of the martensite start temperature M s  as a function of Pt composition for a Ni 50-x Pt x Ti 50  ternary alloy, based on literature reports. As shown in  FIG. 1(   b ), a transformation temperature in the range of 150-250° C. can be achieved with Pt composition in the range of about 13 to 18 atomic %. 
         [0018]    As an example of the present invention, a targeted series of Pt-rich NiTiPt ternary alloys having Pt compositions of 15.5-16.5 atomic % were fabricated and examined in detail. The ternary alloys were processed via arc melting, followed by homogenization. For example, a 50.5 atomic % Ti alloy can be homogenized at 1050° C. for 60 hours. Electro-discharge machining (EDM) was used to make mini-coupons for tensile testing. Likewise, a targeted series of Pd-rich NiTiPd ternary alloys having Pd compositions of 25-26 atomic % were fabricated and examined in detail. 
         [0019]    Transformation temperatures can be directly measured using differential scanning calorimetry (DSC).  FIG. 2  is a DSC plot of the transformation temperatures for the sample composition Ni 33.0 Pt 16.5 Ti 50.5 . The forward transformation from martensite to austenite occurs during heating at A s =222° C. and A f =231° C. The reverse transformation occurs at M s =219° C. and M f =207° C. The DSC traces stabilize in 2-3 thermal cycles. While DSC provides a good measure of phase transformation temperature, it is only indirect evidence of alloy composition. Scanning electron microscopy and transmission electron microscopy can be used to determine whether fine-scale precipitates exist.  FIG. 3(   a ) shows a scanning electron micrograph of a sample that was pulled to failure at room temperature, after which the grip section was imaged.  FIG. 3(   b ) shows elemental maps obtained from energy-dispersive x-ray spectroscopy (EDS) of the pulled sample. The dark precipitates are Ti-rich, likely Ti 2 (Ni, Pt), and the light precipitates are Ti-rich, slightly Pt-rich, and Ni-lean relative to the matrix. The presence of the Ti-rich precipitates confirm that the alloy composition is Ti-rich, which agrees with the indirect DSC data. 
         [0020]    Thermal cycling experiments were conducted to determine if the material exhibits large tensile strain recovery at high temperatures via stressing through transformation (i.e., superelasticity). For these experiments, Ni 33.0 Pt 16.5 Ti 50.5  and Ni 24.5 Pd 25.0 Ti 50.5  samples were heated to at least 40° C. above the A f  temperature at zero stress, cooled to a test temperature, held at this test temperature, pulled to a 2.5% tensile strain at an effective strain rate of 2×10 −4  per second, and unloaded to zero stress. This cycle was repeated four times for each test temperature. For Ni 33.0 Pt 16.5 Ti 50.5  (Pt165), the martensite start temperature is M s =220° C. and the austenite finish temperature was A f =234° C. For Ni 24.5 Pd 25.0 Ti 50.5  (Pd250), M s =195° C. and A f =212° C.  FIGS. 4(   a )-( e ) and  5 ( a )-( f ) show the isothermal stress-strain graphs for test temperatures increasing from 213° C. to 270° C. For all test temperatures, the hysteresis loops stabilized after less than five stress-strain cycles. As can be seen, superelasticity was observed from 219° C. to 270° C. between the compositions. 
         [0021]      FIGS. 6(   a ) and ( c ) are graphs of the stabilized stress-strain curves for each test temperature. Superelasticity with characteristic flag-shape hysteresis is observed. The austenite-martensite transformation is indicated by the kink in each isothermal stress-strain curve during loading. Full recovery upon unloading was observed even up to about 3.3% strain.  FIGS. 6(   b ) and ( d ) are graphs of the transformation stress as a function of the test temperature. The transformation stress increases linearly with temperature. The transformation stress slope was about 4.8 MPa/° C. for the Pt alloy and 4.0 MPa/° C. for the Pd alloy. Superelasticity was observed for slightly Ti-rich NiTiPt ternary alloys with 
         [0022]    Pt concentrations from greater than 13 to less than 18 atomic %. The superelastic behavior is exhibited in the vicinity of the martensite start temperatures of approximately 150-250° C. for these alloys. Superelasticity was also observed for slightly Ti-rich NiTiPd ternary alloys with Pd concentrations from greater than 20 to less than 30 atomic %. The superelastic behavior is exhibited in the vicinity of martensite start temperatures of approximately  120 - 230 ° C. for these alloys. Both ternary alloys show superelasticity with characteristic flag-shape hysteresis in their stress-strain behavior above the austenite-martensite transformation, with full recovery up to about 3.3% strain for the Pt alloy and up to about 2.2% strain for the Pd alloy. 
         [0023]    The present invention has been described as to NiTi-based superelastic high-temperature shape memory alloys. It will be understood that the above description is merely illustrative of the applications of the principles of the present invention, the scope of which is to be determined by the claims viewed in light of the specification. Other variants and modifications of the invention will be apparent to those of skill in the art.