Binary NiTi Alloy Research Articles

Ti-Ni SMA thin film microactuators made on a Si substrate

Among several types of performance materials proposed for fabricating microactuators, Ti-Ni shape memory alloys have great advantages such as large deformation and recovery force. The intrinsic disadvantage of shape memory alloys, the slow response due to the limitation of cooling rate can be greatly improved in micro size actuators, because the surface/volume ratio of materials will drastically increase in thin films so that cooling becomes efficient. Therefore, the development of Ti-Ni shape memory thin films has been proposed in the field of micromachines. In the present investigation, Ti-Ni shape memory alloy thin films were deposited by sputtering on Si substrates covered by SiO2 layer. The Ti-Ni thin films were crystallized by heat-treatment in order to achieve high transformation temperatures. The Ms and As temperatures were 340K and 365K,respectively. This Ms is the highest temperature achievable in the binary Ti-Ni alloy and about 50K higher than room temperature, enabling that a high frequency response is achieved. Square diaphragm type microactuators were fabricated by the photolithography method with anisotropic etching applied to the Si substrate. The dimension of the sides of the squares ranges between 100 and 1,000 micronmeters. The structure of the diaphragm consists of a 1.5 micronmeters thick Ti-Ni film and a 1.0 micronmeter thick SiO2 layer. Joule heating and natural cooling methods were applied to move the actuators. The microactuators moved up and down upon cooling and heating, respectively. The highest position at the center of the actuator was about 30 micronmeters, while the lowest was 0 micronmeters. The highest frequency detected was 100Hz.

Microstructural effect on pseudoelastic behaviour of binary Ni-Ti alloys

The effect of various heat treatments on the microstructure and pseudoelastic property of two binary Ni-Ti alloys with different Ni contents were investigated in this work. Some samples of each alloy were produced with three different heat treatments (a) solution heat treated, (b) solution heat treated and aged (c) marformed and aged,. Marforming process was carried out by cold rolling some of the samples in martensitic state. The temperature dependence and reversibility of transformatio n induced deformation (pseudoelastic behaviour) as well as the conventional yield stress were obtained by tensile test. The formation of Ni rich precipitates {NUTi3) was only observed in the alloy with higher Ni content following the ageing process. It was concluded that the microstructure produced by marforming and ageing result in the optimum pseudoelastic properties.

Comparative study of degradation effects in different shape memory alloy systems

The presented investigations aim at clarifying the influence of microstructure and applied training procedure on the stability of the created intrinsic two-way shape memory effect (TWSME). The investigations are carried out on three different alloy systems in order to get an appropriate variation of the microstructure with respect to dislocation density, second phase particles and grain size. A uniaxial TWSME is induced by a thermomechanical treatment in wire specimens of the alloys NiTi, CuAlNi and a two phase alloy composed of a NiTi matrix and dispersoids of tungsten. Subsequently, thermal cycles are carried out, continuously observing the changes in the deformation behaviour. The results suggest that the training must result in both, a large pseudoplastic strain as well as a pronounced irreversible strain in order to obtain a TWSME of high magnitude. The degradation of the TWSME in the NiTi-based alloys was found to consist of two characteristic stages, which can always be recognised, independent of the preliminary heat treatment. The obvious strengthening effect of the particles in the two-phase alloy NiTi-W has a very positive effect on the stability of the created TWSME. This effect can even be improved by preliminary cold working. The stability of the effect in the CuAlNi alloy is far better then in the binary NiTi alloy. No dramatic change in the degradation rate is observed throughout the investigated period of 5000 thermal cycles.

Stress/temperature phase diagrams as a tool for shape memory alloy selection and processing

One of the difficulties of applying shape memory alloys to real applications is dealing with how applied stress affects the transformation temperatures and phase sequence. This paper presents stress-temperature phase transformation diagrams of the type described by Todoroki, Tamura and Suzuki [1]. The data employed to construct these diagrams relates to a previous study of binary NiTi alloys that have varied alloy compositions, prior cold work and heat treatment temperatures [2]. The diagrams presented in this paper graphically display how phase sequence as well as transformation temperatures vary with processing and alloying parameters. In addition, it is shown that the stress-temperature diagrams change after repeated thermal transformations against applied stress. Increasing M, temperatures and decreasing A, temperatures result in narrower thermal hysteresis and loss of the intermediate R-phase.

