Binary NiTi Alloy Research Articles

Formation of structure of TiNiCu alloys with high copper content upon producing by planar flow casting

The rapidly quenched alloys of the quasibinary intermetallic TiNi-TiCu system with a high copper content (more than 25 at.%) are of great interest as shape memory materials due to the possibility of a significant decrease in the temperature and deformation hysteresis in comparison with the binary TiNi alloy. To obtain alloys with a copper content of 25 to 40 at.%, the planar flow casting technique was used. The alloys were fabricated at a melt cooling rate of about 106 K/s in the form of ribbons 30-50 μm thick and wide in the range from 7 to 20 mm. The study of the structure of the alloys was carried out using X-ray diffraction analysis, scanning and transmission electron microscopy. It was shown that from the ribbon side, contacting the quenching wheel, all alloys are amorphous, while on the non-contact side of the ribbons of alloys with 25 and 30 at.% Cu, a thin surface crystalline layer with a B2 structure is observed. Using energy dispersive X-ray spectroscopy, it was found that the content of the alloy components in the amorphous and crystalline phases coincides.

Assessment of diffusion mobility for bcc phase of Ti–Al–Ni ternary system

The experimental diffusion coefficients of the Ti–Ni binary and Ti–Al–Ni ternary bcc phase were critically evaluated to assess the diffusion mobilities by the DICTRA-type phenomenological treatment. The calculated diffusion coefficient shows reasonable consistency with the diffusion coefficient extracted in the experiment, including the impurity and inter-diffusion coefficients in bcc Ti–Ni binary alloy and the main interdiffusion coefficient in bcc Ti–Al–Ni ternary alloy. The difference between the calculated and experimental ternary cross coefficients is within appropriate relative errors. The predicted composition profiles and diffusion paths reasonably represent the diffusion processes that occur in Ti–Ni binary and Ti–Al–Ni ternary diffusion couples, thereby validating the mobility database assessed in the present work.

First-principles investigation on the adsorption and dissociation of O2 and H2O molecules on the Ni-rich TiNi alloy surface

The excellent corrosion resistance of Ti-Ni binary alloy highly depends on the thin passive films spontaneously formed on the surface. For the Ni-rich 60NiTi, the TiNi3 phase would coexist with the B2 TiNi matrix phase after the final heat treatment, affecting the formation of passive films. In this study, the adsorption and dissociation of O2 molecule on the TiNi (110) and TiNi3 (110) surface are investigated by the DFT method to have an insight into the distinction of oxidation behavior between TiNi and TiNi3 phase. Subsequently, the interaction of H2O molecule with the most stable O2 adsorption configuration is further studied in order to clarify the atomistic evolution process when the oxidized surface is exposed to the wet air and erosion solution. The calculated results implied that the O2-TiNi/H2O system exhibits the physisorption nature of the H2O molecule. However, the H2O molecule, when interacted with the O2-TiNi3 configuration, can be dissociated into OH group and H atom. These then were captured to create the Ti-O, Ni-O and H-O bond. This study may provide the useful details involved in the formation of oxide/alloy interface on the Ni-rich TiNi alloy as well as the interaction of oxide/alloy with the water solution.

Large thermal hysteresis NiTi Belleville washer fabricated by metal injection moulding

ABSTRACT In this work, Ni49.4Ti50.6 Belleville washers are prepared by metal injection moulding (MIM). The martensitic transition temperatures and service performance of washers were investigated. The thermal hysteresis has reached 63.1–71.6°C, which is much higher than documented data in binary NiTi alloys. Ti2Ni and Ni4Ti3 phase are observed in XRD patterns, which is responsible for the large thermal hysteresis. In the service tests, the restoring forces are higher than 40 kN, which are sufficient to ensure a tight connection of the fasteners. The loop resistance and high-current test have proved the safety of MIM NiTi Belleville washers compared to traditional steel spring washers, which can meet the challenge in complex outdoor circumstances.

Volume effect upon martensitic transformation in Ti29.7Ni50.3Hf20 high temperature shape memory alloy

Thermoelastic martensitic transformation of Ti29.7Ni50.3Hf20 alloy was studied by high-resolution and in situ real-time neutron diffraction in the temperature range from 25 °C to 250 °C upon heating and cooling. It was shown that B19′ martensite phase is characterized by strong anisotropy of thermal expansion, lattice parameters of aB19′ and bB19′ increase while cB19′ and β angle decrease with increasing of temperature. Coefficients of thermal expansion for martensite and austenite were determined. The volume effect of transformation in Ti29.7Ni50.3Hf20 alloy is 0.75% which is six times larger than that of binary TiNi alloy.

