Binary NiTi Alloy Research Articles

The nonexistence of an order–disorder transition in near-stoichiometric TiNi alloy

It is usually believed that there is an order–disorder transition at 1090 °C in binary Ti–Ni alloys, as Honma et al. reported previously. The presence or absence of such transition is very important, because the result may greatly affect the martensitic transformation behavior, and the associated shape memory characteristics. In the present paper, we question whether such transition really exists or not. One of the difficulties in such experiments, including the work of Honma et al., lies in the fact that Ti can be easily oxidized at such high temperatures, which complicates the alloy behavior greatly and may lead to erroneous interpretation of experimental data. After doing carefully designed experiments on this point, we did not find any differential thermal analysis peak up to the melting temperature, or any drastic change in electrical resistance in a wide temperature region including 1090 °C, which is the characteristic of order–disorder transition. These observations lead to the conclusion that such order–disorder transition does not exist in Ti–Ni binary shape memory alloys.

Comparative X-ray and time-of-flight neutron diffraction studies of martensite crystal lattice in stressed and unstressed binary Ti–Ni alloys

An X-ray and time-of-flight neutron diffraction study of a Ti–50.0 at.% Ni alloy was performed. The B19′ martensite lattice parameters are different for the quenched martensite and for the martensite formed from the austenite containing a well-developed dislocation substructure due to transformation-induced hardening. A tensile stress increases the “underrecovery” of the d h k l values of planes perpendicular to the axis of tension (and of the “apparent” martensite lattice parameters calculated from these d h k l values) after a cycle of transformation-induced hardening compared with the corresponding values of the initial quenched martensite. A probable cause for the differences between d h k l and lattice parameters of martensite formed from highly dislocated austenite and corresponding parameters of quenched martensite formed from recrystallized austenite are tensile components of stress fields generated by the dislocation substructure and/or residual stresses of different origin which are present in the initial austenite.

Shape memory behavior of Ti–Ni–Cu thin films

Three kinds of Ti-rich Ti–Ni–Cu thin films (Ti 51.9Ni 41. 6Cu 6.5, Ti 51.6Ni 36.8Cu 11.6, Ti 51.5Ni 33.1Cu 15.4) and three kinds of (Ni, Cu)-rich Ti–Ni–Cu thin films (Ti 48.9Ni 44.9Cu 6.2, Ti 48.5Ni 40.0Cu 11.5, Ti 48.6Ni 35.9Cu 15.5), numbers indicate at.%, were prepared by sputtering and annealed at 773, 873 and 973 K for 1 h. X-ray diffraction patterns showed that the Ti-rich thin films contain Ti 2Ni and Ti 2Cu phases, while the (Ni, Cu)-rich thin films contain a TiNiCu phase. The B2 → B19′ martensitic transformation occurred below 10 at.% Cu, while the B2 → B19 martensitic transformation occurred above 10 at.% Cu. The transformation temperature increased with increasing Cu content and annealing temperature. The temperature hystereses were small compared with those of Ti–Ni binary alloy thin films. The maximum recoverable strain decreases with increasing annealing temperature. It also decreases with increasing Cu content when the Cu content is more than 10 at.%. The critical stress for inducing plastic deformation increases with decreasing annealing temperature and increasing Cu content.

Ripening of L12Ni3Ti precipitates in the framework of the trans-interface diffusion-controlled theory of particle coarsening

Abstract Data are presented on the kinetics of coarsening of γ′-type Ni3Ti precipitates (L12 crystal structure) in three binary Ni–Ti alloys containing 10.31, 11.84, and 13.72at.% Ti aged at 720°C for times up to 64 h. Data on the distributions of particle sizes (PSDs) are also presented. These data, as well as previously published data, are analyzed in light of a new theory of coarsening in which diffusion is controlled by transport through the non-sharp interface between the matrix and precipitate phases. The new theory, called the trans-interface diffusion-controlled (TIDC) theory of coarsening, predicts time (t)-dependent behavior of the type 〈r〉n ∝ t for the growth of precipitates of average radius 〈r〉 and XTi ∝ t−1/n for the depletion of the solute concentration of the matrix, XTi. The exponent n is intimately related to the width of the interface between the precipitate and matrix phases, δ, which is assumed to depend on the particle radius as δ ∝ m, where n = m + 2. The shape of the scaled distrib...