Internal structures and shape memory properties of sputter-deposited thin films of a Ti–Ni–Cu alloy

Low-temperature heat treatments of the sputter-deposited amorphous films, which were previously proved to be a new method to produce very good shape memory properties for Ti-rich Ti–Ni alloys, have been applied to a ternary Ti–43.0Ni–6.2Cu alloy (at.%). The basically same nanometric structures as in the binary alloy are formed, i.e. the nanometric structures consist of extremely thin plate precipitates of bct structure, which are formed on {100} planes of the parent B2 structure and have the c-axis normal to the habit planes. High-shape recovery stresses of about 500 MPa with recoverable shape strains of 5% are obtained without accompanying any permanent strains. A shape recovery stress of more than 870 MPa is attained if it is allowed to involve about 1% permanent strain. Although these bct precipitates have large tetragonalities, they are perfectly coherent with the parent bcc lattice. The maximum shape recovery stress is nearly twice that of the Ti-rich Ti–Ni binary alloy having a similar nanometric structure. It is suggested that this remarkable increase in recovery stress may be attributed to the change in Burgers vector of dislocations caused by partial disordering in Ti–Ni–Cu alloys. It is emphasized that the shape recovery stress in this ternary alloy is four times that of the Ti 2Ni containing samples and 10 times that of a bulk Ti–45Ni–5Cu alloy.

Microstructure and tensile properties of two binary NiTi-alloys

The shape memory effect (SME) and superelasticity (SE) after marforming (deformation in the martensitic phase at 203 K, followed by annealing at 713 K, 0.5 h) were studied in the equiatomic TiNi crystals under compression. Marforming was carried out along [0 0 1]B2 and [0 1 1]B2 directions of B2-phase after strain of ε = 1.5ε0 (ε0 is the lattice deformation, which depends on the crystal orientation). SME and SE after marforming were studied along [0 0 1]B2 direction. It was shown that the maximum stress level of B2-phase σcr(B2) = 750 MPa, the lowest values of mechanical Δσ = 100 MPa and thermal ΔTh = 32 K hysteresis and the maximum temperature range of SE ΔTSE = 77 K were observed when marforming under compression was realized along the [0 0 1]B2 direction.

Phase transformation behavior and wire drawing properties of Ti-Ni-Mo shape memory alloys

Transformation behavior and wire drawing properties of Ti-Ni-Mo shape memory alloys have been investigated by means of differential scanning calorimetry (DSC) measurements, X-ray diffraction, electron microscopy, tensile tests and wire drawing tests. Mo addition to a Ti-Ni binary alloy induced the R phase transformation, and consequently Ti-Ni-Mo alloys showed two stage transformation, i.e., from the B2(cubic) parent phase to the R(rhombohedral) phase, and then from the R phase to B19′(monoclinic) phase. In the thermo-mechanically treated 51Ti-48.3Ni-0.7Mo alloy, reverse transformation temperature, Af, kept constant, irrespective of thermo-mechanical treatment conditions, while it changed in the thermo-mechanically treated 51Ti-49Ni and 51Ti-48.5Ni-0.5Mo alloys. Mo addition to Ti-Ni binary alloy decreased wire drawing stress. Wire drawing stress decreased with raising intermediate annealing temperature monotonously when the annealing treatment was made in vacuum. When the annealing treatment was made in air, however, it decreased with raising annealing temperature up to 923 K, and then increased. Optimum intermediate annealing temperature of Ti-Ni-Mo alloys for wire drawing was 823 K, above which a thick oxide film which reduced the drawability of the alloys was formed on the surface of alloy wires.

Recent development of TiNi-based shape memory alloys in Taiwan

TiNi-based alloys are considered to be the most important shape memory alloys (SMAs) because of their salient shape memory effect (SME), pseudoelasticity (PE) and high damping capacity (HDC). Recent investigations focus on their transformation behaviors, thermal-mechanical treatments, manufacturing processes, and industrial applications based on the characteristics of SME, PE and HDC. The two-stage martensitic transformations of B2↔R-phase↔B19′ and B2↔B19↔B19′ have been clarified for both TiNi binary and ternary alloys. The deformation behaviors have been investigated by cold-rolling, hot-rolling and wire drawing. Both SME and PE can be improved by thermal-mechanical treatments, and the damping characteristics of TiNi and TiNiX SMAs have been systematically studied. Both B19′/B19 martensite and R-phase have high damping capacities due to the easy stress-induced movement of twin boundaries. The high temperature SMAs, Ti–Ni–X with X=Au, Pd and Zr alloys, have also been intensively studied. The ion nitriding technique has been used to improve the wear and corrosion resistance of TiNi and TiNiX SMAs. TiNi thin films have been successfully fabricated using the sputter-deposition technique. These investigations on the TiNi-based SMAs have attracted much attention and their important characteristics will be applied widely in the near future.