Hydrogen diffusion and the effect of hydrogen on structural transformations in binary TiNi based alloys

The paper analyzes the effect of electrolytic hydrogenation on martensite transformation temperatures in binary TiNi alloys. The analysis shows that this effect can be strong or weak depending on the phase state of TiNi. Research data are presented on the diffusivity of hydrogen in binary TiNi alloys in martensite and austenite states. The diffusion coefficient of hydrogen is estimated from its distribution measured by glow discharge spectroscopy in TiNi after hydrogenation. The experimental results about the formation of TiNiH hydride in the martensitic and austenitic state in binary TiNi based alloys are also presented.

Martensitic transformation of Ti50(Ni50\u2212xCux) and Ni50(Ti50\u2212xZrx) shape-memory alloys

Martensitic transformation and phase stability of Ti50(Ni50−xCux) and Ni50(Ti50−xZrx) shape memory alloys are investigated based on density functional theory (DFT). According to the results of formation energy we calculated, upon substitution of Ni by Cu at levels of about 10.4 at.%, Ti50(Ni50−xCux) alloys lose the monoclinic martensite in favor of the orthorhombic martensite structure. The martensite monoclinic B19´ structure of Ni50(Ti50−xZrx) becomes more stable with increasing of the Zr content. The energy difference between austenite and martensite decreases when Cu < 10.4 at.%, and then increases slightly, which suggesting that Cu addition reduces the composition sensitivity of martensitic transformation temperature comparing with binary NiTi alloys. The energy difference decreases slightly firstly when Zr < 10.4 at.% and then increases sharply, which indicates that Zr addition increases martensitic transformation temperature dramatically. Furthermore, a geometric model is used to evaluate the thermal hysteresis. More interestingly, it is found that the lowest thermal hysteresis is achieved at 10.4 at.% for Cu-doped NiTi; whereas the thermal hysteresis increases with increasing of Zr. The electronic structures of austenite phase are also discussed in detail.

Effect of Ternary Addition of Cobalt on Shape Memory Characteristics of Ni–Ti Alloys

Shape memory alloys (SMAs) are a class of metallic materials that exhibit two unique characteristics: shape memory effect and superelasticity. These alloys are capable of responding to stimuli like heat when they are subjected to thermo-mechanical treatment. Therefore, these alloys have proven their utility in many areas and are very common in applications such as sensors and actuators, various life-saving medical devices and stents. Ni–Ti alloys are in the forefront with respect to shape memory materials and have been employed in many practical applications. In recent times, there have been attempts to improve their shape memory characteristics, mechanical properties and corrosion resistance through alloying addition, thermo-mechanical processing, severe plastic deformation, etc. Among these methods, ternary alloying addition seems to have a greater effect in comparison with other methods. In this work, cobalt was added to a binary Ni–Ti alloy and its influence on transformation temperatures and microstructure was studied. Ternary NiTiCo alloys with varying cobalt contents (1, 2, 3 at.%) were prepared by vacuum induction melting, and the alloys were then hot-rolled and homogenized. Various characterization techniques such as differential scanning calorimetry, transmission electron microscopy and X-ray diffraction were employed to study the transformation temperatures, the microstructure and the phase structure of the ternary SMAs. It was seen that cobalt addition decreased the phase transformation temperatures and influenced the functional properties of the binary Ni–Ti alloy. Elaborate results have been discussed in this paper.

Influence of Cobalt on the Hot Deformation Characteristics of an NiTi Shape Memory Alloy

Among shape memory alloys (SMAs), Ni–Ti alloys are more widely used in multifarious sectors because of their functional properties of shape memory effect and superelastic effect. With an emphasis on miniaturization and higher performance of actuators and biomedical devices these days, there is a necessity to develop SMAs with high stiffness, i.e., high upper/lower plateau stress, and high modulus. In recent times, there have been attempts to improve their shape memory characteristics, mechanical properties and corrosion resistance through alloying, thermal treatment, etc. In this context, addition of Co has been made to binary Ni–Ti alloys. But till date, there has been a dearth of literature on hot deformation characteristics of NiTiCo SMAs. This work therefore investigates the role of cobalt on hot deformation characteristics of an Ni–Ti SMA. The addition of cobalt results in raising the yield strength and reducing the Ms temperature of the alloys. In this study, elemental forms of high-purity nickel, titanium and cobalt were melted together in a vacuum induction melting furnace to produce cylindrical rods. These rods were homogenized at 900 °C for 3 h and quenched to room temperature in water and subsequently machined so as to obtain a length-to-diameter ratio of 1.5, which was ideal to carry out hot deformation studies on a Gleeble thermomechanical simulator. The samples were deformed at different strain rates (0.01, 0.1, 1 and 10 s−1) at temperatures of 800, 900, 1000 and 1100 °C, respectively. Elaborate results on the deformation characteristics of an NiTiCo SMA have been discussed in this paper.