Two-Way Shape Memory Effect and Its Stability in a Ti–Ni–Nb Wide Hysteresis Shape Memory Alloy

The effect of training strain, deformation temperature and number of training cycles on the two-way shape memory effect (TWSME) and its stability have been investigated systematically in a Ti 46.3 N1 44.7 Nb 9 (at%) wide hysteresis shape memory alloy (SMA). The experimental results indicate that two-way shape memory strain increases with increasing the training strain up to 14.5% and then decreases with further increasing the training strain. When the training strain is less than 12%, the two-way shape memory strain increases with the increase of number of training cycles in the Ti 46.3 Ni 44.7 Nb 9 alloy deformed at the room temperature. Compared with TiNi binary alloy, the TWSME is smaller and the training strain to get the maximum TWSME is larger in the Ti-Ni-Nb alloy due to the deformation of β-Nb soft particles during training. However, the Ti-Ni-Nb alloy exhibits excellent stability of TWSME during the thermal cycling.

Vacuum Induction Melting of Ternary NiTiX (X=Cu, Fe, Hf, Zr) Shape Memory Alloys Using Graphite Crucibles

In the present study we investigate vacuum induction melting (VIM) of prominent ternary NiTiX (X ¼ Cu, Fe, Hf, Zr) shape memory alloys using graphite crucibles. We apply a melting procedure which was recently developed for binary NiTi alloys and which keeps the carbon pick-up during melting at a minimum. We investigate the microstructures of the as-cast and homogenized alloys using scanning electron microscopy (SEM) in combination with energy dispersive X-ray analysis (EDX). Differential scanning calorimetry (DSC) was performed to study the phase transformation temperatures of the as-cast and homogenized materials. The results show that VIM processing in graphite crucibles provides ternary NiTiX shape memory alloys with good chemical homogeneity and acceptable impurity contents.

Recent development of TiNi-based shape memory alloys

Recent developments in China on TiNi-based shape memory alloys (SMAs), including TiNi binary SMAs, TiNiNb wide hysteresis SMAs, TiNiCu narrow hysteresis SMAs, TiNiHf high temperature SMAs and TiNiRE SMAs were concisely reviewed. The damping characteristics and corresponding mechanisms of TiNi and TiNiNb alloys were described and discussed. Some surface modifications of TiNi binary alloys were employed to improve the corrosion resistance and the biocompatibility, which are very useful for the medical applications. Shape memory effect and mechanical properties of TiNiHf alloys were enhanced by aging Ni-rich TiNiHf alloys, while the Ms still remain enough high. The authors found that the addition of RE to TiNi alloys increases the phase transformation temperatures and even changes the transformation sequence. Fundamental research on the TiNi-based alloys is still in the ascendant, leading to increasingly extending of the applications.

Effect of Hydrogen on Damping Capacity of Ti<SUB>50</SUB>Ni<SUB>25</SUB>Cu<SUB>25</SUB> Alloy

Ternary Ti–Ni–Cu alloy is known to exhibit higher damping capacity than binary Ti–Ni alloy. Hydrogen doping was used to further improve the damping capacity of this ternary alloy. Considering ease of manufacture, the Ti50Ni25Cu25 alloy was produced by a lamination process. As a result of hydrogen doping, a new damping peak appeared at around 260 K. The peak temperature shifted with changes in oscillation frequency. The activation energy (H) and the relaxation time constant (� 0) of the damping peak were calculated according to the Arrhenius plot as H ¼ 57:5 kJ/mol, and � 0 ¼ 2:2 � 10 � 13 s, respectively. The damping capacity due to hydrogen doping increased with increasing hydrogen concentration up to 0.45 at%, showing the maximum peak value of tan � ¼ 0:05, but thereafter decreased. On the other hand, the peak temperature increased monotonically with increasing hydrogen concentration, and the peak shape was broader than the single Debye relaxation peak. From these results, we concluded that this damping peak was not the Snoek peak but the Snoek-Koester peak caused by hydrogendislocation interaction. The decrease of the Snoek-Koester peak with increasing hydrogen concentration is explained by formation of hydrogen clusters or hydrides. Schoeck’s theory for the Snoek-Koester peak may be applicable. On further hydrogen doping, both the Snoek-Koester peak and the transformation peak were hardly visible above 5 at% hydrogen. This was found to be due to destruction of the martensitic phase by excess hydrogen doping.