Microstructure and Transformation Behavior of Ni<sub>50</sub>Ti<sub>50-x</sub>Ta<sub>x</sub> Alloys

The effect of Ta addition on the microstructure, phase transformation temperature and mechanical properties of Ni 50 Ti 50-x Ta x alloys have been studied in this paper. Beginning with a base composition of Ti 50 Ni 50 , Ti was substituted by Ta up to 15at%. A DSC, an optical microscope and SEM were used to characterize the constituent phases in those alloys. The results showed that the ternary Ni 50 Ti 50-x Ta x (x=2∼15at%) shape memory alloys have lower transformation temperatures than the binary NiTi alloy. The microstructure of Ni 50 Ti 50-x Ta x alloys changes as a function of Ta content as follows: B 19 , phase → B 2 + β-Ta phase + Ti-rich dendrites.

Study of Ti-Ni-Nb Shape Memory Alloys with a Wide Hysteresis

It has been shown that TiNiNb shape memory alloys exhibit a wide transformation hysteresis compared to TiNi binary alloys (about 145°C versus 30°C). This wide hysteresis has important engineering utility. The present paper was undertaken to review of the microstructure. mechanical behavior, deformation-induced martensitic transformation and the effect of deformation on the stability of martensite and its reverse transformation in TiNiNb alloys. Simultaneously, the strain recovery characteristic and interface structure of martensite were explained, and the mechanism for increasing martensite stability by deformation was discussed. In addition, the microstructure and interface structure inside the stress-induced martensite (SIM) variants were evaluated. It is found that the morphology and substructure of SIM depend on deformation condition. The tensile deformation at parent phase condition has great influence on the substructure and interface structure of SIM variants. The most distinctive substructural feature of the stress-induced martensite is the presence of the various secondary twins. Based on the analysis of the stability and shape memory effect of various kinds of martensite, the concept of stability of deformed stress-induced martensite was proposed and explained by the mechanism for the formation of several secondary deformation twins.

Inversion of acoustic emission asymmetry accompanying martensitic transformations in titanium nickelide alloys

Asymmetry of the acoustic emission indicates that the processes of nonchemical energy scattering differ in the direct and reverse martensitic transformations. An analysis of the inversion of the emission asymmetry with increasing nickel concentration in binary Ti-Ni alloys and in the course of multiple transformation cycles suggests that in a direct transformation nonchemical energy is dissipated by plastic relaxation whereas in a reverse transformation it is dissipated by dynamic relaxation. Significantly plastic relaxation degrades to saturation and dynamic relaxation begins to appear exclusively.

Marforming and tempering of binary Ni–Ti alloys including precipitation effects

The influence of plastic deformation in the martensitic state followed by tempering was studied. The amounts of true plastic deformation up to ϕ=1,1 were obtained by cold rolling. To achieve complete transformation into the martensitic state, Ni rich alloys with low transformation temperatures were cooled in a solution of liquid nitrogen and alcohol below Mf. The effects of workhardening on strength and transformation behaviour were investigated by differential scanning calorimetry, tensile tests and micro-hardness measurements, as well as by light and electron microscopy. Increasing deformation leads to higher reverse transformation temperatures (As, Af). Finally the alloys lose their martensitic retransformability (dead martensite), i.e. As and Af reach the temperature range in which diffusion effects become considerable. By tempering at 200<TMT<800°C the austenitic state, consequently reversible martensitic transformability and shape memory, is regained by passing through several stages. These are: recovery in martensite, nucleation and the coarsening growth of austenite. In addition to these processes, the precipitation in martensite or austenite was examined by transmission electron microscopy. The tensile tests provided evidence for the improvement of strength properties and fatigue resistance by some thermomechanical treatments.

