Nb Shape Memory Alloy Research Articles

A phenomenological study on the microstructure dependence of the internal friction in U–Nb shape memory alloy

Dynamic mechanical analysis was used to investigate the damping behavior of a U–Nb shape memory alloy in various states, including water quenched (WQ), aging (AG), and cold rolling (CR). Internal friction peaks in the U–Nb alloy were identified, as well as their sensitivity to microstructure. The effects of amplitudes on internal friction and storage modulus were more pronounced in the WQ and AG samples than in the CR samples. A relaxation peak at 200 K was discovered in the U–Nb alloy, and its intensity decreased with aging, as did the activation energy for the relaxation event. It may be that the interaction between H and twin boundaries became more active as a result of the local concentration of Nb with aging. There are three transformation peaks in the current U–Nb alloy in the WQ and AG states, corresponding to phase transformations of α’’→γ0 and γ0→γ, respectively. The damping behavior of the coarse-grained and fine-grained samples differs only slightly, and grain size (grain boundaries), is not believed to be directly responsible for the difference; instead, twin boundaries as damping sources may dominate the damping behavior of the U–Nb alloy.

Effect of Si Content on Microstructure and Mechanical Behavior of Porous Ti-22 at% Nb Shape Memory Alloys for Biomedical Applications

Our research novel Ti-22Nb biomedical alloys made by powder metallurgy and analyze the effect of adding silicon at various weight ratios to the base alloy (0.4, 0.8, and 1.2 %at Si). In this work, the wear characteristics of Ti-Nb-Si alloys in dry conditions are examined, as well as the wear process. To measure the wear rate, a pin-on-disc wear testing apparatus was employed. The optical microstructure analysis showed that the microstructure had a mixture-like appearance and included only a small quantity of another phase. XRD results showed that the stability of the β phase increased with silicon concentration. The (Ti-22Nb) alloy's hardness and compressive strength both increased once silicon is added. Hardness also rises as the amount of silicon additions increases, with the maximum percentage (1.2 percent Si) resulting in the highest hardness and compressive strength

Assessment of Biocompatibility and Physical Properties of Ni–Ti–Zr–Nb Shape Memory Alloys

Assessment of Biocompatibility and Physical Properties of Ni–Ti–Zr–Nb Shape Memory Alloys

Damping Capacity of Ti–Nb Shape Memory Alloys Evaluated Through DMA and Single-Impact Tests

Damping Capacity of Ti–Nb Shape Memory Alloys Evaluated Through DMA and Single-Impact Tests

Optimization of a thermomechanical treatment of superelastic Ti-Zr-Nb alloys for the production of bar stock for orthopedic implants

Ti-19Zr-14 Nb (at%) shape memory alloy was subjected to low- and high-temperature thermomechanical treatments to fabricate long-length bar stock for the production of load-bearing orthopedic implants. The phase composition, structure, texture, uniaxial tensile and three-point bending fatigue behavior were studied. Low-temperature thermomechanical treatment (LTMT), combining cold rotary forging and post-deformation annealing at 550 °C led to the formation in the peripheral zone of the bar cross-section of a statically recrystallized fine-grained structure with a relatively strong [001] crystallographic texture and of a mixed statically recrystallized and polygonized substructure, in the central zone of the cross-section. In this state, the alloy exhibited an excellent combination of the static functional and mechanical properties: relatively high strength (UTS=680 MPa), low Young’s modulus (E < 40 GPa), and high superelastic recovery strain (εrsemax=3.4%). High-temperature thermomechanical treatment (HTMT), consisting in hot rotary forging at 700 °C, led to the formation of a dynamically polygonized substructure with a uniform crystallographic texture close to the [101] direction; this direction corresponding to the maximum theoretical recovery strain limit. As compared to the LTMT bar stock, its HTMT equivalent manifested slighly inferiour static properties but superior fatigue resistance in bending, thus being an optimal candidate for the production of orthopedic implants.

