Papers by Keyword: Superelasticity

Abstract: This paper deals with the martensitic transformation and functional properties in the quenched single crystals of the Co₃₅Ni₃₅Al₂₈Fe₂ medium-entropy alloy, oriented along the [001]_B2-direction. The microstructure and chemical composition of the single crystals have been studied in detail using transmission and scanning electron microscopy. The {111}_L10 martensite twins up to 10-20 nm width and γ/γ′-phase precipitations larger than 100 μm are detected. The thermoelastic B2-L1₀ martensitic transformation upon stress-free cooling/heating in single crystals of Co₃₅Ni₃₅Al₂₈Fe₂ alloy is characterized by the accumulation of elastic energy, which is the driving force of the reverse martensitic transformation, and the low dissipation energy. The reverse transformation starts at lower temperatures than the forward transformation M_s>A_s. The regularities of the stress-induced B2-L1₀ martensitic transformation change due to an increase in the contribution of the dissipated energy and M_s^σ<A_s^σ. There is shape memory effect with the reversible strain (3.2±0.3)% and high temperature superelasticity with the reversible strain (3.3±0.3)% in the temperature range from 323 K to ≥548 K in the [001]_B2-oriented single crystals. These crystals withstand stress up to 1200 MPa in compression without destruction.

Abstract: This study proposes to design and fabricate the peripheral stent using NiTi shape memory alloy wires by braiding technique. The 0.2 mm diameter NiTi wire was attached to the automatic braiding machine and fabricated the braided stent under the different height of circular rod holder. Heat-treatment was carried out at 450°C, 500°C, 550°C and 600°C for 10 and 20 minutes, respectively. Then transformation temperature, the shape of cell area, and mechanical properties were evaluated. The results show that complete superelasticity can be confirmed under body temperature in stent prototypes with heat-treatment temperature over 500°C irrespective of heat-treatment time. The prototype with the height of circular mold of 70 mm shown a good braiding pattern with high kink resistance. The radial force shown no significant difference at any section along the length profile. It is noted that stent with the height of circular mold at 70 mm, carrier speed of 30 rad/sec and take-up speed of 4.86 mm/s which shown radial resistive force (ROF) of 2.44 ± 0.05 N/mm, and chronic outward force (COF) of 0.533 ± 0.03 N/mm which is sufficient for vessel expansion.

Abstract: Over the recent years, Nitinol (Ni-Ti) shape memory alloys have gained popularity in the medical, aerospace and energy sectors, due to their superelasticity, shape memory effect, low stiffness, good biocompatibility and corrosion resistance. Compared to steels and other common metallic materials, it is difficult to model the mechanical behavior of Ni-Ti due to the inherent functional properties caused by the diffusion-less solid-state phase transformations. With the help of Laser Powder Bed Fusion (L-PBF) process, these transformational characteristics can be controlled. This will ultimately lead to controlling the mechanical and thermal properties for specific applications. In this work, Finite Element Analysis (FEA) was conducted to replicate the actual mechanical phenomenon occurring in Nitinol. Models were generated for simulating the superelastic and plastic behaviors, and were validated against actual experimental data. The ability to model the complex mechanical response of Nitinol will enable exploration into the sensitivity of material response to phase volumes, material composition, and strain rate. Robust models of these phenomenal also provide the potential for tailoring in-silico the microstructure required for specified desired macroscopic material properties.

Abstract: Nowadays, NiTi shape memory alloys have been extensively used for medical applications especially as transcatheter [6]. This research aims to investigate the effect of heat-treatment temperature on radial force of NiTi self-expanding atrial depressurized device. The function of this ADD is to expand and sustain the hole in the atrial septal and release the pressure in the heart chamber to an acceptable level, and subsequently, prevent it from closing again, without a repeating surgery. The device was made by braiding 72 NiTi wires with diameter 0.1 mm and formed to double-disc shape with 8 mm fenestration size. Heat-treatments were carried out at 560°C, 580°C, and 600°C for 10 min, respectively to obtain suitable mechanical properties and shape setting. In order to evaluate the basic performance of the device, phase transformation temperatures and radial force were investigated. The results show that ADD device can be formed perfectly and excellent superelasticity can be confirmed in all specimens at room temperature, irrespective of heat-treatment temperature. It is confirmed that both radial resistive force and chronic outward force greatly increase while the fenestration size decreases with increasing of compression load. This ensures that the device is able to resist compression of the heart wall. It can be concluded that the optimum heat-treatment temperature is 560°C since the low heat-treatment temperature produces less oxidation and the greatest recovery force.