Strain dependence of pseudoelastic hysteresis of NiTi

This work investigated the transformation-strain dependence of the stress hysteresis of pseudoelasticity associated with the stress-induced martensitic transformation in binary NiTi alloys. The strain dependence was studied with respect to the deformation mode during the stress-induced martensitic transformation, which was either localized or homogeneous. It was observed that the apparent stress hysteresis of pseudoelasticity was independent of the transformation strain within the macroscopic deformation range, for the specimens deformed in a localized manner. For specimens macroscopically deformed uniformly, the stress hysteresis of pseudoelasticity increased continuously with increasing strain from the beginning of the stress-induced martensitic transformation. The transformation-strain independence of the stress hysteresis for localized deformation is ascribed to be an artificial phenomenon, whereas the transformation-strain dependence of the hysteresis for uniform deformation is believed to be intrinsic to the process of stress-induced martensitic transformation in polycrystalline materials. This intrinsic behavior is attributed to the polycrystallinity of the materials.

A study of TiNiV ternary shape memory alloys

The TiNiV ternary shape memory alloys, obtained by equal substitution of V for both Ti and Ni, are investigated focusing on their basic transformation behavior, shape memory effect, pseudoelasticity and wear characteristic. Experimental results reveal that the Ti49.25−x/2Ni50.75−x/2Vx (x=0–4 at%) alloys exhibit a B2↔B19′ one stage martensitic transformation. The transformation temperatures will drop down about 10°C by adding 1–2 at% V due to the effect of solid-solution strengthening. There appear many (Ti,V)2Ni second-phase particles within the matrix of Ti47.75Ni49.25V3 and Ti47.25Ni48.75V4 alloys. The oxygen atoms in the matrix will be easily absorbed by the (Ti,V)2Ni second-phase particles to form the (Ti,V)4Ni2O oxide. This decreased oxygen content in the matrix will contribute to raise the transformation temperatures for both Ti47.75Ni49.25V3 and Ti47.25Ni48.75V4 alloys. The shape memory effect and pseudoelasticity of Ti49.25Ni50.75 alloys can be improved by the addition of 1–2 at% V due to solid-solution strengthening. The energy storage efficiency (E2/E1+E2) can also be increased by the addition of V. However, the Ti47.75Ni49.25V3 and Ti47.25Ni48.75V4 alloys exhibit a worse shape memory effect due to the formation of second-phase particles. The wear mechanisms of Ti49.25−x/2Ni50.75−x/2Vx alloys are found to be similar to those of TiNi binary alloys. The Ti49.25Ni50.75, Ti48.75Ni50.25V1, Ti48.25Ni49.75V2 alloys have a better wear resistance than the Ti47.75Ni49.25V3 and Ti47.25Ni48.75V4 alloys, due to their higher hardness and pseudoelastic behaviors of the B2 structure.

The Effect of Hafnium and Thermal Cycling on the Transformation Temperatures of NiTi-Based Shape Memory Alloys

AbstractThe effect of thermal cycling on the austenite to martensite (A→M) and martensite to austenite (M→A) transformation temperatures (TTs) of Ni49.8Ti50.2 and Ni49.8Ti40.2Hf10 shape memory alloys is investigated. Test specimens are from arc melted buttons homogenized at 900°C for 100 hours. Results show that the A→M and M→A TTs for the alloys decrease with increasing number of thermal cycles through the phase transformations. The A→M TT of the NiTiHf alloy stabilizes more quickly than the A→M TT of the NiTi alloy when cycled. However, the M→A TT of the binary NiTi alloy stabilizes more quickly than the M→A TT of the NiTiHf alloy when cycled.

Interaction between microstructure and multiple-step transformation in binary NiTi alloys usingin-situtransmission electron microscopy observations

Abstract The multiple-step transformation is a special case of martensitic transformation observed in NiTi alloys. It is characterized by the appearance of several transformation steps during the transformation of the R phase into martensite. The present study shows that this effect can be observed after the sample has been solution treated at 1173K and further aged at 793 K and is accompanied by a special microstructure which consists in a well recrystallized matrix containing small coherent precipitates of the Ni4Ti3 type. These precipitates have been observed to be the source of a local stress field which influences both the nucleation and the growth of the R and martensitic phases. Based on the present observations, a model has been developed which explains the basic of the mechanisms involved in this phenomenon. According to this model, the additional transformation step can be considered to be a locally stress-induced martensitic transformation. This model is extended to interpret some results found in the literature.

