Papers by Keyword: TiNi Shape Memory Alloy

Abstract: The present work deals with the optimization of micro-WEDM process parameters for machining Ti_49.4-Ni_50.6 shape memory alloy (SMA) for orthopedic implant application. Effect of micro-WEDM parameters viz. Gap voltage, capacitance, wire feed and wire tension on the response variables such as material removal rate, surface roughness, kerf width and dimensional deviation is determined. As Ti-Ni SMA has fascinating properties and bio-compatibility, have been considered for present work. Nine experiments have been performed on micro-WEDM based on an orthogonal array of Taguchi method. Subsequently, the grey relational analysis (GRA) method is applied to determine an optimal set of process parameters. It is observed that optimized set of parameters A3B3C3D1 viz. 140 V gap voltage, 0.4 µF capacitance, wire feed 30 µm/sec and 30% of wire tension determined by using GRA offers maximum MRR and minimum SR, KW and DD. From the Analysis of Variance, it is seen that the process parameter capacitance is the most significant parameter for multi-response optimization with a percentage contribution of 77.91%. Young’s modulus also checked for biocompatibility. Also, SEM images are taken to confirm the results offering better surface quality. Heat treatment process like annealing is found to be the most suitable to recover shape memory effect of WEDMed samples.

Abstract: The effect of rare earth element Ce addition on the microstructure and martensitic transformation behavior of Ti₅₁Ni₄₉ shape memory alloy was investigated by scanning electronic microscope (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The results show that the microstructure of TiNiCe ternary alloy consists of Ti₂Ni phase, CeNi phase and the matrix. One-step martensitic transformation is observed in quenched TiNiCe ternary alloys, which is the same as that in quenched TiNi binary alloys. The martensitic transformation temperatures of Ti-rich TiNi alloy hardly increase with Ce addition.

Abstract: Dissimilar materials between TiNi shape memory alloy (SMA) and stainless steel of 200μm mm thick were butt welded by micro impulse laser and the electrochemical behavior of the weld joints was studied. The results show that the crack sensitivity of the welded joint is very high and good joining between TiNi SMA and stainless steel can be realized with pure Ni wire as filler material presented between TiNi SMA and stainless steel. The tensile strength of the welded joint with pure Ni wire as filler material is 580MPa. The electrochemical tests of the welded joints in different concentrations of NaCl solutions and different pH value solutions show that, the maximum of the corrosion potential Ecorr in the three typical corrosion systems (0.9%NaCl, pH=7.3; 3.5%NaCl, pH=7.3; 0.9%NaCl, pH=4) is the weld seam, which indicates that the corrosion tendency of the weld is smaller than that of TiNi alloy and stainless steel in the three corrosion systems. The pitting corrosion potential of TiNi alloy, stainless steel and the weld gradually decreased with the increase of Cl-concentration; the pitting corrosion potential decreased with the increase of H+ concentration, while TiNi alloy decreased faster than that of the weld and stainless steel, the surface stability of TiNi alloy is low when pH value dropped below 7.

Abstract: A Ni-rich Ti-50.7.at%Ni alloy was processed by Equal Channel Angular Extrusion (ECAE) at 500°C. After 8 passes ECAE, microstructure was refined to sub-micron scale, approximately 0.2 μm～0.3 μm. TEM observation reveals that Ti3Ni4 phase precipitated in Ni-rich Ti-50.7.at%Ni alloy during the preheating treatment before each ECAE pass, but re-dissolved during sequent ECAE processes. After ECAE treatment, the B2R transformation occurred within a larger temperature range. Comparing with the solution-treated TiNi specimen, the martensitic transformations start (Ms) and peak temperatures (Mp) of TiNi specimens ECAEed were dramatically lowered. Super-elasticity characteristics of TiNi alloy were tested by tensile loading and unloading cycles. The results reveal that at a tensile strain of 4% or smaller, ultrafine-grained (UFG) TiNi alloy processed by 4 passes ECAE shows better super-elasticity than solution-treated sample. Microstructure evolution and its effect on phase transformations and super-elasticity characteristics have been discussed.