Correlative SANS and TEM investigation on precipitation kinetics of H-phase in Ni50.3Ti29.7Hf20 high temperature shape memory alloy

NiTiHf alloys are emerging as promising candidates for high-temperature shape memory actuator applications. The H-phase precipitation in these alloys significantly influences their mechanical strength and transformation temperatures. In this paper, we report a complementary study using time-resolved Small-Angle Neutron Scattering (SANS) along with transmission electron microscopy, to probe the H-phase evolution as a function of aging conditions in the Ni50.3Ti29.7Hf20 alloy. While, electron microscopy provided direct structural information about the precipitates, SANS analysis allowed in probing quantitatively the temporal evolution growth and coarsening of H-phase. It is demonstrated that while the diffusion of Ni atoms in the matrix predominantly governs the growth regime, the coarsening kinetics is primarily controlled by Hf diffusion. Further, relatively sluggish coarsening of the H-phase in this alloy, as compared to Ni4Ti3 precipitates in Ni-rich binary NiTi alloy, has been attributed to the slow migration of Hf in the matrix.

High temperature deformation behavior and processing map for a nickel-titanium shape memory alloy

The hot deformation behavior of 49.2Ti-50.8Ni shape memory alloy was studied using hot compressive deformation testing in the temperature range of 1023-1323 K and at strain rates of 0.01-10 s-1. The work-hardening rate was induced to analyze the stress-strain curves, and the critical stress σc and the dynamic recovery saturation stress σsat were measured which can be specified approximately by the expressions: σsat-1.12σp and σc-0.86σp. An Arrhenius model was calculated to describe the relationship between peak stress and the Z parameter. The relationship between deformation activation energy, the deformation conditions and the effect of Ni component in a binary TiNi alloy on the activation energy were discussed in this work. With the help of electron backscattering diffraction, a connected mode dynamic recrystallization microstructure was confirmed in peak efficiency regimes (850 °C & 0.01 s-1 and 1050 °C & 10 s-1) of the processing map.

First principles study of the effect of Cu doping on the martensitic transformation of TiNi alloy

As is well known,copper is such an unbelievable element that it can affect the phase transition behaviors of binary TiNi alloy when it displaces Ni element up to near upon 25%.The martensitic transition behaviors of TiNi1-xCux alloys appear from high-temperature cubic B2 phase to intermediate B19 structure with orthorhombic system and then finally to low-temperature B19' phase with monoclinic system with x 10% on cooling,so called two-stage martensitic phase transformation.Whereas,it directly transforms into orthorhombic B19 phase withx 20% on cooling,so called one-stage martensitic phase transformation.The orthorhombic B19 phase becomes final low-temperature phase while monoclinic phase will be unstable on cooling.The electronic structures and the formation energies of various point defects, Mulliken bond orders,etc.are studied for TiNi1-xCuxx alloys,however,the phase transition pathway at an atomic level has not been described at all,and further,the difference in transition pathway between TiNi and Ti1Ni1-xCuxx has not been understood so far.In this work,we optimize the crystal structures of TiNi and Ti50Ni25Cu25 alloys with initial geometry from experimental data.In order to choose the proper positions of Cu atom,we calculate the total energy of each doping system and find the most stable configuration.To study the transformation mechanism of TiNi,we calculate the phonon-dispersion spectra of each phase with both frozen-phonon method and linear response method,and then find the atomic vibrations with the imaginary frequency.Finally,with the help of this atomic vibration direction with negative frequency,we find the intermediate structures by the linear interpolation method and calculate their total energies.The phase transformation of TiNi from cubic to orthorhombic phase is driven by the phonon softening at the M point (0.5,0.5,0) of Brillouin zone.For orthorhombic and monoclinic phase,TiNi has real phonon frequencies for all k points and modes.A barrier of 1.6 meV is calculated between orthorhombic and monoclinic phase while no barrier is found between cubic and orthorhombic phase of TiNi,so it is easy to transform from cubic to orthorhombic and then to monoclinic phase.There exists a potential energy barrier of 10.3 meV at least between orthorhombic and monoclinic phase for Ti50Ni25Cu25,which is too high for its transition to overcome the maximum value of potential energy which corresponds to =93.4.The difference in transition pathway between TiNi and Ti50Ni25Cu25 accords well with the experimental measurement,so that the copper concentration with 25% in binary TiNi alloy will offer a new transition path from cubic to orthorhombic phase.