Ｔｉ‐Ｎｉ‐Ｃｕ合金の減衰能に及ぼす水素の影響

Ti-Ni shape memory alloy is one of the most promising materials among high damping alloys. Ti-Ni alloys show a transient damping peak due to martensitic transformation and an almost temperature independent damping in the martensitic phase. Since the transient damping peak is useless due to many limitations, we can utilize only the damping capacity of martensitic phase. In the present work, we doped hydrogen into a ternary Ti-Ni-Cu alloy, which is known to exhibit higher damping capacity than binary Ti-Ni alloy, to improve further the damping capacity. Ti50Ni25Cu25 alloy was produced by a lamination process from a high workability point of view. As a result of hydrogen doping, a new damping peak occurred at around 260 K. The temperature of damping peak was shifted with change in oscillation frequency. The activation energy (H) and relaxation time constant (τ0) of the damping peak were calculated according to the Arrhenius plot as H=57.5 kJ/mol. τ0=2.2×10-13 s. The damping capacity due to hydrogen doping increased with increasing hydrogen concentration up to 0.45 at% hydrogen showing tan φ=0.05, but thereafter decreased. On the other hand, the peak temperature increased monotonously with increasing hydrogen concentration, and the peak shape was broader than single Debye relaxation peak. From these results, we conclude that this damping peak is not the Snoek peak but the Snoek-Koster peak possibly caused by hydrogen and dislocation interaction effect; G. Schoecks theory for Snoek-Koester peak may be applicable.

Phase diagram of binary Ti-Ni alloys on Ti-rich side under a pressure of 2.2 GPa

Phase diagram of binary Ti-Ni alloys on Ti-rich side under a pressure of 2.2 GPa

Effect of pre-oxidation treatment on the corrosion behaviour of Ni–Ti alloys in eutectic (Li, K)2CO3 mixture at 650°C

The effect of pre-oxidation treatment at 650°C in air on the corrosion behaviour of binary Ni–Ti alloys containing 5, 10, and 15 wt-%Ti in eutectic (Li, K)2CO3 at 650°C in air has been investigated. The results indicate that the pre-oxidation of the alloys can produce an almost pure TiO2 scale. Nevertheless, this TiO2 scale cannot hinder the formation of NiO outside and beneath the TiO2 scale during subsequent molten salt corrosion. The corrosion rates of the alloys decrease with increasing content of titanium in the alloys. The corrosion of the alloys proceeds mainly by inward diffusion of oxygen through the TiO2 layer, forming large quantities of NiO beneath the TiO2 layer. Compared with Ni–5Ti and Ni–10Ti, the formation of NiO on Ni–15Ti is inhibited significantly, especially in the initial stages.

Ultrasonic investigation of the B2 ↔ B19 martensitic transition in a Ni40Ti50Cu10 alloy

Abstract The elastic constants C′, C44 and CL of a single crystal of the Ni40Ti50Cu10 (numbers indicate at.%) alloy have been measured in the premartensitic region (320–350 K) and have been found to exhibit softening on approaching the B2→B19 transition from high temperature. This means that these elastic constants decrease with decreasing T. The anisotropy factor A=C44/C′ is relatively small (≅2.2) over all the temperature range and slightly increases with decreasing temperature down to the transition start-temperature Ms. Thus, its temperature coefficient ∂A/∂T has opposite sign of that exhibited by the binary Ni50.8Ti49.2 alloy prior to the B2→B19′ transition. The present results support the view recently expressed by some authors that the B2→B19′ transition occurring in binary Ni–Ti alloys is a consequence of concomitant lattice instabilities against {1 1 0}〈1 1 0〉 B 2 and {0 0 1}〈 1 1 0〉 B 2 shear strains respectively corresponding to the C′ and C44 ultrasonic propagation modes.

Microstructures and mechanical properties of Ti-Ni-Cu shape memory alloy melt-spun ribbons

Ti-Ni shape memory alloy (SMA) thin films are suitable for microactuators due to their large shape recovery strain and stress. In this study, rapidly solidified Ti-Ni-Cu SMA ribbons containing 10at% Cu and 25at%Cu were fabricated by melt-spinning method followed by systematic heat treatment at temperatures below 1173K. Deformation behavior and microstructures were investigated. It was found that the as-spun ribbons were amorphous regardless of alloy composition. Such amorphous ribbon showed a high value of yield stress at 1.35GPa. The decrease of crystallization temperature was confirmed when increasing Cu-content from 10at% to 25at%. However, transformation temperature increased about 40K by increasing Cu-content to 25at%. It was also found that the ribbons heat-treated below 1023K were brittle regardless of alloy composition, while the ductility was improved when heat-treat above 1073K. Transformation temperature hysteresis was reduced to 10K by adding Cu to the Ti-Ni binary alloy: this characteristic is suitable for enhancing cooling rate in microactuators.