Insights into the martensitic transformation kinetics and mechanical properties of quaternary Ti–Ni–Nb–V shape memory alloys

The effect of V addition on the microstructural evolution, martensitic transformation behaviors and mechanical performances of Ti–Ni–Nb shape memory alloys was systematically investigated. The results reveal that the phase remains almost equal in Ti–Ni–Nb–V shape memory alloys, irrespective of the V content. With increasing V content, the amount of β-Nb phase is increased, which further leads to a reduction in the interspace and width of the β-Nb phase. When the V content changes, Ti–Ni–Nb–V shape memory alloys possess a single stage B2⇌B19´ martensitic transformation. In addition, the martensitic transformation temperatures first increase and then decrease owing to the variation in the chemical composition of the TiNi matrix and microstructural features as the V content increases. The higher yield strength of Ti–Ni–Nb shape memory alloys can be obtained by adding a moderate V content. In conclusion, the lower martensitic transformation temperature and the higher yield strength can be tailored by optimizing the V content in Ti–Ni–Nb shape memory alloys, which would further widen their application fields.

Size effect on the martensitic transformation of Ti–Nb shape memory alloy

The effect of grain size on martensitic transformation was investigated by transmission electron microscopy in a nanocrystalline Ti–16Nb thin film with a broad range of grain sizes. The results show that the larger nanocrystals are filled with multiple variants with the {111}α" type I twinning relationship. As the grain size decreases to about 40 nm, the martensite changes to the single variant morphology, and the shape memory effect is believed to deteriorate significantly then. Finally, the martensitic transformation is totally suppressed and the β parent remains when the grain size drops below 25 nm.

Structure and Superelasticity of Novel Zr-Rich Ti-Zr–Nb Shape Memory Alloys

Two novel superelastic Ti-41Zr-8(10)Nb alloys were studied and compared to the reference Ti-18Zr-15Nb alloy (all in at. %) in terms of their microstructure and mechanical properties. A thermomechanical treatment consisting of cold rolling and post-deformation annealing was applied to all the alloys to create conditions for their superelastic behavior at room temperature. X-ray diffraction analysis demonstrated considerably larger lattice distortions of both parent and martensitic phases in Zr-rich alloys that resulted in significantly higher theoretical limits of recovery strain in these alloys (~ 8.0%) as compared to the reference alloy (~ 5.5%). However, during room temperature fatigue testing, the novel alloys accumulated considerable residual strains and showed a relatively weak fatigue resistance caused by the presence of notable quantities of α″-phase at this temperature. Conversely, the reference alloy containing only β-phase at the temperature of testing and therefore, a more favorable phase composition at the testing temperature manifested a better superelasticy, and therefore, a better fatigue resistance. Nonetheless, an excellent combined effect of shape memory and superelasticity in Zr-rich alloys indicates that their room temperature superelasticity could be improved via an additional adjustment of their chemical composition.

Strengthening mechanism and micropillar analysis of high-strength NiTi–Nb eutectic-type alloy prepared by laser powder bed fusion

Although laser powder bed fusion (LPBF) can produce complex structures with promising properties, there still exists a concern on the diversity in materials, especially composites, for LPBF. This work innovatively used NiTi and Nb powders to prepare NiTi–Nb eutectic-type alloy by LPBF, which provides a new method to prepare NiTi–Nb shape memory alloy and could be an alternative for tailoring microstructure. The semi-molten Nb particle phase was retained during LPBF, so that β-Nb and the eutectic structure display a gradient distribution in the Nb diffusion concentration. The heat treatment process (annealed at 850 °C for 0.5 h) makes NiTi–Nb samples have high yield strength (~1640 MPa), high compressive strength (~2380 MPa), and high compressive strain (~39%), which are superior to the corresponding ones of its as-built and as-cast counterparts. Large Nb phase particles with a size of 10–30 μm are retained and accelerated the formation of eutectic phase and β-Nb precipitate phase. Overall, the presence of β-Nb phase has a beneficial effect on the alloy, and a large number of intertwined dislocations and stacking faults around the β-Nb phase promote the formation of martensite under stress loading. The eutectic phase makes an important contribution to the high strength and plasticity of the NiTi–Nb alloy.

