Functional Stability of the Ni₅₁Ti₄₉ Two-Way Shape Memory Alloy as Artificial Anal Sphincter During Thermo-Mechanical Cycling

Ni₅₁Ti₄₉ alloy strip with optimal two-way shape memory effect to be potentially used in a purpose-designed artificial anal sphincter (AAS) was prepared by means of rapid solidification process followed by constraint-aging treatment. The functional stability in terms of phase transformation behavior and deformation performance during thermo-mechanical cycling (TMC) was studied. Results show that the forward and reverse R-phase transformation temperatures of the alloy remain in the required operation temperature range of 35–55 °C with small shifts during TMC. The alloy strip exhibits stable deformation performance with steady recovery ratio during TMC, and its microstructure after TMC remains featured fine and stable Ni₄Ti₃ precipitates together with limited number of dislocations, indicating that the external force and cycling temperature have no influence on the size of Ni₄Ti₃ precipitates and coherent stress field. Such excellent stability of microstructure and corresponding functionalities are attributed to the stabilized Ni₄Ti₃ precipitates formed through optimal constraint-aging treatment and the small lattice distortion of R-phase and reverse R-phase transformations during TMC. Nevertheless, the Ni₅₁Ti₄₉ alloy strip has the maximum displacement of at least 10 mm within 35–55 °C and an irreversible displacement of 4 mm.