Microstructure and pseudo-elastic low-cycle high amplitude fatigue of NiTi

A binary NiTi alloy consisting of 50.7 at. % Ni was studied under cyclic tension. This alloy can show pseudo-elastic behaviour at room temperature (M s < RT < M d ). The following microstructural conditions were compared: I. homogenous β-phase (SHT: solution heat treated), 2. p-phase with small coherent particles (AHT: age heat treated), 3. p-phase with defects (dislocations and particles)(TMT: thermo mechanically treated). All structures show stress induced phase transformation during mechanical cycling at ambient temperature. They exhibit heterogeneous strain. The amount of heterogeneity is reduced by the TM-treatment. The change in the pseudo-elastic (PE) stress-strain curves, as well as of the transformation temperatures were analysed as a function of the number of mechanical cycles. The changes were interpreted in terms of formation and accumulation of secondary (i.e. fatigue induced) defects and / or residual martensite. The introduction of primary defects (i.e. as introduced by TMT) has the strongest effect on the stability against mechanical fatigue and change of the transformation temperatures.

Damping characteristics of TiNi binary and ternary shape memory alloys

High damping materials are currently attracting much attention in engineering applications. TiNi shape memory alloys (SMAs) could exhibit high damping capacity, as well as excellent shape memory effect and superelasticity. The high damping capacity of TiNi SMAs is mainly related to the hysteretic movement of martensite variants, twin planes and parent–martensite interfaces. The addition of a third element has a substantial effect on the damping capacity of TiNi SMAs. In this paper, the damping characteristics of the binary and ternary TiNi SMAs are systematically investigated. Also, effects of alloy’s microstructures and crystal defects on damping characteristics of TiNi SMAs are discussed.

On the in vitro biocompatibility of Elgiloy, a co-based alloy, compared to two titanium alloys.

The aim of the present investigation was to contribute to an understanding of the effects of surface topography and chemical composition on the corrosion behavior and thus the biocompatibility of Elgiloy (RMO, Denver, CO, USA), a common Co-based alloy. The results are compared with those obtained for a binary NiTi alloy, Neo Sentalloy (GAC, Central Islip, NY, USA) and a beta-III-Ti alloy, TMA (Ormco, Glendora, CA, USA). In the present study, the surface topography and the chemical composition of two different grades of Elgiloy, Blue Elgiloy (soft) and Yellow Elgiloy (ductile), were examined by means of scanning electron microscopy (SEM) and energy-dispersive spectroscopy analysis (EDS). Their corrosion behavior in half-strength Ringer solution and in an artificial saliva solution according to Barrett [1] was investigated using potentiodynamic corrosion testing (PDC). The photometry-based PAN method was used to quantify the released Ni and Co ions. The in vitro biocompatibility of the two grades of Elgiloy was tested in three different cell cultures: in L929, a commercially available mouse fibroblast cell line, and in primary human epithelial cells and fibroblasts. The results of the corrosion testing showed satisfactorily high pitting corrosion potentials but lower repassivation potentials and a strong increase in current density once pitting had occurred. The photometric results revealed the release of Ni and Co ions in both tested electrolytes. The tested native surfaces exhibited numerous grinding and polishing grooves, inclusions and inhomogeneities of the microstructure. After corrosion testing the same surfaces displayed numerous signs of corrosion, especially in areas with microstructural inhomogeneities. In vitro biocompatibility testing showed a substantially reduced dehydrogenase activity in the presence of Elgiloy. The reduced quality of surface finish resulting from the manufacturing process led in the case of the tested Elgiloy types to decreased corrosion resistance with consequently reduced in vitro biocompatibility. In this context it is also conceivable that patients with a proven allergy to nickel, cobalt or chromium may react sensitively to the deployment of this alloy, at least in the surface quality tested by us. From this aspect, the introduction of a binding standard for the surface quality of materials used in orthodontic appliances is urgently recommended.

