NiTi Shape Memory Research Articles

Multi-strengthening effects on the martensitic transformation temperatures of TiNi shape memory alloys

The multi-strengthening effects, introduced by combining cold rolling, aging and thermal cycling, on the martensitic transformation temperatures of Ti50Ni50 and Ti49Ni51 shape memory alloys have been studied by using DSC and microhardness measurements. Experimental results show that both pre-cold-rolling of Ti50Ni50 alloy and aging treatments of Ti49N51 alloy can impede the further introduction of dislocations during the thermal cycling and hence effectively reduce the effect of thermal cycling on transformation temperatures. For Ti49Ni51 alloy, whether the cold rolling is conducted before or after the aging treatment, the strengthening of cold rolling can significantly increase the specimen hardness but only slightly affect the following thermal cycling effect. The multi-strengthening effects on the martensitic transformation temperatures are also found to follow the equation of Ms = T0−KΔσy.

Preliminary investigation of the effects of surface treatments on biological response to shape memory NiTi stents

Preliminary investigation of the effects of surface treatments on biological response to shape memory NiTi stents

Preliminary investigation of the effects of surface treatments on biological response to shape memory NiTi stents

Preliminary investigation of the effects of surface treatments on biological response to shape memory NiTi stents

Reduction in KI by Shape Memory Effect in a TiNi Shape-Memory Fiber-Reinforced Epoxy Matrix Composite. Dependency of Stress Intensity Factor on Crack-tip Domain Size.

Shape memory TiNi fiber reinforced/epoxy matrix composite is fabricated to demonstrate the suppression effect of crack-tip stress concentration and the control of fracture toughness K value of the composite. The decrease of fracture toughness was attributed to the compressive stress field in the matrix which was induced when the prestrained TiNi fibers contract to the initial length upon heating above austenitic finish temperature of TiNi fiber (>Af). The dependencies of K values on the prestrain value of TiNi fibers as well as crack-tip compressive stress domain size are discussed based on the analytical equivalent inclusion model for composite.

Microstructural modifications induced by swift ions in the NiTi intermetallic compound

The conditions for track formation by irradiation with swift ions, in the shape memory NiTi metallic compound are studied. We observe that tracks are only induced in the monoclinic structure but not in the cubic one and only when the linear rate of energy deposition by electronic excitation ([d E/d x] e) exceeds 46 keV/nm. We show that the tracks are amorphous in their centre and that a decrease of the monoclinic towards cubic transformation temperature is observed at the periphery of the track region. For [d E/d x] e=32 keV/nm, no individual tracks can be observed at low fluences, only a monoclinic → cubic structure transformation is observed at high fluences. If [d E/d x] ⩽ 17 keV/nm, swift ions are unable to induce any visible structural modification by electronic excitations. The track formation in this alloy is discussed within the framework of the classical Coulomb explosion and thermal spike models.

Ti-NiおよびTi-Pd形状記憶合金のマルテンサイトにおける第II種双晶の界面構造

Ti-NiおよびTi-Pd形状記憶合金のマルテンサイトにおける第II種双晶の界面構造

Reduction in K1 by Shape Memory Effect in a TiNi Shape-Memory fiber-Reinforced Epoxy Matrix Composite.

Shape-memory TiNi fiber-reinforced/epoxy matrix composites have been fabricated, and the suppression of crack-tip stress intensity and the change in fracture toughness have been systematically investigated. Stress-strain data for these composite specimens with notches at various angles and different crack lengths in the transverse direction have been measured in tensile tests. The stress intensity factor at the crack tip is experimentally determined from photoelastic fringe patterns. The decreases inK values are attributed to the compressive stress field in the matrix induced when the pre-strains of the TiNi fiber contract to their initial length upon heating above the austenitic final temperature. We present the influences of the pre-strain of TiNi fibers and the compressive domain size between a crack tip and fiber on theK value.

Material Properties and Strengthening Mechanism in Shape Memory TiNi Fiber Reinforced Al Matrix composite

In the present paper, it is attempted to reconfirm the Intelligent material properties using both the sintered TiNi/Al(1100) matrix composite made by powder metallurgy method and the squeeze-casted TiNi/Al6061 specimens. A metal matrix composite is, its fault has been considered to deteriorate a strength of composite by heating residual stress of the matrix. Therefore, it is necessary to remove a tensile residual stress, to produce the strength of a composite better. On the contrary, if compressive residual stress happens in matrix of composite in place of tensile residual stress, it will make the strength of composite better. So that, this paper introduce the development of a high strength of composite, by using compressive residual stress well, on the study. By using these specimens, shape memory strengthening effects in tensile strength and fatigue crack propagation above inverse transformation temperature of TiNi fiber were investigated. We occurs the prestrain and volume fraction for to discuss the effects of a composite strength. Moreover, by SEM observation, the effect of the residual stress at the interface between Al matrix and TiNi fiber and some brittle precipitation layers such as inter metallic compounds on fracture mechanisms was discussed metallurgically.urgically.

