Fabrication Of Rapidly Solidified Ni₅₁Ti₄₉ Two-way Memoryalloy For Artificial Sphincter And Its Functional Fatiguebehavior

The NiTi alloy with two-way shape memory effect（TWSME）,which enables the repetitive deformation of the alloy between high-temperature and low-temperature shapes,is expected to be used to make artificial anal sphincter（AAS）for treating fecal incontinence.According to the human body environment,the NiTi alloy must have enough deformation strain and recovery ratio in a narrow temperature range（35–55°C）so as to realize the“open”and“close”functions of the rectum.Further,the alloy has to possess excellent functional stability,enabling long-term and reliable use of the AAS.This study aims at developing the NiTi alloy most suitable for making AAS,by using advanced fabrication method and optimized training process.The functional fatigue behavior of the NiTi alloy was studied to evaluate its functional stability,and sophisticated microanalysis approaches were used to reveal the correlation and interaction between microstructures and properties of the alloy.Rapid solidification technology through vacuum suction casting was utilized to fabricate Ni₅₁Ti₄₉ alloy strip.The high cooling rate during rapid solidification enables the as-fabricated alloy strip to have fine columnar grains（radial diameter³.9μm）with high solid solubility,little segregation and strong fiber texture of<001>_B2 parallel to thickness direction.Such fine-grained and strongly textured microstructure increases the nucleation,limits the orientation and restricts the growth of Ni₄Ti₃ precipitates.Massive fine（with average size of³06.7 nm）,well-aligned and dispersively distributed（with average inter-particle distance of¹66.5 nm）Ni₄Ti₃ precipitates introduce strong coherent stress field along the direction of constraint stress,leading to large deformation and high recovery ratio of the alloy during phase transformation.Besides,homogeneous composition and massive interfaces in the B2matrix promote the simultaneity of local phase transformation,which embodies the narrow temperature ranges of phase transformations.As a consequence,the rapidly solidified Ni₅₁Ti₄₉alloy has superior TWSME to the conventional cast one.Afterwards,the TWSME of the alloy was obtained through constraint-aging treatment,and the coupling effect of aging parameters,such as aging temperature,time and constraint stress,on the phase transformation and deformation behavior of the Ni₅₁Ti₄₉ alloy was studied.Results show that with increasing aging temperature and time,the phase transformation path evolves from one-step（B2?R）to two-step（B2?R?B19’）.Meanwhile,the R-phase transformation temperature and recovery ratio increase at first and then decease.Hence,the rapidly solidified Ni₅₁Ti₄₉ alloy followed by constraint-aging at 400°C for 100 h exhibits an optimal TWSME.With a high recovery ratio of 92.9%and a pre-designed transformation temperature range of 35–55°C,the so-obtained Ni₅₁Ti₄₉ alloy has great potential to make the AAS.The optimal TWSME results from the nano-scaled（with average size of¹31.4 nm）and dispersively distributed（with average inter-particle distance of⁵7.9 nm）Ni₄Ti₃precipitates,which contribute to a significant improvement of coherent stress field.Furthermore,an abnormal TWSME induced by low-temperature aging（approximately below 350°C）was found in the present thesis work,which refers to the situation where the deformation direction of the Ni₅₁Ti₄₉ alloy strip is toward the constraining position on cooling.With increasing aging temperature and time,the abnormal TWSME decays and gradually changes to normal TWSME.From the existence of R-phase and the increase of R-phase transformation temperature with aging temperature/time,it can be deduced that the abnormal TWSME is controlled by residual stress rather than coherent stress.Thus,the transition from abnormal to normal TWSME can be attributed to the increase of coherent stress field caused by the growth of Ni₄Ti₃ precipitates.Finally,the functional stability of the fabricated Ni₅₁Ti₄₉ alloy was evaluated using a self-designed fatigue testing system.Results show that the alloy exhibits excellent stability in phase transformation and deformation behavior during thermal and thermo-mechanical cycling for 30000 cycles.After thermal cycling,the fluctuations of forward and reverse R-phase transformations are 1.7°C and 2.4°C,respectively,together with small displacement degradation（1.2%）.Comparatively,the Ni₅₁Ti₄₉ alloy exhibits slightly higher displacement degradation（6.7%）during thermo-mechanical cycling,while the fluctuation of R-phase transformation temperature is still very small（1.2°C）.Such a robust functional stability stems from the comprehensive effect of cycling temperature range,phase transformation type and microstructure.The Ni₄Ti₃ precipitates were already stabilized during the constraint-aging treatment,whose further growth can not be activated during thermal cycling between 30–90°C or thermo-mechanical cycling within 25–80°C.The stable and nano-sized Ni₄Ti₃precipitates in the fine-grained Ni₅₁Ti₄₉ alloy not only ensure the existence of steady coherent stress field,but also effectively prevent generation and multiplication of dislocations during alternate forward and reverse R-phase transformations.Nevertheless,the Ni₅₁Ti₄₉ alloy strip functions stably in the temperature range of 35–55°C with enough displacement（>10 mm）during thermal cycling,the output of strip is insufficient during thermo-mechanical cycling.