Einfluss von Gefüge und Wärmebehandlung auf das Ermüdungsverhalten von Formgedächtnislegierungen / Influence of Microstructure and Thermal Treatment on the Fatigue Behavior of Shape Memory Alloys

Development and stability of the intrinsic two-way shape memory effect on three alloys with different microstructure are studied in this work. Wire specimens of a binary NiTi alloy, a CuAlNi alloy and finally a shape memory alloy with W-dispersoids in a NiTi matrix are investigated. After a training procedure the samples are subjected to 4000 two-way cycles. Changes in deformation behavior are discussed. Whereas the influence of work hardening on the magnitude and stability of the effect should be aim of the work with the binary alloy, results of different grain size are investigated in the CuAlNi alloy. The NiTiW alloy should deliver information about the effect of internal stress fields caused by the W-dispersoids on fatigue behavior. It is shown in this work that quite good stability of the two-way effect can be achieved by optimizing the microstructure. Certainly it must be admitted that any measure to improve the stability reduces the magnitude of the effect. Consequently an appropriate compromise has to be realized by combining a suitable microstructure and thermal treatment.

Relation between Tensile Deformation Behavior and Microstructure in a Ti-Ni-Co Shape Memory Alloy

Relation between tensile deformation behavior and microstructure in variously aged Ti–49.7 at%Ni–1.3 at%Co shape memory alloys has been investigated. Stress-strain curves for the alloys aged at 623 K and 723 K for 1.8 ks were of a work-hardening type, but those for the alloys aged at the two temperatures over 28.8 ks were nearly of a constant-stress type, as for binary Ti–51 at%Ni aged alloys. Lenticular precipitates were observed in both the 623 K and 723 K aged alloys, and the distance among the precipitates increased with increasing aging temperature and period, as well as the size of the precipitates. The lenticular precipitates were identified to be of the composition of Ti3(Ni, Co)4 from an EDX analysis. Based on these observations, the constant-stress type stress-strain behavior for the alloys aged over 28.8 ks was attributed to some composition change accompanied with the aging progress by which Ti:(Ni+Co) composition ratio in the matrix of the Ti–Ni–Co alloys approached 1:1, as in the equi-atomic Ti–Ni binary alloys.

Phase Transformation of Ti-Ni Containing Platinum-Group Metals

ABSTRACTSince the maximum shape recovery temperature of the binary Ti-Ni alloys is limited to be around 400K, the increase in martensitic transformation temperature (Ms) of Ti-Ni should be done by alloying for the demand of high temperature applications. Although most of additional elements are known to decrease Ms of Ti-Ni, substitutional elements having large atomic size are expected to increase Ms. In this study, phase constitution, phase transformation temperature, lattice parameter of B2 phase and Vickers hardness were investigated for Ti-Ni alloys containing several platinum-group metals (PGM). The alloy systems investigated were the pseudobinary systems of TiNi-TiRh, TiNi-TiIr and TiNi-TiPt where the PGM atoms substitute for the Ni-sites of TiNi. The phase transformation and phase constitution were assessed by differential scanning calorimetry (DSC), X-ray diffraction analysis (XRD) and transmission electron microscopy (TEM). It was found by XRD that TiNi can contain a large amount of the PGMs as Ti(Ni, Rh), Ti(Ni, Ir) and Ti(Ni, Pt). Lattice parameters monotonously increase with increasing amount of PGMs. With increasing Pt content, Ms slightly decreases when less than 10mol%Pt while continuously increases as the rate of 26K/mol%Pt when more than 10mol%Pt. On the other hand, Ms decreases and then disappears with increasing Rh or Ir content. Hardness ranges from HV180 to HV570 and the maximum values in the pseudobinary systems lie around 20–30mol%PGM, suggesting solid solution hardening caused by the substitution of PGMs.

Corrosion of Ni–Ti alloys in the molten (Li,K) 2CO 3 eutectic mixture

The corrosion of pure Ni and of binary Ni–Ti alloys containing 5, 10, and 15 wt.% Ti respectively in molten (0.62Li,0.38K) 2CO 3 at 650°C under air has been studied. The corrosion of the single-phase Ni–5Ti alloy was slower than that of pure Ni, forming an external scale composed of NiO and TiO 2. The two-phase Ni–10Ti and Ni–15Ti alloys underwent much faster corrosion than pure Ni, producing an external scale containing NiO and TiO 2, and a thick internal oxidation zone of titanium mainly involving the intermetallic compound TiNi 3 in the original alloys. The rates of growth of the external scales for the Ni–Ti alloys were reduced with the increase of their titanium content, while the internal oxidation was significantly enhanced. The corrosion mechanism of the alloys is also discussed.