Relationship between crystallographic compatibility and thermal hysteresis in Ni-rich NiTiHf and NiTiZr high temperature shape memory alloys

The relationship between the crystallographic compatibility of austenite and martensite phases and the transformation thermal hysteresis (ΔT) of Ni-rich Ni50.3Ti29.7Hf20 and Ni50.3Ti29.7Zr20 alloys undergoing B2–B19′ martensitic transformation was studied as a function of microstructure, via differential scanning calorimetry, transmission electron microscopy, and X-ray diffraction. An experimental linear relationship of ΔT vs λ2 (the second eigenvalue of the transformation stretch matrix) was observed for these NiTi(Hf/Zr) alloys, but with a shallower slope as compared to the universal behavior followed by alloys showing B2–B19 martensitic phase transformation. Several ternary NiTiCu and binary NiTi alloys undergoing the B2–B19′ transformation were also found to deviate from the universal behavior attributed to alloys that undergo a B2–B19 transformation, and instead, follow the trend revealed for the present alloy systems. Aged NiTi(Hf/Zr) samples, which consist of very fine nano-precipitates, followed the newly established ΔT vs λ2 linear relationship, due to only minor changes in the microstructure. In contrast, samples with large precipitates, exhibited a large deviation from this relationship due to much more drastic changes in microstructure. Finally, despite the poor crystallographic compatibility of the austenite and martensite lattices observed in the present alloys, rationalized by large deviation of λ2 values from 1, relatively low ΔT values were measured. This behavior is actually consistent with the newly established relationship for ΔT vs λ2 for B2–B19′ transforming alloys, which is much less sensitive to compatibility (shallower slope). It is concluded that such a difference in the ΔT vs λ2 slope must be a consequence of the crystallography of monoclinic martensite formation in NiTi-based alloys as long as other factors such as plasticity or major constraints to the martensitic transformation do not intervene.

Deformation of a super-elastic NiTiNb alloy with controllable stress hysteresis

Room temperature deformation of a Ni46.7Ti42.8Nb10.5 alloy was studied by in-situ synchrotron X-ray diffraction. Compared to binary NiTi alloy, the Nb dissolved in the matrix significantly increased the onset stress for Stress-Induced Martensite Transformation (SIMT). The secondary phase, effectively a Nb-nanowire dispersion in a NiTi-Nb matrix, increased the elastic stiffness of the bulk material, reduced the strain anisotropy in austenite families by load sharing during SIMT, and increased the stress hysteresis by resisting reverse phase transformation during unloading. The stress hysteresis can be controlled over a wide range by heat treatment through its influences on the residual stress of the Nb-nanowire dispersion and the stability of the austenite.

Investigation on Ce Addition on Microstructure and Martensitic Transformation of a Ti51Ni49 Shape Memory Alloy

The effect of rare earth element Ce addition on the microstructure and martensitic transformation behavior of Ti51Ni49 shape memory alloy was investigated by scanning electronic microscope (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The results show that the microstructure of TiNiCe ternary alloy consists of Ti2Ni phase, CeNi phase and the matrix. One-step martensitic transformation is observed in quenched TiNiCe ternary alloys, which is the same as that in quenched TiNi binary alloys. The martensitic transformation temperatures of Ti-rich TiNi alloy hardly increase with Ce addition.

Radiopaque Shape Memory Alloys: NiTi–Er with Stable Superelasticity

Binary NiTi alloy is one of the most important biomaterials currently used in minimally invasive procedures and indwelling devices. The poor visibility of intermetallic NiTi under X-ray could be an unsatisfactory feature especially for developing low-dimensional implantable devices for the body. It is a matter of fact that the alloying of a third radiopaque element, such as noble or heavy metals, in NiTi can significantly enhance the alloy’s radiopacity. Recently, it was demonstrated that the addition of a rare earth element such as Erbium has led to an equivalent radiopacity at a much lower cost than the equivalent addition of noble metals. This work reviews the main physical aspects related to the radiopacity of NiTi alloys and compares the radiopacity of NiTi–Er compositions with other NiTi-based alloys containing Pd, Pt, W and Cr. Furthermore, a NiTi–6Er alloy is produced by spark plasma sintering, and successfully processed by conventional hot and cold working procedures to a continuous wire showing stable superelastic behaviour (up to 4 % in strain), suitable for developing biomedical devices.

