Papers by Keyword: NiTi

Abstract: The pseudoelasticity (PE) and shape memory effect (SME) are the two main behaviors presented by the shape memory alloys (SMA's) and are associated respectively to mechanical and thermally induced martensitic transformations. The aim of this work is to investigate the effects of heat treatment temperature on the microstructure properties and phase transformation temperatures of a NiTi alloy with 57 w.t. % Ni. The X-ray diffraction (XRD) was carried out to obtain the phases present. The phase transformation temperatures were measured by differential scanning calorimetry (DSC). The alloy chemical composition and hardness were obtained by X-ray fluorescence (XRF), hardness (HRC) and microhardness (HV) tests, respectively. The analysis were performed in the state as received and after aging treatment at different temperatures between 350 °C and 600°C. The samples as received contained a fully austenitic microstructure at room temperature and the DSC analysis showed the presence of a phase transformation in multiple steps (B19'-R-B2). After aging at 350 °C the R phase was observed at room temperature with the austenite. With the aging treatment at 600 °C the R phase was solubilized and the alloy phase transformation occurred in a single step, ie, direct transformation from austenite to martensite and vice versa. The transformation temperatures Af, As, Ms and Mf changed with the aging treatment temperatures.

Abstract: The purpose of this study is to identify machining difficulties of nickel-titanium based shape memory alloys. Nickel-titanium (Nitinol) is one the widely used shape memory material which is applied in many products in the aerospace, medical, and biomedical fields. NiTi alloy cannot be machined easily because of high tool wear, high cutting force, huge hardness and surface defects are made many problems into their machining. Investigation in micron precision shows plenty surface defects in machining process, something like debris of microchips, feed marks, tearing surface, deformed grains, material cracking and chip layer formation which consists the main problem in the machining of shape memory alloys. Moreover, these defects can be reduced or eliminated by changing the cutting parameters such as: feed rate, cutting speed and cutting depth so that, machining of nickel-titanium alloys would be improved.

Abstract: The desired compression-shear loading were applied by the beveled ends with different angles, and this new technique based on split Hopkinson pressure bar can be used to investigate the dynamic response of materials. A series of experiments of NiTi shape memory alloy were performed at different impact velocities and different loading angles, and the dynamic equivalent pressure curves were given. The initial phase transition points under different impact velocities were plotted on σ-τ stresss space, and the phase transition surfaces predicted by a dynamic phase transition criterion were given.

Abstract: Objective: To study the surface characteristics, chemical composition and Ni release from simulated standard fixed orthodontic appliance ligated with two differently priced nickel titanium (NiTi) archwires in artificial saliva at pH 5.14 and 6.69 for 4 weeks at 37oC. Materials and Methods: Two commercial NiTi rectangular wire (Ormco and Smart), 0.016 x 0.022 in size were studied. Their surface characteristics were evaluated: surface morphology by scanning electron microscope, surface roughness by surface roughness tester and grain structure analysis by optical microscope. Their chemical composition was analyzed by an energy dispersive X-ray spectrometer (EDC). For Ni release, twenty-eight simulated standard fixed orthodontic appliance samples sets, each set corresponding to one half-maxillary arch were used. Sample sets were divided in 2 groups (14 sets per group). The first group was ligated with Ormco NiTi archwires (USA) and the second with Smart NiTi archwires (China) with elastomeric ligatures. Half sets of each group were immersed in 50 ml artificial saliva at pH 5.14, and the other half at pH 6.69. Ni release was quantified with the use of flame atomic absorption spectrophotometry. Statistical analysis of variance was determined on days 1, 4, 7, 9, 14, 21 and 28 comparing Ni release between the groups and t-test was determined the difference between pH 5.14 and pH 6.69. Results: The suface morphology showed striations along the longitudinal axes. The Ormco NiTi wire had more surface roughness than Smart NiTi wire and the diameter of grain sizes were 2-8 μm. The chemical composition of the two NiTi wires was Ni, Ti, Cu, Al, and Cr but there was difference in the percentage of elements. Both Ormco NiTi and Smart NiTi wires continuously increased Ni release at time intervals at both pH levels. The Ormco NiTi wire had more Ni release at pH 6.69 than pH 5.14 but Smart NiTi wire had more Ni release at pH 5.14 than 6.69. At 4 weeks, the Ni release of one half-maxillary arch was 1.221 ppm (1221 μg/l) at pH 5.14, 1.267 ppm (1267 μg/l) at pH6.69 for Ormco NiTi wire and 2.175 ppm (2175 μg/l) at pH 5.14, 0.676 ppm (676 μg/l) at pH 6.69 for Smart NiTi wire. No significant difference was found in Ni release from Ormco and Smart NiTi wires at pH 5.14. At pH 6.69, no significant difference was found in Ni release from Ormco NiTi wires while Smart NiTi wire showed significant difference (p <0.05) on days 14, 21 and 28. Conclusion: Ni release depends on surface characteristics and chemical composition of archwires and pH.

