Study On High Strength Of Low Temperature Shape Memory Alloys

Shape memory alloys of exhibiting superelasticity at cryogenic temperatures,over wide temperature windows and with high superelastic stress have potential application background in aerospace,for example,Moon buggys or Mars probes require shape memory alloys to be serviced from-190℃to+60℃.However,existing shape memory alloys are difficult to meet this demand.In this paper,a variety of NiTi-based shape memoy alloys and its composites of wide temperature range at cryogenic temperature were created by controlling composition design,using melting,forging,wire drawing and annealing（traditional metallurgical methods）.The microstructures of NiTi-shape memory alloys and their mechanical properties at different temperatures were studied,which exhibiting high superelastic stress at cryogenic temperatures over wide temperature range.The main research contents and results are as follows:A series of nanocrystalline low-temperature NiTi-based shape memory alloys（NiTi Fe₃,NiTi Cr₃,NiTiNb Fe,NiTi Fe₁Mo₂,NiTi Fe₂Mo₁,NiTi Fe₁Cr₂,NiTi Fe Cr Mo,NiTiNb₃Fe₄,etc.）were created by melting,forging,wire drawing and annealing through alloying element doping（such as Fe,Cr,Mo,Nb,Co,etc.）.Their low temperature range and superelastic stress are superior to the shape memory alloys ever reported.Moreover,superelastic stress as a function of temperature shows a“V”curve,which deviates from thermodynamic Clausius-Clapeyron equation.It is preliminarily believed that high superelastic stress originates from elemental doping and nanocrystalline strengthening,and the wide temperature range is derived from the low martensitic transformation temperature.In order to further broaden the temperature range of the superelastic properties,the design concept is proposed by using the coupling between phase transforming component and high stress phase transforming component to realize dual phase synergy.NiTi/Ni₄₆Ti₄₇Nb₃Fe₄ laminated composite plate was obtained by vacuum hot-pressing and forging,and NiTi/high superelastic stress Ni₄₆Ti₄₇Nb₃Fe₄ dual phase transforming component composite was created by wire drawing and annealing subsequently.It is found that the composite exhibites high superelastic stress over a wide temperature range（-196℃–+180℃）,which are far superior to those of previously reported shape memory alloys.Aiming at the problem that the nanocrystalline metals lose yield stress due to premature plastic instability or fracture because of lacking of microscopic uniform deformation mechanism,the design idea is proposed by using the collective atomic shear characteristics of phase transforming component to provide microscopic uniform deformation mechanism for nanocrystalline metal to improve the strength of metal nanocrystals.Moreover,based on the previous research results,Fe and Nb were selected to reduce the phase transition temperature of NiTi and make it not easy to phase change.By vacuum hot-pressing,forging,wire drawing and annealing,we fabricated phase transforming NiTi/nanocrystalline Ni₄₃Ti₄₇Nb₃Fe₇ composite wire.In-situ synchrotron radiation high-energy X-ray diffraction experiment confirmes that under the effect of martensitic transformation（lattice shear）of NiTi component,the nanocrystalline Ni₄₃Ti₄₇Nb₃Fe₇ component can exhibit lattice strain（elastic strain limit）as high as 3.4%,which is 2-3 times the elastic strain limit of nanocrystalline metals reported in the past,conforming the design idea of large elastic strain/strength of nanocrystalline metals achieved by the phase transforming component.For the challenge that the nanowires cannot reproduce superelastic strain in the dislocation slip matrix（referred to as“the valley of death”）,and based on the previous research results,Fe and Nb were selected to reduce the phase transition temperature of NiTi,in which NiTiNb Fe matrix mainly exhibited dislocation slip（a small amount of phase transition）under external loading,Nb nanowires were used to compose the composite material.In situ tensile synchrotron radiation high energy XRD detection shows that the Nb nanowires can exhibit an elastic strain of up to 5%during the NiTiNb Fe matrix dislocation slip（phase transformation is only 7%）,thus crossing“the valley of death”.The strength（2230 MPa）and plasticity（17%）of the composite are better than previous literature reports,which may provide new idea for the development of high strength and high plasticity materials.