Memory Alloy NiTi Research Articles

Electrophoretic deposition of polyetheretherketone (PEEK) and PEEK/Bioglass® coatings on NiTi shape memory alloy wires

Polyetheretherketone (PEEK) and PEEK/Bioglass® coatings were produced on shape memory alloy (NiTi, Nitinol®) wires using electrophoretic deposition (EPD). Best results were achieved with suspensions of PEEK powders in ethanol in the range (1–6 wt%), using a deposition time of 5 minutes and applied voltage of 20 Volts. EPD using these parameters led to high quality PEEK coatings with a homogeneous microstructure along the wire length and a uniform thickness of up to 15 μm without development of cracks or the presence of large voids. Suspensions of PEEK powders in ethanol with addition of Bioglass® particles (0.5–2 wt%) (size < 5 μm) were used to produce PEEK/Bioglass® coatings. Sintering was carried out as a post EPD process in order to densify the coatings and to improve the adhesion of the coatings to the substrate. The sintering temperature was 340 °C, sintering time 20 min and heating rate 300 °C/h. Sintering led to uniform and dense PEEK and PEEK/Bioglass® coatings without any cracks. The bioactive behaviour of PEEK/Bioglass® composite coatings was investigated by immersion in acellular simulated body fluid (SBF) for up to two weeks. As expected, hydroxyapatite crystals formed on the surface of the coated wires after 1 week in SBF, confirming the bioactive character of the coatings. The results have demonstrated for the first time that EPD is a very convenient method to obtain homogeneous and uniform bioactive PEEK and PEEK/Bioglass® coatings on Nitinol® wires for biomedical applications.

The influence of a poly-Si intermediate layer on the crystallization behaviour of Ni-Ti SMA magnetron sputtered thin films

Shape memory alloy Ni-Ti thin films as sputtered are amorphous if the substrate is not intentionally heated during deposition. Therefore, these films have to be heat treated to induce crystallization in order to exhibit the shape-memory effect. Several films have been prepared by dc magnetron sputtering and then studied concerning the influence of the type of substrate (single-crystal Si, polycrystalline Si) on the crystallization kinetics and the final structure. The structural development of the films during crystallization (at a constant temperature of 430 °C) has been studied by X-ray diffraction in grazing incidence geometry off-plane (GIXD) at a synchrotron-radiation beamline. These experiments allow us to establish a correlation between the deposition conditions and the kinetics of crystallization. For films deposited at an electrode distance of 70 mm on a Si(100) substrate, a longer crystallization time is needed compared with films obtained at 40 mm, for otherwise fixed deposition parameters. The analysis of the nucleation kinetics by using the Johnson–Mehl–Avrami equation leads to exponents between 2.6 and 3. The presence of an intermediate layer of poly-Si drastically enhances the crystallization process. Additionally, ex situ annealing of identical samples at 500 °C during 1 h and complementary characterization of the structure and morphology of the films by cross-sectional transmission electron microscopy and selected-area electron diffraction were performed. The temperature dependence of the electrical resistivity was measured, identifying the phase transformation temperature ranges. An increase of the overall resistivity with the precipitation of Ni4Ti3 has been detected. Results obtained by X-ray reflectometry and GIXD suggest that during crystallization excess nickel is driven into an amorphous region ahead of the crystal/amorphous interface, thus leading to a higher concentration of Ni at the surface and further precipitation of Ni4Ti3.

Property Change During Fixtured Sintering of NiTi Memory Alloy

ABSTRACT Fixtured sintering of up to 5 hours at 1223 to 1323 K is successfully used to diminish the dimensional change of NiTi memory alloys produced from mixtures of elemental Ni and Ti powders packed and pressed at room temperature under 400, 500, and 600 MPa pressure. The improvements obtained can help near-net-shape technology via powder metallurgy that overcomes the traditional casting problems such as oxygen, nitrogen, hydrogen, and carbon absorption and intermetallic compound precipitation, which lower the workability and the homogeneity of the final alloy. The effects of compaction pressure, sintering time, and sintering temperature on dimensional change, porosity, hardness, and morphology of the produced TiNi intermetallic alloy are investigated. Phase transformation temperatures and mechanical properties of the sintered samples were determined and compared for the influence of compaction pressure and sintering conditions. It was concluded that suitable thermoelastic and superelastic effects are achievable via appropriate selection of fixtured sintering conditions.