Abstract: Ni-Ti-Nb system alloys show wide shape memory hysteresis, suitable for assembly applications. The microstructure is composed by NiTi matrix (with some dissolved Nb) and Nb dispersed particles (with some Ni and Ti content). These particles are to cause the hysteresis widening. This work evaluates the microstructure evolution during wire fabrication process of equiatomic Ni and Ti alloys with increasing Nb content (1.5, 3.0, 6.0 and 9,0%at.). It is shown that as-cast alloys with up to 9% at.Nb and near equiatomic Ni:Ti relation show three main microconstituents: NiTi matrix phase, interdendritic eutectic phase (NiTi + β-Nb) and Ti₃(Ni,Nb)₂ compound precipitates. It was observed that NiTi matrix phase and eutectic phase (NiTi + β-Nb) have ductile behavior while Ti₃(Ni,Nb)₂ compound have fragile behavior. There was not much hardness variation during hot swaging (200-300 HV) due to recovery and recrystallization processes. Mechanical hardening prevailed as the mechanism for increase hardness of cold worked samples from 200 to 450 HV.

Abstract: NiTi shape memory alloy was potentiostatically anodized in a molybdate electrolyte containing ammoniac ion to form a thick and uniform oxide layer. The oxide layer was characterized by SEM, XRD, EPMA, XPS and potentiodynamic polarization test. It was found that the as-prepared coating is a thick (approx. 40 μm), smooth and dense oxide layer, and is free of cracks and porous defects. Moreover, the surface analyses revealed that the anodic oxidation at 60 mV potential (SCE) reduces significantly the proportion of Ni in the outmost surface layer, and the atomic ratio of Ti and Ni elements is raised from 1:1 to 9.7:1. The as-prepared coating was primarily composed of noncrystalline TiO₂ on the outermost surface with a small quantity of Ni (OH)₂. Potentiodynamic polarization tests in Fusayama artificial saliva pH 6.2 demonstrated that the oxide layer presents a significant increase in breakdown potential due to titanium enrichment.

Abstract: In this study, the electrosynthesis method of the zinc oxide films on NiTi shape memory alloy substrate from zinc acetate solution, has been studied. The optimum conditions for production of the ZnO thin films were found at a voltage of 20 V for 60 s. Experimental data were presented on the deposition process, structure and composition of the ZnO thin films. It was found that depending on the deposition parameters, the structure of the obtained zinc oxide films varies from amorphous to coarse-grained. It was ascertained that the examined electrosynthesis of the ZnO films appears as a useful method for the surface modification of the NiTi alloy towards biomedical applications.

Abstract: Two shape memory alloys with nominal chemical composition: Ni50Ti50 and Ni47Ti50Co3 were produced using twin roll casting technique (TRC). Relatively high cooling rate realized during solidification causes directional heat flow. In result of that, high textured strips were produced. However, the total amount of the preferentially oriented grains differs between surface and cross-section of the strip. At surface, where the crystallization of the grains is the most intensive amount of the grains can reach about 77%. At the cross-section of both strips, where the solidification rate decreased, the total amount of the textured grains was not higher than 57%.

Abstract: In order to improve a biocompatibility of a NiTi shape memory alloys used as a long-term implant, surface was covered by protective multi-layers coating. First, alloy was passivated in autoclave, and then hydroxyapatite was deposited using electrophoresis at room temperature. In result of that multi-layer was formed on the top of the surface consisted of titanium oxide and hydroxyapatite. Phase identification done with use of grazing incidence beam X-ray diffraction proved sequence of obtained coatings. Applied procedure for multi-layer deposition allowed avoiding of the B2 parent phase decomposition. Moreover, the martensitic transformation occurs in two-steps.

Abstract: The influence of increased dislocation density; dispersity of Ni-rich (Ti₃Ni₄ and Ti₂Ni₃) particles and volume fraction of Ti-rich (Ti₂Ni) particles on the low-cycle (high amplitude) and high-cycle (low amplitude) fatigue resistance of nitinol has been considered in this paper. It was revealed that the fatigue resistance of nitinol in low-cycle conditions may be improved by increasing the part of deformation which is realized by martensitic mechanism. This part may be estimated by measuring ε_cr, which can reflect the influence of the structure parameter both on σ_M and σ_slip. It was found that in high-cycle fatigue conditions the substructure of nitinol predominantly determine its fatigue resistance, which is being the better in samples that had higher dislocation density or high dispersity of Ni-rich particles (up to 30 nm).