Design and Material Evaluation of Shape Memory Composites

Shape memory alloy (SMA) is regarded as one promising intelligent material element natively with thermo-sensor, shape memory actuator functions. A fundamental design concept for intelligent materials using SMAS is firstly proposed. Two types of composites which show some intelligent functional properties relating to the shape memory phase transformation were fabricated. One is the shape memory TiNi fiber/Al matrix plate. Another is the plaster matrix composite as a model of building material which contains both shape memory TiNi fibers and high damping TiNi particles having different phase transformation temperatures with each other. The former (TiNi)f/Al composite showed unique properties such as self-strengthening by the compressive stresses in Al matrix due to shape memory shrinkages of the embedded TiNi fibers. The increases of tensile property (yield stress) as well as fatigue resistance (crack growth retardation) at higher temperature were experimentally confirmed. The latter (TiNi),f/plaster showed the increases of resistance to fracture as well as vibration damping.

A new shape memory Ni-Ti alloy stapes prosthesis coated with Teflon®

A new shape memory Ni-Ti alloy stapes prosthesis coated with Teflon®

Martensitic Transformation in Thin Foil Specimen of a Shape Memory TiNi Alloy

Since shape memory alloys are applied in various fields, especially in the micro-machine field, and the dependence of size effect on shape memory phenomena of the alloys has been reported, it should be important for using the alloys actually to clarify the quantitative relationship between the thickness and the occurrence of martensitic transformation of the alloys. In the present paper, the relationship for TiNi (its Ms temperature is 248 K) thin film specimens suitable for TEM observation was investigated by in-situ TEM and SEM methods using cooling stages, and a convergent beam electron diffraction method. The results obtained are as follows: (1) At 98 K, martensitic transformation does not occur in areas thinner than about 100 nm; (2) The Ms temperature of the thin film is considerably lower than that of the alloy in the bulk-state; and (3) The interval between As and Af temperatures decreases with decreasing specimen thickness, and in the case of a thin film specimen suitable for TEM observation, the interval becomes almost zero.

Shape memory TiNi synthesis from elemental powders

Commercially, the shape memory alloy TiNi is produced either by vacuum induction melting or by vacuum arc remelting of the pure metal ingots. Powder metallurgy techniques provide an alternative fabrication route but problems arise in achieving chemical homogeneity. In this study TiNi compacts were cold-pressed from the blended elemental powders and sintered in vacuum for varying times at temperatures from 800 °C to 1000 °C. Two heating rates were used, 5 K min −1 and 10 K min −1. A TiNi microstructure could be produced after annealing at 1000 °c for 6 h, although some TiNi 3 was still observed. This is likely to be difficult to completely remove as TiNi 3 is thermodynamically more stable than TiNi. Thus, homogenization is unlikely to be completed by solid-state diffusion processes. The martensitic B19′ structure was observed to be highly oriented after processing.

Study of the martensitic transformation in shape-memory Nitinol proton-irradiated alloy by electrical resistivity measurements

The results of a study of proton-induced martensite and its parent structure in the shape-memory Ni-Ti alloy (Nitinol) by electrical resistivity measurements are reported. The irradiation was carried out with 2 MeV protons with a flux of 1.4 × 10 18 protons cm −1 for 26 h. The electrical resistivity results for the irradiated sample, measured under similar conditions as for the unirradiated sample, indicate that irradiation sharpens the transformation and increases the observed value of the transformation onset. The observations are discussed in terms of the interaction of martensite nuclei with lattice defects produced by irradiation. Our results also indicate that the structure changes produced by proton irradiation in this alloy are not permanent.

Enhanced Mechanical Properties of TiNi Shape Memory Fiber/Al Matrix Composite

A design concept of shape memory TiNi fiber reinforced/Al metal matrix composite (SM-MMC) was proposed. Mechanical tensile properties such as stiffness and yield strength, were improved by the strengthening mechanisms: back stress in the Al matrix induced by stiffness of TiNi fibers and the compressive stress in the matrix caused by shape memory shrinkage of TiNi fibers. Damping capacity of the composite was also increased. These results suggest that this composite with prestrain can be applicable and is suitable for machinery, especially engine components where the material becomes stronger at higher temperatures owing to the shape memory effect

A new thermal shape memory Ti-Ni alloy stent covered with silicone.

An attempt was made to develop an airway stent for nonsurgical transluminal implantation using a bronchofiberscope. The stent is composed of a single wire with a diameter of 0.5 mm. The wire is made of thermal shape memory titanium nickel alloy, with a transition temperature of 25-30 degrees C. To avoid direct contact between the metal and the tissue, the wire is covered with a 300 microns thick coating of silicone. The stent is horseshoe shaped in cross-section. It is 15 mm in outer transverse diameter and 40 mm long. Ten stents were implanted in 10 dogs whose tracheal cartilages had been previously broken. The stents were first cooled in ice water to reduce their diameter and then inserted into the target site. The wire was warmed to body temperature and recovered its initial shape within 10 sec. The dogs were killed for examination between 1 week and 6 months after implantation. All the stents were located at the implantation sites and were patent. Tissue reactions, such as ulceration and granulation formation, were less severe than in those with previously implanted non-silicone covered stents. Microscopic observation showed that the wires became gradually covered with epithelium within 2 months. This transluminal technique for preserving airway patency shows promise for clinical application.