Abstract: The effect of thermal cycling of the Ti - 49.8 at. % Ni alloy deformed by rolling at 500 оС on dilatation of the material was investigated. It was shown that R → B2 and B19' → B2 phase transformations take place during heating due to retaining of R-phase at the lowest temperature of cycling. The evolution and subsequent stabilization of two-way shape memory effect (TWSME) upon thermal cycling are caused by decreasing of the residual B19'- martensitic phase formed during rolling. It was revealed that recoverable strain of the alloy doesn’t exceed 0.8 % even after forty cycles.

Abstract: Butt welding of 0.2mm thick TiNi shape memory alloy sheet was successfully realized by using micro impulse laser whose average power is 80W and the microstructure of welded joint was study in this paper. The results show that, the welded joint of micro laser welding can be divided into four zones according to grain size and microstructure. The microstructure in base metal zone is rolled structure and the grains are coarse and heterogeneous. The microstructures of welded seam center zone are fine equiaxed crystals and the microstructures of both lower surface and upper surface edge zone are columnar crystals. There is almost no obvious coarse grain heat-affected zone at the edge between welded seam and base metal. There is obvious segregation layer in local area of welded seam because the content of Ti and Ni elements is changed and different with base metal during the crystallizing course of welding pool.

Abstract: Superelastic 0.1mm diameter Ti-Ni filaments are used to manufacture braided orthopedic cable for bone fixation. Biomechanical conditions for this application generally have a cyclic nature, and therefore it becomes important to evaluate the influence of the installation (mean) strain on the fatigue life of these filaments. Uniaxial tension cyclic testing of Ti-Ni filaments is performed in a water bath at 37°C with a 2Hz frequency of to 100 000 cycles. Strain-controlled testing conditions are as follows: alternating strain magnitude varies between 0.64 and 3.64% with mean strain range between 1.32 and 7.1%. Based on the premises that the minimum strain should be high enough to prevent any loss of tension in the tested specimen and that the maximum strain should not bring the specimen to failure during the first loading cycle, the total strain magnitude is encompassed between 0.68 and 8.94%. The results obtained provide a better understanding of the impact mean strain has on the fatigue life of superelastic Ti-Ni alloys.

Abstract: In this study, a current density contour map of TiNi shape memory alloy in sodium sulfate solutions was constructed by potentiodynamic polarization technique. The current density contour map of TiNi alloy, constructed in a 0.5 M sodium sulfate solution over the pH range 1 to 13 at 37oC, showed only hydrogen evolution region, passive region, and oxygen evolution region. The localized corrosion region did not exist and the susceptibility of localized corrosion was not exhibited. We also investigated the effects of chloride ions on the pitting corrosion of TiNi alloy in sodium sulfate solutions. Pitting corrosion occurred with addition of 0.2 M NaCl in a sulfate solution. And it was observed that pitting potential got lower as the concentration of chloride ion went up. However, repassivation potential was not affected by the addition of chloride ion up to 0.5 M. It is concluded that a TiNi shape memory alloy exists as a passive state and a good corrosion resistance in sulfate solutions over the pH range 1 to 13, and the sulfate ion performs a role of inhibiting localized corrosion in chloride environment.

Abstract: In order to investigate the corrosion behavior of TiNi shape memory alloy, especially electrochemical behavior of pitting and crevice corrosion in a human body, current density contour(CDC) map of TiNi alloy was constructed by potentiodynamic polarization technique in simulated physiological sodium chloride solutions of pH ranging from 1 to 13 at 37oC. Morphology of pits and corrosion products in sodium chloride solutions of various pH were analyzed by SEM and EDX, and susceptibility and mechanism of localized corrosion were also discussed.

Abstract: In this research, we investigated the fabrication conditions of Ti-Ni alloy powder by MA process and the shape memory characteristics of the sintered alloy by a pulse-current sintering equipment using mechanically alloyed powder (MA powder). The microstructure of the sintered alloy of the MA powder was more homogeneous than that of the alloy of the elemental powders. The application of the MA powder makes the width between transformation temperatures of the shape memory treated alloy of the MA powder became narrower, that is, it improves the temperature response of the compact. The tensile strength and elongation of the shape memory compact of the MA powder were approximately 780 MPa and 7.5 %, respectively. This is one of the superior tensile properties of SMA compact fabricated by powder metallurgy process. The superelastic behavior took place in the alloy of the MA powder. Thus, the MA process in short process time resulted in lower contamination of the MA powder and the application of the MA powder resulted in superior shape memory characteristics of the sintered alloy.