Improvement Of Corrosion Resistance AndBio Compatibility Of Ni-Ti Shape MemoryAlloy By Various Surface Modifications

Nickel-titanium shape memory alloy (Ni-Ti alloy) has unique mechanical properties such as shape memory effect and super elasticity along with good wear and corrosion resistances. For this reason Ni-Ti alloy is expected to be used as implant material in the biomedical field. However, the biocompatibility of the Ni-Ti alloy is not fully examined. The purpose of this research is to evaluate in vitro cytocompatibility and to try to improve corrosion resistance for Ni-Ti alloy by various surface modifications. Ni-Ti alloy showed good cytocompatibility compared with that of a Ti-6Al-4V alloy, judging from the small amount of hIL-1β and LDH generations. Biocompatibility of the Ni-Ti alloy is superior compared with pure nickel. Also, it was made clear that the biocompatibility of the Ni-Ti alloy showed almost the same level as that of stainless steel, which gave a lot of actual results as a biomaterial until now. Therefore, it was understood that there is no problem in the biocompatibility of the Ni-Ti alloy for application as a biomaterial. Moreover, corrosion resistance of in air oxidized Ni-Ti alloy was recognized to be greatly improved.

Investigation on the fracture behavior of shape memory alloy NiTi

This work investigates the fracture behavior of shape memory alloy NiTi (50.7 at. pct Ni) at room temperature. Macroscopic mechanical tests, microscopic in situ observations of tensile fracture processes by scanning electron microscopy (SEM), and detailed analyses of fracture surfaces were carried out. The results reveal that specimens with different thicknesses show various shape memory effects and superelasticities. The main crack with a quasi-cleavage mode that combines cleavage with ductile tearing is initiated at the notch tip and is stress-control-propagated in line with the direction of the maximum normal stress. The microstructure has little effect on the direction of crack propagation, but coarser substructures show lower resistance to the crack propagation. In specimens with various types of notches, various notch acuities present different effects on the crack initiation and propagation and result in different fracture behaviors.

Behavior of the shape memory alloy NiTi during one-dimensional shock loading

The response of alloys based on the intermetallic compound NiTi to high-strain-rate and shock loading conditions has recently attracted attention. In particular, similarities between it, and other shape memory materials such as the alloy U–6%Nb in the propagation of the plastic wave in Taylor cylinders are of significant interest. In this article, the Hugoniot is measured using multiple manganin stress gauges, either embedded between plates of the NiTi alloy, or supported with blocks of polymethylmethacrylate. In this way, the shock stress, shock velocity, and details of the shock wave profile have been gathered. An inflection at lower stresses has been found in the Hugoniot curve (stress-particle velocity), and has been ascribed to the martensitic phase transformation that is characteristic of the shape memory effect in this alloy. In a similar way, the variation of shock velocity with particle velocity has been found to be nonlinear, contrary to other pure metal and alloy systems. Finally, a break in slope in the rising part of the shock profile has been identified as the Hugoniot elastic limit in NiTi. Conversion to the one-dimensional stress equivalent, and comparison to quasistatic data indicates that NiTi exhibits significant strain-rate sensitivity.

Superelastic NiTi Memory Alloy Micro-Tube under Tension— Nucleation and Propagation of Martensite Band

Superelastic NiTi Memory Alloy Micro-Tube under Tension— Nucleation and Propagation of Martensite Band

