Effects Of Limited Solubility Metals On Ti-Ni-Hf Shape Memory Alloy

The microstructure, martensitic transformation behavior, mechanical properties and shape memory effects of Ti-Ni-Hf-Sn, Ti-Ni-Hf-Ag alloys and Ti-Ni-Hf-Ag ribbon were investigated in this paper. And the influence of heat treatment on the Ti-Ni-Hf-Ag ribbon was also clarified.The results show that, Ti-Ni-Hf-Sn alloy consists of B19’martensite,(Ti,Hf)2Ni and Sn-containing compounds at room temperature. The B19’martensite variants show strip or mosaic morphologies and its substructure is(001) compound twins. The martensite variants are(011) type twins.While in addition to B19’martensite and(Ti,Hf)2Ni, Ag-containing compounds were also observed in Ti-Ni-Hf-Ag alloys and ribbon. The B19 ’martensite is in the shape of strip. The precipitated phase is of disperse distribution. The volume fraction and distribution of the precipitate and the grain size were affected by different annealing process.The results of DSC test show that, with the increase of Sn and Ag content, the martensite transformation temperatures of Ti-Ni-Hf alloys dercrease. During cooling, Ti-Ni-Hf based alloys and Ti-Ni-Hf-Ag ribbon undergo a two-stage B2-R- B19’transformation. The phase transformation temperature decrease with the annealing temperature increase or the annealing time extends. When the annealing temperature is lower than 700℃ or the annealing time is too short, there would be more than one transformation peaks in the DSC curves of the Ti-Ni-Hf-Ag ribbon, and that was attributed to the difference of grain size and the existence of the precipitates.Tensile and compression tests show that elongation of Ti-Ni-Hf-Sn alloys decreases with the increas of Sn content; After 7% deformation, when Sn content is 3at.%, the alloy shows the maximun recoverable strain of 6.25%. Elongation of Ti-Ni-Hf-Ag alloy decreases with the increase of Ag content decreased, while the recoverable strain shows the same variation tendency.For the Ti-Ni-Hf-Ag ribbon, the elongation of the as-spun ribbon decreases with increasing Ag content. After 6% deformation, when Ag content is 0.3at.%,the maximal recoverable strain is up to 4.25%. With the annealing temperature increases or the annealing time extends, the elongation of the ribbon decreases. After annealing at 500℃ for 40 min, the maximal recoverable strain is up to 4.10%.