Abstract: In this comparative study, the structural and superelastic characteristics of two thermomechanically treated metastable Ti-Nb based (Ti-22Nb-6Zr) and Ti-Zr based (Ti-18Zr-14Nb and Ti-18Zr-13Nb-2Ta (at. %)) alloy systems were studied. To study the influence of room temperature storage on the functional properties of these two alloy systems, the alloys were subjected to a multistage testing routine consisting of four ten-cycle loading-unloading testing series alternated with three room temperature ageing periods (1, 5 and 20 days). Based on microstructure-properties relationships, it was shown that for each alloy system, the forward stress-induced martensitic transformation was essentially dependent on the material microstructure, whereas the subsequent reverse martenstic transformation was controlled by the material composition. The Ti-Zr based alloys demonstrated more stable functional behavior than their Ti-Nb based counterparts. More specifically Ti-18Zr-13Nb-2Ta, subjected to a combination of cyclic training alternated with room temperature ageing showed a significant improvement in superelastic behavior with small accumulated strains and narrow stress hysteresis.

Abstract: The paper presents the results of a study the hydrogen effect on the structural-phase transformations and the superelasticity in binary ultrafine-grained (UFG) TiNi based alloy after diffusion redistribution hydrogen as a result of aging at room temperature. The redistribution of hydrogen in the process of long-term aging after electrolytic hydrogenation of UFG wire specimens the Ti_49,1Ni_50,9 (at.%) stabilizes the B2 structure. Superelasticity in samples aged at room temperature after hydrogenation is significantly deteriorated.

Abstract: Actuation response of NiTiHf high temperature SMAs can be enhanced by means of suitable heat treatment on the material through precipitation hardening. Heat treatments can be chosen carefully to improve the performance of the NiTiHf SMAs in order to meet the requirements of targeted applications to design more robust and efficient high temperature solid-state actuator systems. The present work aims to develop a novel approach to model and predict the behavior of heat-treated NiTiHf SMAs. The predictions of the thermomechanical response of NiTiHf SMAs are based on Representative Volume Elements (RVEs). The precipitated NiTiHf SMA is modeled as a composite consist of of thermo-elastic non-transforming precipitates and a polycrystalline SMA matrix. The structural effect of precipitates and the effect of Hf-concentration gradient resulted from Hf depletion during precipitation are included. The composition distribution resulting from the elemental depletion and the transformation temperature distributions in the SMA matrix are related. In the present work, these relations are developed from experimental measurements on several NiTiHf compositions. Thermo-mechanical responses of Ni_50.3Ti_29.7Hf₂₀ heat-treated at 500°C for 48h at different loading conditions are predicted and the correlations with experimental results demonstrate the validity of the proposed framework.

Abstract: It was shown in Ni₅₃Mn₂₅Ga₂₂ single crystals that the annealing at 1273 K for 1 h followed by slow cooling, at which the (L2₁+γ)-structure is formed, provides the increase of crystals plasticity as compared with the brittle initial L2₁-single crystals. The increase of critical stresses by more than 2 times (up to σ > 150 MPa), at which the cooling/heating cycles under stress are accompanied by sample destruction, is observed. Moreover, the value of thermal hysteresis is increased by 1.5 times and the perfect curve of superelasticity is observed at T=423 K.

Abstract: Recently, Ni-free shape memory Ti-based alloys (composed of the biocompatible β-stabilizing elements such as Ta and Nb) are extensively studied. In this work, new Ni-free Ti-17Nb-6Ta is presented as a candidate for shape memory alloys with high biocompatibility. This alloy produced using arc-melting in argon atmosphere, followed by solution annealing at 900° C for 30 min. β-phase is the predominant phase beside α” martensite phase. Stress induced martensitic transformation is observed after cold rolling and during bending tests as measured by XRD. The hardness of the bended wire in the solution treated condition was around 330HV. While the cold rolled wire hardness before bending was 300 HV. The superelasticity and shape memory effect was investigated through bending tests of alloy wires. The cold rolled wire showed higher superelasticity than shape memory effect. But superelasticity and the shape memory effect were almost similar with the solution treated wire. Also, the total spring back in cold rolled wire is higher compared with solution treated wire.

Abstract: In this paper investigation results of behavior of promising superelastic alloys of Ti-Nb-Zr system in blood-vessel are presented. The possibility of their use in manufacturing of medical stents is examined. Based on analytical review of present scientific papers, four different alloys of Ti-Nb-Zr system are taken in consideration. A finite element modelling of stent behavior during delivery and opening stages is considered. These processes are done for two typical stent geometries and four alloys possessing different mechanical properties. Finite modelling results are analyzed to show the distribution of internal stresses, mechanical aspects of stent installation and effectiveness of various configurations to widen the narrowed vessel. Modeling has allowed to formulate recommendations for optimal mechanical characteristics of the superelastic alloy used for the manufacture of medical stents.