Computational Methods for Smart Structures and Materials

SECTION 1: ANALYSIS TOOLS - On the modelisation of the piezoelectric effect with control system and some applications Simulation of flexible fibers by discrete cyllindrical segments An adaptive fuzzy logic control synthesis Fundamental solutions for infinite transversely isotropic piezoelectric media. SECTION 2: PHYSICAL SYSTEMS MODELLING AND ANALYSIS - Design and analysis of a travelling wave ultrasonic motor for space applications Finite element based modelling and analysis of structronic systems A finite element approach for multilayered plates including piezo-layers Tuned liquid damper (TLD) using heavy mud On the analytical solution of a cubic nonlinear structure controlled by linear tuned mass damper. SECTION 3: SENSOR AND ACTUATOR TECHNOLOGIES - Piezoelectric stiffener-actuators for the suppression of the vibration of laminated composite beams Thick-film strain transducers made using piezoelectric paint Evaluation of piezoelectric active control systems for vibrations suppression. SECTION 4: DAMAGE DIAGNOSIS - Smart structure integrity monitoring using transient response Truss bridge fatigue evaluation using acoustic strain gauges Diagnosis of structural damage in smart structures Cumulative damage diagnosis of smart structures under dynamic loading. SECTION 5: ACTIVE AND PASSIVE CONTROL - On the optimal placement of piezoelectric elements for active flutter suppression Active noise control by modal state feedback using piezo-electric actuators Passive structural control strategies using seismic steel energy dissipators On the sound radiated by electrical equipment:an active structural acoustic control of harmonic sound. SECTION 6: INTELLIGENT CONTROL SYSTEMS - Applications of quasi-optimizing control method to structural response control systems for seismic excitations Wind-resistant active structural response control by installing active rotor fin system. SECTION 7: ADAPTIVE MATERIALS AND STRUCTURES - Optimum design of deployable structures using genetic algorithms The effect of shape memory alloys in vibration suppression of a cantilever beam Generation and stability of reversible shape change in NiTi memory alloys Electrical discharge machining of smart materials parts.

The use of shape memory alloy NiTi particles in SnPbAg matrix: interfacial chemical analysis and mechanical characterisation

Abstract Uncoated and copper electroless coated fine NiTi superelastic particles have been incorporated into a liquid SnPbAg matrix. The wettability of the Cu coating by the molten alloy was determined with a wetting balance. The composite interfaces have been investigated by Auger electron spectroscopy (AES). Uncoated NiTi reacted with the matrix to form Ni–Sn intermetallics, whereas the Cu layer of coated particles behaved as a sacrificial layer, leading to a small interfacial zone containing neither Cu–Sn nor Ni–Sn intermetallics. The mechanical characterisation of Cu-coated NiTi/SnPbAg composites shows a reinforcement of the composite material compared to the monolithic matrix, and also an unusual increase of the elongation at rupture. These results can be interpreted with the superelastic properties of the NiTi shape memory alloy (SMA) particles.

Calorimetric analysis of the two way memory effect in a NiTi alloy—experiments and calculations

In addition to the well known one-way memory effect in heating, some shape memory alloys may also exhibit a shape recovery upon cooling, i.e. they show the two-way memory effect (TWME). This is not an inherent property of such alloys, and to obtain it, the alloy must be trained. Besides the two-way memory effect, the training processes can also cause changes such as a shift of the transformation temperatures. In this work a calorimetric investigation of a one-way and two-way NiTi memory alloy is presented. The heat flow was measured by use of a Differential Scanning Calorimeter. The differences in the temperatures and enthalpy of transformations between one-way and two-way memory samples are presented, compared and discussed. A mathematical prediction of the heat and temperatures of transformation is presented.

Acoustoplastic effect in the shape memory alloy Ni–Ti–Re at ultrasonic frequency

The influence of ultrasonic vibrations on the deformation and phase transformation in the shape memory alloy Ni–Ti–Re was studied. The decrease in the sample's length under superimposed ultrasonics in the interval of phase transformation temperatures was measured. Acoustoplastic effect in the shape memory alloy was investigated. The increase in the hardening coefficient and the decrease in the stage of the pseudoelastic deformation under superimposed ultrasonics was found.

Generation and stability of reversible shape change in NiTi memory alloys

Shape Memory Alloys (SMA) are promising materials for actuator components. This work focuses on the principal aspects underlying the deformation phenomenon known as Two-Way Shape Memory Effect (TWSME). On wire specimens of a near equiatomic NiTi alloy such an effect is achieved by a thermomechanical treatment. Depending on the training parameters and heat treatment conditions, different magnitudes of reversible shape change are obtained. Further experimental work concentrates on the stability of this special effect throughout cyclic application. Again a strong influence of the initial microstructure on the degradation characteristic is observed.

Crystal structures of the shape memory alloy NiTi: Comments on recent publications in Acta metall. mater

Crystal structures of the shape memory alloy NiTi: Comments on recent publications in Acta metall. mater

Deformation behaviour of NiTi shape memory alloys in bending

The deformation behaviour of memory alloys is exactly that of a linear-elastic and ideal-plastic material. A short report on the description of bending deformation of such a material in continuum mechanics is given. According to the parameters of geometry and material the bending deformation of NiTi memory alloy is first illustrated and later characterized by its parameters and limitations.

