Papers by Keyword: NiTi

Abstract: The main goal of present paper is to obtain porous coatings enriched in copper by Plasma Electrolytic Oxidation on titanium and niobium as well as on NiTi and Ti6Al4V alloys. Performed SEM and EDS studies confirmed the hypothesis that it is possible to create the porous surfaces with pores, which shapes and size are different. In order to show the copper enrichment inside the surface layers, the copper-to-phosphorus ratios were used. Based on these ratios it may be concluded that average value of Cu/P is maximal for NiTi alloy after oxidation in electrolyte containing 300 g of copper nitrate in 1 liter of phosphoric acid and equals 0.26. The minimum of Cu/P ratio equaling to 0.12 was recorded for pure titanium and pure niobium treated in electrolyte containing 300 g of Cu (NO₃)₂ in 1 L H₃PO₄.

Abstract: NiTi is categorized as a Shape Memory Alloy (SMA) that has been commercially studied and used in biomedical industry due to two main unique properties, Pseudoelastic (PE) and Shape Memory Effect (SME). Combined with biomimetic properties to human bone, NiTi has the potential to be applied as implants by engineered manufacturing process. The common manufacturing by casting has some challenges in order to obtain intrinsic and miscellaneous design of NiTi parts leash to explore more using powder metallurgy (PM) method that expected to get the porous structure. This paper aims to provide an overview of processing NiTi by conventional PM method which could contribute in focusing porous part that suits for biomedical and implants.

Abstract: NiTi shape memory alloy thin film sputter-deposited on a large scale silicon wafer was characterized by means of instrumented (depth-sensing) indentation technique. Thickness of deposited thin film was measured by calotest device. Microstructure of thin film was observed using differential interference (Nomarski) contrast. It was shown that the local mechanical properties are different in areas containing different phases (austenite and martensite) according to different deposition conditions (kinetic energy of deposited atoms when impacting the substrate surface).

Abstract: The integration of functions in lightweight structures features great potential for future applications in diagnosis and control. The combination of shape memory wires or ribbons made of NiTi embedded in aluminium and manufactured by composite extrusion offers the possibility to produce a composite actuator material in a single production step. The extrusion process allows a wide range of shapes and provides higher versatility than actuators made of bi-metals. The transformation temperature of NiTi varies depending on the composition of the alloy, between -100 °C and 100 °C. However, NiTi can also transform stress-induced. In the designated application, a force is applied via the interface onto the matrix material to deform it. Due to the resulting stress, the transformation temperature rises to temperatures higher than those of the unloaded material. Furthermore the production of composite extrusions leads to a significant heat input on the shape memory alloys followed by another increase of the transformation temperature.Therefore it is essential to reproduce the heat treatment and the stress-induced transformation to predict the transformation temperature in the resulting composite influenced by the interface. For that purpose, the wire gets annealed in a furnace with different durations at a temperature similar to that of the bar extrusion process. After this, the transformation temperatures can be observed at various stresses to evaluate their applicability for aluminium composite actuators.

Abstract: The addition of Cu to near equiatomic NiTi shape memory alloys (SMAs) can provide some modifications of their shape memory properties by affecting their transformation behavior. The same effect was obtained in the case of Ni₃Ti₂ and Ni₄Ti₃ precipitates presence in the microstructure of NiTi. Also the substitution of Cu to NiTi alloys increases the hardness of the materials. This paper presents the microstructural and mechanical investigations of NiTi and NiTiCu alloys obtained by spark plasma sintering (SPS) process that represents a great potential for researchers as a new process for the fabrication of intermetallic compounds.

Abstract: The aim of this work was to describe the dependence of microstructure of NiTi shape memory alloy on the conditions of powder metallurgy processing route. The technology consisted of blending of elemental Ni and Ti powders, uniaxial cold pressing and reactive sintering. The effects of reactive sintering temperature, heating rate, holding duration and particle size were determined. The proposed technology can be used as the alternative production route of NiTi to minimize the contamination of the alloy.

Abstract: The radiotracer technique was used to measure the grain boundary diffusion of ⁴⁴Ti and ⁶³Ni in slightly Ni-rich polycrystalline NiTi compound in the temperature range of 673 - 923 K. The temperature dependence of the grain boundary triple product P (P = sδD_gb, s is the segregation coefficient, δ is the grain boundary width, and D_gb is the grain boundary diffusion coefficient) for Ti and Ni was determined. The triple products of both Ti and Ni grain boundary diffusion in NiTi reveal a unique behavior with significant deviations from an Arrhenius-type dependence. Probable evolution of the grain boundary structure with temperature was used to interpret this phenomenon.

Abstract: Nickel titanium is a near-equiatomic intermetallic that possesses distinctive and desirable thermomechanical properties. Micro-arc oxidation (MAO) treatment of NiTi can effectively prevent the release of Ni ions from NiTi. In this paper, NiTi is treated with MAO method in Na₂SiO₃ electrolyte. MAO process of NiTi in Na₂SiO₃ electrolyte contains two stages: “growth period”, “jumped period”. During the process of MAO, Ni in NiTi is oxidized to Ni ion, and the Ni ions are dissociated in electrolyte. Ti was left in NiTi, which generate much Ti content appearing on the surface of the sample, and contribute to prerequisites for the reaction. After the applied voltage reaches a certain value, Na₂SiO₃ electrolyte participate in the reaction and form insulating amorphous silicon oxide layer. With the increase of thickness of insulating layer on NiTi, the anodic voltage increase. When applied voltage excess certain threshold, discharge spark appear on the surface of NiTi.

Abstract: This work demonstrates particularly the basic properties of shape memory alloys and gives a brief review about their basic crystallographic processes. The attributes of shape memory alloys will be presented through the NiTi alloys. The crystallographic principles of three unique properties were investigated and the functional mechanism described. One of the three essential mechanisms, the one-way effect was demonstrated through an experiment. The change of length depending temperature was tested and documented. The hysteresis behavior of shape memory alloys was recorded.

Abstract: Shape memory alloy (SMA) actuators have drawn much attention and interest due to their unique and superior properties, and are expected to be equipped in many modern vehicles at competitive market prices. The key advantage is that SMA actuators do not require bulky and complicated mechanical design to function, where the active element (e.g. SMA wire or spring) can be deformed by applying minimal external force and will retain to their previous form when subjected to certain stimuli such as thermomechanical or magnetic changes. This paper describes the SMA attributes that make them ideally suited as actuators in automotive applications and to address their limitations, feasibilities and prospects.