Papers by Keyword: NiTi Alloy

Abstract: Nowadays, NiTi shape memory alloy plays an important role in medical applications, especially in intervention devices such as Cardiovascular stent, Retriever stent, etc. This research aims to design and fabricate the self-expanding and self-centering Atrial Septal Defect (ASD) closure device from NiTi shape memory alloy wires. The device was made by braiding 72 NiTi wires and forming to double-disk shape. Heat-treatment was carried out at the temperature range between 510°C to 550°C for 30 min, respectively. DSC measurements and pull tests were done in order to evaluate the transformation and mechanical behavior of the devices. The results show that excellent superelasticity can be confirmed in all specimens at room temperature. Recovery forces obtained from pull test increase with increasing heat-treatment temperature. Geometrical control is success with good consistency and the recovery force obtained can be compatible with those of commercial products.

Abstract: NiTi alloy is usually prepared by casting and forming. As an innovative process, reactive sintering powder metallurgy is tested worldwide, aiming to prepare pure NiTi alloy easily from nickel and titanium powders. This process enables to prepare both porous and low-porosity alloy, depnding on the process conditions. However, the formation of NiTi phase in this process is always accompanied by the Ti₂Ni phase, which is hard, brittle, less corrosion resistant and does not have the shape memory. In this work, various alloying elements (Al, Si, Mg, Fe, Nb, V) were added to Ni-Ti alloy in order to lower the amount of Ti₂Ni phase or at least to minimize its undesirable effect on the alloy properties. The reactive sintering behaviour, phase composition and mechanical properties of Ni-Ti-X alloys were described.

Abstract: In this paper, the TiO₂ film and Ti/TiN/TiO₂ multilayers were prepared by anodic oxidation and RF magnetron sputtering technology on surface of NiTi shape memory alloy, respectively. The structure and surface morphology of films were systematically studied by X ray diffracmeter (XRD) and scanning electron microscope (SEM); blood compatibility was researched by kinetic clotting time and platelets adsorption experiment. The result showed that, blood compatibility was improved in varying degrees after surface treatment. And the multilayer which was deposited on NiTi alloy by RF reactive magnetron sputtering after the method of constant voltage anodization had optimal Anti-clotting properties.

Abstract: In this present work, NiTi alloy has been successfully produced by metal injection moulding (MIM) starting with elemental powders mixture of Nickel (Ni) and Titanium (Ti) mixed with a composite binder of palm stearin (PS) and polyethylene (PE). Two different atomic fractions of Ni-Ti were investigated; 50-50 and 50.8-49.2 and the powder loading used was 65.5vol%. The green parts were successfully injection moulded at an optimum temperature of 130°C. The samples were then underwent solvent extraction using n-heptane solution to remove the primary binder of PS. It was followed by thermal debinding to completely remove the backbone binder of PE and subsequently sintered in high vacuum at 1050°C to allow diffusion of the elemental Ni and Ti powders to form NiTi alloy. The phase constituents of the sintered specimens were characterized by X-Ray Diffraction (XRD) and by Scanning Electron Microscopy (SEM), equipped with EDX analysis under back-scattered electron (BSE) mode. The reversible austenite to martensite phase transformation temperatures (PTTs) correspond to shape memory effect (SME) was determined by Differential Scanning Calorimeter (DSC) with heating and cooling cycle in the range of-50 and 200°C. The influence of Ni-Ti ratio and the processing conditions on phase constituents and SME was then analyzed and discussed.

Abstract: The Nickel Titanium (NiTi) alloy is a shape memory alloy. It presents two distinct properties: shape memory effect (martensite phase at room temperature) and the pseudoelasticity (austenite phase at room temperature). These materials are frequently used in industries like engineering, dentistry, medicine and aeronautics, and for certain applications it is important to know the wear behavior of these alloys.Thus, this study aims to evaluate the micro-abrasive wear behavior of pseudoelastic NiTi alloys at the as received and aged conditions, and compare with results obtained for NiTi alloy with shape memory effect. The aging heat treatment was performed at temperatures of 350, 450, 500 and 600 °C with an isotherm of 30 minutes and subsequent quenching in water. The wear tests were carried out in a free ball machine. After wear testing was observed that the wear coefficient obtained were lower for all treatment temperatures when compared to pseudoelastic NiTi alloy as received, reducing wear by 30 % in case of treatment at 350 °C. The alloy with shape memory effect showed good wear behavior, with a wear coefficient about 14 % less than the untreated pseudoelastic NiTi alloy.

Abstract: This paper presents the processing of Nickel Titanium (NiTi) alloy from elemental powders of Nickel and Titanium by conventional powder metallurgy process. Two batches of feedstock containing 2 different formulations (1) Ti-50.4at% Ni and (2) Ti-50.8at% Ni were prepared. The feedstocks for each composition were warm-pressed and subsequently sintered in vacuum at 1150°C. Based on the XRD result, it clearly showed that the conventional sintering following warm-press method is promising in producing greater amount of the predominant NiTi (B2) phase and a minor fraction of martensitic NiTi (B19’) phase. The reversible phase transformation and the microstructure of the sintered samples have also been observed and discussed.

Abstract: NiTi alloys are used for both medical and veterinary purposes, and also for production of surgical instruments. Sulphate-reducing bacteria (SRB) colonize various anaerobic environments, including human oral cavity. Desulfovibrio desulfuricans is the SRB species responsible for corrosion of many metals including highly alloyed steels as well as titanium and its alloys. The aim of this work was to compare growth of D. desulfuricans biofilms on NiTi alloy submerged in artificial saliva or in inflammatory saliva. The results of investigations showed differences between D. desulfuricans biofilms formed on NiTi alloy in the presence of artificial saliva and inflammatory saliva. The growth medium influenced biofilm structure; inflammatory saliva promotes its formation. The biofilms grown on samples immersed in inflammatory saliva were much thicker as compared with samples emerged in artificial saliva. After 28 days of incubation in inflammatory saliva, plentiful mature biofilm was present on alloy surface.

Abstract: Bacteria of Desulfovibrio genus belong to group of widespread sulphate-reducing bacteria (SRB). D. desulfuricans is considered one among many bacterial species involved in microbiologically influenced corrosion (MIC) of metals, mainly of stainless steels and other alloys. SRB can produce gaseous hydrogen sulphide. This gas is released into the environment leading to formation of metal sulphides that significantly influence electrochemical processes and ultimately enhance the corrosion of materials. Biofilms formed by these bacteria are especially harmful for highly alloyed steels and many alloys. The aim of this work was to compare the character of growth and biofilm formation by three strains of D. desulfuricans (standard soil strain DSM and two wild intestinal strains: DV/A and DV/B) on the surface of NiTi alloy.

Abstract: The pitting potential, E_pit, of the passive layer on the implant alloy can be treated as an accelerated laboratory test to assessment a susceptibility to pitting corrosion of metallic biomaterials in simulated body fluids. This study deals with an evaluation of E_pit of the self-passivated TiO₂ layer formed on the surface of the NiTi implant alloy as a function of the scan rate of polarization. Cyclic potentiodynamic studies were performed in Ringer’s solution at 37°C. It was found out that the more noble value of E_pit in the range of 0.99-2 V was registered at a given polarization scan rate that ranged from 0.16 to 2 mV s^-1, the lower susceptibility of the self-passivated NiTi implant alloy to the initiation of pits was detected.