Interaction between microstructure and multiple-step transformation in binary NiTi alloys using in-situ transmission electron microscopy observations

Abstract The multiple-step transformation is a special case of martensitic transformation observed in NiTi alloys. It is characterized by the appearance of several transformation steps during the transformation of the R phase into martensite. The present study shows that this effect can be observed after the sample has been solution treated at 1173K and further aged at 793 K and is accompanied by a special microstructure which consists in a well recrystallized matrix containing small coherent precipitates of the Ni4Ti3 type. These precipitates have been observed to be the source of a local stress field which influences both the nucleation and the growth of the R and martensitic phases. Based on the present observations, a model has been developed which explains the basic of the mechanisms involved in this phenomenon. According to this model, the additional transformation step can be considered to be a locally stress-induced martensitic transformation. This model is extended to interpret some results found in the literature.

Effects of additions of iron and nickel on structure and properties of aluminium alloy : K.Matsuki et al. (Toyama University, Japan.) J. Jpn Inst. Light Metals, vol 46, no 4 1996, 189–194. (In Japanese.)

Nanocomposite particles of Fe-TiN, Ni-TiN, and Co-TiN were synthesized by arc melting, vaporization, and deposition of binary Fe-Ti, Ni-Ti, and Co-Ti alloys in an atmosphere of a nitrogen-hydrogen mixed gas at 0.1 MPa. The morphology of the nanocomposite particles is dumbbell-like and/or dice-like particles. Thermal stability of the nanocomposite particles is extremely superior to that of the metal particles. The hydrogen desorption characteristics of the nanocomposite particles are entirely different from those of the metal. Temperature-programmed desorption (TPD) of hydrogen for the nanocomposite particles show the spillover of hydrogen. The amount of hydrogen adsorption is dependent on exposing temperature.

Superelasticity and Impact Properties of Two Shape Memory Alloys : Ti50Ni50 and Ti50Ni48Fe2

Superelasticity phenomena and Impact behaviour have been studied on Ti 50 Ni 50 and Ti 50 Ni 48 Fe 2 shape memory alloys. Both alloys have been made by powder metallurgy (combustion synthesis). The main objective of the present work was to induce superelastic properties for temperatures ranging from -25°C to 0°C on a TiNi-based alloy by the addition of 2% Fe. It is well known that superelasticity properties are enhanced by the occurence of premartensitic R phase transition. On the other hand, Fe addition decreases the martensitic transformation temperature (Ms) promoting the occurence of intermediate premartensitic R phase. For binary TiNi alloys, cold working and partial recovery heat treatment is necessary to obtain such as properties. This paper describes the microstructure, superelasticity and Charpy Impact behaviour of both Ti 50 Ni 50 and Ti 50 Ni 48 Fe 2 . Results have shown that: Impact energy curves display unexpected trend for testing temperatures ranging from - 196 to + 50°C. In particular, for TiNiFe alloy, typical Impact energy values are: 25J/cm 2 at -196°C, 4J/cm 2 at -25°C (the minimal value obtained) and 20J/cm 2 at +20°C. Complementary fractographic examinations have shown a brittle fracture mode for test temperatures ranging from -25 to 0°C. On the other hand, these temperatures correspond to the maximum of superelasticity, that is 1.5 to 2% of reversible strain measured by loading and unloading during tensile test. These results show a high brittleness probably induced by the intermediate R phase. Consequently, the optimization of superelasticity should be detrimental to the ductility/toughness of the TiNi alloys.

Sputter deposition of TiNi, TiNiPd and TiPd films displaying the two-way shape-memory effect

TiNi, TiNiPd and TiPd films exhibiting the one-way and two-way shape-memory effect have been prepared by d.c. magnetron sputtering onto unheated substrates followed by annealing and training processes. In the case of TiNi, films could be prepared showing the R-phase transition, the important feature of which is its small hysteresis of about 1 K. By the Ni-Pd substitution the transition temperatures (austenite/martensite finish temperatures) could be increased from 32 °C/–38 °C for the binary NiTi alloy to a maximum of 570 °C/498 °C for TiPd films. The films have been subject to further microstructuring for developing micro-actuators displaying the two-way shape-memory effect.