Design And Preparation Of Two-Way NiTi Shape Memory Alloy Used For Artiifcial Anal Sphincter

This thesis work aims at fabrication of a kind of two-way NiTi shape memory alloy forapplication of the substitution material for human body’s dysfunctional natural anal sphincter.In this work, the striped NiTi alloy of the tailored composition was prepared with pure nickeland titanium via vacuum arc melting and rapid solidification through suction casting,followed by constraint aging treatment. Preparation parameters were carefully optimized toobtain the NiTi alloy stripes for making the artificial anal sphincter, which keep straight format room temperature and turn to bending spontaneously with a large enough radian uponheating, and exhibit a narrow temperature range of phase transformations and small hysteresis.Phase constitutent, microstructure and phase transformation behavior of the as-prepared NiTialloy were investigated systematically via phase analysis (XRD), microstructure analysis(SEM) and thermal analysis (DSC). Furthermore, the shape memory effects of the NiTi alloyat different temperatures were quantitatively characterized and the optimized preparationparameters were thus obtained. Finally, two NiTi alloy stripes after the same processingtreatment were assembled by hinges to consist of an artificial anal shipincter device. Thedeformation behavior of the as-designed device was studied under electrical loading of directcurrent circuit.The present study shows that the surface layer of the as-casted NiTi specimen consists offine equiaxed grains, while the cross-sectional part mainly exhibits twin columnar grains.XRD analysis results indicate that the major phase in the as-homogenized specimens is NiTiof B2structure with a small amount of NiTi2and Ni3Ti phases. After constraint agingtreatment, Ni4Ti3phase precipitates from the matrix of the NiTi alloy.Both the constraint aging temperature and dwelling time have significant influence onthe phase transformation behavior and two-way shape memory effect of the NiTi alloy. NiTialloy undergoes B2→R→B19’ two-step phase transformation after aging. Moreover, NiTispecimens aged at low temperatures (~300℃) show smaller transformation hysteresis andbetter two-way shape memory effect than that aged at high temperatures.Initial bending strain εthas little influence on R phase and B19’ martensite transformation temperature under low temperatures and the long term (~100h) aging. Whenspecimens were aged under different initial strains of εt, the values of εpi(two-way reversiblestrain between boiling water and icy water) and εpr(two-way reversible strain between boilingwater and room temperature) tend to be constant. The range of εpiis between0.95%and1.17%with an average of1.07%, while the range of εpris0.42~0.53%with an average of0.49%. Moreover, NiTi alloy can obtain straight state at the room temperature after aging withεtin the range of0.97~1.33%.The artificial anal sphincter device is heated up rapidly when subjected to the electricalloading of direct current supply. Thus, the NiTi alloy stripes start to deform ahead of theAustenite transformation temperatures (Asand Af) when loading. On the other hand, thedeformation of the NiTi alloy stripes is consistent with the R phase transformationtemperatures (Rsand Rf) when unloading. The artificial anal sphincter device does not showobvious hysteresis of defomation on heating and cooling. After several electrical-thermalcycles, the device remains an excellent repeatability of switching between opening andclosing.