Shape Memory Response of Polycrystalline NiTi12.5Hf Alloy: Transformation at Small Scales

The transformation behavior of NiTiHf alloys is intriguing. In NiTiHf alloys, the experimental transformation strains have been reported to be considerably lower than theoretical transformation strains. In this study, the transformation strain is established with very careful strain measurements at small scales in isobaric and isothermal experiments. Because of the heterogeneity of strain distributions, the results depend on the sub-region considered. The measured local transformation strain can be as high as 6.0 % in compression which is in very good agreement with theoretical calculations for NiTi12.5Hf. The comprehension of NiTi12.5Hf alloy was furthered upon extensive microstructural characterization including high-resolution electron microscopy, establishing the volume fractions of precipitates and twin type. The volume fraction of precipitates is similar to that of Ni-rich binary NiTi alloys. Meanwhile, the twinning modes in the martensite are compound and Type I twins which were used in the theoretical calculations of transformation strains. This material also generates a high work output and represents a foundation for understanding higher Hf compositions.

Shape Memory Effect in Cast Versus Deformation-Processed NiTiNb Alloys

The shape memory effect (SME) response of a deformation-processed NiTiNb shape memory alloy is benchmarked against the response of a cast alloy. The insoluble Nb element ternary addition is known to widen the hysteresis with respect to the binary NiTi alloy. Cast microstructures naturally consist of a cellular arrangement of characteristic eutectic microconstituents surrounding primary matrix regions. Deformation processing typically aligns the microconstituents such that the microstructure resembles discontinuous fiber-reinforced composites. Processed alloys are typically characterized for heat-to-recover applications and thus deformed at constant temperature and subsequently heated for SME recovery, and the critical stress levels are expected to facilitate plastic deformation of the microconstituents. The current work employs thermal cycling under constant bias stresses below those critical levels. This comparative study of cast versus deformation-processed NiTiNb alloys contrasts the strain–temperature responses in terms of forward ΔT F = M s − M f and reverse ΔT R = A f − A s temperature intervals, the thermal hysteresis, and the recovery ratio. The results underscore that the deformation-processed microstructure inherently promotes irreversibility and differential forward and reverse transformation pathways.

Study on Mechanism of Phase Forming in Ni&lt;sup&gt;+&lt;/sup&gt; Implantation into Ti6Al4V at Elevated Temperature

Recently, due to the prediction of the forming phase only focuses on the case which a simple metal ion implants into a single metal matrix, we will study that a simple metal ion is implanted into alloy matrix in this paper. The forming phases in Ni+implantation into Ti6Al4V at elevated temperature will be predicted by the effective heat of formation (EHF) model. The results show that the prediction is consistent with the experimental findings, the first-phase is TiNi with the most negative EHF, the second-phase is Ti2Ni which component concentrations of Ni limited element is lower and locates on the left in the Ti-Ni binary alloy phase diagram, and the TiNi3phase is not found in the experiment. Simultaneously, we explain correctly why the Ni-Al or Ni-V phases don’t generate in Ni+implantation into Ti6Al4V at elevated temperature.

Mechanical and structural aspects of high-strain-rate deformation of NiTi alloy

The mechanical behavior of the binary polycrystalline NiTi alloy with a quasi-equilibrium structure has been considered in the course of the high-strain-rate extension in a temperature range of 20–300°C. The quasi-equilibrium structure, which is necessary to ensure the long-term stability of special properties of the alloy, was achieved using aging, after which both the forward and reverse martensitic transformations exhibited a multistage character and the phase composition at room temperature was characterized by the presence of R and B19′ martensites. To separate the contributions that come from the equilibrium structure and from the high rate of tension to the mechanical behavior of the alloy, a comparative analysis of the diagrams of high-strain-rate and quasi-static tension has been performed. It has been shown that the action of several mechanisms of reversible deformation is determined by the specific features of the equilibrium structure, and the level of stresses at which these mechanisms are developed is controlled by the rate of tension. The results of the X-ray diffraction study of the phase composition of the alloy samples after high-strain-rate tension, which make it possible to conclude that the mechanical behavior of martensite and austenite upon the dynamic tension of the alloy is determined by the development of stress-induced R → B19′, B2 → R, and B2 → B19′ transformations and by the processes of the detwinning and reorientation of crystals of B19′ martensite, are given.