Adaptive Damping in Shape Memory TiNi during Cavitation

AbstractRecent studies by this author and others has demonstrated that cavitation-erosion of NiTi coatings or bulk NiTi is exceptional. Studies were undertaken to ascertain whether this property is a consequence of either the general intermetallic properties of NiTi or by an adaptive stress-dissipation mechanism of the cavitation-generated shock wave by A microstructural mechanism related to the shape memory effect.In cavitation, an oscillating pressure field causes the formation and implosion of air bubbles. As a surface easily nucleates bubbles, the subsequent implosion of the bubbles generates stresses approaching several MPa, which are large enough to ablate material, and are also high enough to generate stress-induced Martensite or Austenite, depending on whether the applied stress is tensile or compressive. The implication is that the stress wave may be partially accommodated by the stress-induced transformation, which can dissipate the energy as heat on retransformation to the materials unstressed phase.Calculations concerning the variation of the cavitation-induced stresses and temperature distribution with depth into the TiNi coupled with the associated problems of heat transfer will be presented. It will be shown that an adaptive mechanism is plausible.

MECHANICAL AND ELECTRICAL PROPERTIES OF A NiTi SHAPE MEMORY ALLOY

Mechanical properties of shape memory NiTi alloys have been widely investigated: though a reasonable understanding of the micromechanisms underlying macroscopic behaviour is known, these sometimes can hardly be inferred from macroscopic stress- strain curves. Electrical Resistance (ER) measurements, extensively used to investigate thermal martensite, are here performed on NiTi alloys under stress state. Attention is here focused on the mechanical and electrical behaviour of a NiTi alloy in one start single phase: -in martensitic phase, at T Af or within the hysteresis cycle. ER, sensitively affected by electronic structure modifications, lattice structure defects, residual impurities, can profitably be exploited to enlighten similar macroscopic stress-strain behaviour. ER results here obtained allow to discriminate different active micromechanisms, specifically at a same temperature within the hysteresis cycle where similar stress- strain curves are obtained.

Study of the martensitic transformation in shape-memory nitinol alloy by time-of-flight neutron diffraction techniques

The present paper reports a study of thermally induced martensite (M) and its parent (P) structure in the shape-memory NiTi alloy (Nitinol) by neutron time-of-flight diffraction. The specimen was a 0.6-mm-dia wire in which the martensitic reaction occurred at 33 ± 7°C with zero applied stress. We made seven measurements in a closed cycle of temperature between − 18 to 90°C, using neutrons with wavelengths between 0.72 and 4.15 Å, sorted into 120 channels of 1.5% resolution. Neutron scattered at angles between 30 and 60° were detected by an area detector. The provided scattering data were used in these programs for obtaining the spectrum of the Bragg peaks for given angle θ and T, the lattice constants of the unit cells of both P and M phases, and the specimen texture at a given T. We found, as did previous authors, that the P phase has a CsCl-type cell with the lattice constant 3.032 ± 0.5%, and the M phase is monoclinic which can be formed with both P112 1/ m and P12/ c 1 unit cells. The lattice constants of these cells were found to deviate ±0.4 to ±1.4% from their average values. Although the martensite is predominantly long-range ordered with either Ni or Ti atoms at the cell corners, our data lead us to suggest that in our specimen there are some small fractions of this structure in which the cell corners are occupied by both Ni and Ti atoms in four different ordered arrangements. These domains, together with the atomic positions deduced from our results, eliminate the controversies among the models proposed by previous X-ray and TEM workers, and unifies their findings with our experimental results. The Bragg peak line spectra obtained with a given angle θ, reveal that during the P → M reaction, each peak of the P phase is replaced by two peaks, each of which is composed of a number of peaks of the M phase. We have phenomenologically formulated this so-called “Bp transition”, and identified some families of habit planes. The “transformation twins” are discussed, and it is found that the habit planes {100} are the sources of the two twin systems (1 10) [111] and (1 12) [111] which are formed on {111} planes. The branching angles of all twins are estimated to be within the range 57–62°. The cycle of M → P → M transformations revealed a hysteresis of about 15°C, but no anomaly or premartensitic effects in the Bragg peak spectra.

Shape Memory NiTi Alloys - Applications in Dentistry

Shape Memory NiTi Alloys - Applications in Dentistry

SHS synthesized Ni-Ti based shape memory alloys for both low- and high-temperature applications

The thermal explosion mode of self-propagating hightemperature synthesis (SHS), or combustion synthesis, was used to successfully produce NiTi (Nitinol) shape memory alloys (SMA) for the first time [1]. The explosion mode of ignition of the powder pellet mix is preferred to the combustion mode because it can provide combustion temperatures higher than the melting point of the synthesized intermetallic, thereby providing a cast product [2]. Compared with the conventional process of melting the two metals in an induction furnace, the SHS method provides advantages with respect to both time and energy-savings and easier processing. In this letter, the results of producing shape memory alloys with a range from low- to high-transition temperatures using the SHS technique are reported. Patent