HREM observation of interface between precipitates and martensite of reversible shape memory alloy Ni-Ti

Since the reversible shape memory (RSM) alloys were found in 1960, a number of investigations and applications have been carried out and the mechanism have been discussed extensively. Ni-Ti is one of the typical RSM alloys. Many researches were concentrated on martensite structure and the relationship between martensite and austenite. However, the interface structures between martensite and precipitates have been rarely studied. In the present paper, the interface structures between precipitates ( NiTi2 and Ni3Ti) and martensite were investigated in atomic scale by using nigh resolution electron microscopy.49.5at%Ni-50.5at%Ti alloy whose martensitic transformation temperature is 75°C was annealed at 900°C for 2 hours to allow the components to become uniformly distributed, then quenched into water at room temperature. The specimens for electron microscopy observation were prepared by spark cutting and electropolishing. A JEM-2000EX and JEM-4000EX operated at 200kv and 400kv, respectively were used in the present observations.

Structural and Electrical Properties of Titanium-Nickel Films Deposited Onto Silicon Substrates

AbstractThin films of the shape memory alloy NiTi have been sputter-deposited onto ptype silicon substrates. Films that are initially amorphous may be crystallized by vacuum annealing. The crystalline films exhibit the B2-&gt;B19′ phase change associated with the shape memory effect while remaining in contact with the silicon substrate. Transition temperatures were determined by resistance measurements and x-ray diffraction. The NiTi - Si contacts are diodes, as evidenced by their current-voltage characteristics; however, the effect of the phase change on the barrier height could not be determined.

High resolution TEM observation of shape memory alloy Ni-Ti at room temperature to high temperature

Many investigations and applications of the reversible shape memory (RSM) alloys have been carried out since they were found in 1960 and the mechanism have been discussed extensively.In order to see the structure change of RMS-alloys in atomic scale on either side of the temperature of phase transformation, Ni-Ti alloy whose transformation temperature is 74 C has been heated inside of the 400kV (JEOL) electron microscope from room temperature to 230 C and atomic structure change has been recorded in TV and photographic films.Fig. 1(a) shows a structure of the electro-polished thin film of Ni-Ti alloy in (001) projection at room temperature, in which a martensitic structure is seen. As indicated in the figure, repeating twin structure whose twin boundary (T.B) is (100) and periodic black and white (B-W) contrast anomaly parallel to (110) in each twin bands are seen.Fig.1(b) is the enlargement of a region S in (a), in which the lattice fringes of (010) are seen on either side of T.B and B-W. Very small fringe shift is seen between B-W bands.

Temperature-induced precipitations in the memory alloy NiTi

The structure and growth conditions of aging-induced precipitations in polycrystalline near equiatomic NiTi are examined by neutron powder diffraction. The molar amount of the precipitates after different heat treatments is determined. The influence of stoichiometry on crystallisation and other physical properties are discussed.

Powder neutron diffraction study of nickel-titanium martensite

The structure of the martensitic phase of the shape memory alloy NiTi has been determined from a power neutron diffraction study at 10K. The lattice is monoclinic, space group P21/m(C2h2) with a=2.884(2) AA, b=4.110(2) AA, c=4.665(3) AA, beta =98.10(4) degrees and Z=2. The structure has been refined to an RWP=0.12; remaining discrepancies are associated with shape memory effects and are discussed in relation to the phase transition.

Lattice imaging study of a martensite-austenite interface

The lattice planes up to, and through, a martensite-austenite interface are revealed directly for the first rime by the high resolution, lattice image technique of transmission electron microscopy. Both conventional and unique data may be derived by suitable analysts of such images. Thus, in the shape memory alloy NiTi, the orientation relationship and habit plane are found to be identical to those established by conventional experiments. The two phases are seen to be fully coherent at the end of the growing martensite plate with the first interfacial dislocation occurring about 260 A back from the plate tip. Lattice distortions adjacent to the plate are detected but no significant features are demonstrated at the end of the plate where growth had ceased. Characteristic premartensitic contrast could not be associated with significant static lattice displacements. Whether or not transformation dislocations exist in the martensite-austenite interface requires further, more detailed investigation.