Influence of the Combined Radial Shear Rolling and Rotary Forging on the Deformation Mode of the Small-Diameter Rod Billet Made of Titanium Alloys

This work is devoted to the finite-element modeling of the deformation mode of a small-diameter rod billet during pressure hot treatment in the combination of radial shear rolling (RSR) and rotary forging (RF). Modeling is performed using a rheological model of the Ti–6Al–4V titanium alloy using the QForm VX program. The combination of RSR for one passage of a billet 15 mm in diameter to a rod 12 mm in diameter and subsequent RF in 1, 2, and 3 passages with the formation of rods 11, 10, and 8 mm in diameter is modeled. The operation-by-operation accumulation of the plastic deformation in nonuniformity conditions of its distribution is taken into account. Intermediate and final plastic deformation fields, deformation rates, and average stresses are found. It is shown that the distribution of the plastic deformation after RSR has the pronounced gradient character with a minimal value (3 and larger) at the cross-section periphery and a minimal value (about 1) in the center. Due to this, RF, even with small reductions, leads to a substantial increase in uniformity of the deformation mode when compared with the billet after the same diameter after radial shear rolling only. In addition, residual tensile stresses decrease due to compressing stresses during RF. The direct experimental approval of the combined deformation method is performed for the promising Ti–Zr–Nb shape memory alloy of medical prescription when fabricating rods 7–8 mm in diameter under pilot production conditions. The results of modeling are qualitatively confirmed by the metallographic analysis. The prospects of a combination of RSR and RF for the development of industrial fabrication technologies of small-diameter rods with a high uniformity of finely dispersed structure are shown.

Fabrication of porous samples from a high-temperature Cu–Al–Ni–Mn–Nb shape memory alloy by freeze-drying and partial sintering

Shape memory alloys (SMAs) find important applications in many fields, including the biomedical, aerospace, and civil construction industries. The increasing interest in these materials arises from their mechanical properties such as shape memory effect, superelasticity, and damping capacity. In this work, porous samples of a Cu–Al–Ni–Mn–Nb alloy were successfully fabricated. It is the first time that porous samples of this alloy are obtained by freeze-drying and partial sintering. The SMA powder was initially mixed with deionized water, citric acid, and poly(vinyl alcohol). After freezing the prepared suspensions on a cold finger, the as-obtained ice crystals were removed by freeze-drying. The green bodies were subsequently heat-treated at 980 °C for 6 h and quenched to room temperature under argon flow. β1’ martensite was the major phase observed in the fabricated scaffolds. It was observed that both the structural properties and damping capacity of the obtained samples depend on the size of the particles used for preparing them. The prepared scaffolds showed interesting damping properties in the martensitic field, which could allow their use in high-temperature applications.

Advanced Technology for Preparing Bar from Medical Grade Ti-Zr-Nb Superelastic Alloy Based on Combination of Radial-Shear Rolling and Rotary Forging

Prospects are presented for the technology of preparing biomedical Ti–Zr–Nb shape memory alloy 3–8 mm diameter bars more than 2000 mm long using an advanced combination of radial shear rolling and rotary forging. Radial shear rolling parameters are calculated on the basis of technological limitations of true degree and rate of deformation of 0.05–0.5 and 1–5 sec–1 respectively per pass. Full-scale experimental verification of the technology is performed in NITU MISiS 1030 and 1440 radial shear rolling mini-mills with subsequent rotary forging using an RKM2 machine. Bars are obtained with a grain size of ∼ 40 μm demonstrating superelastic behavior at room temperature with a favorable combination of relatively low Young’s modulus (E = 39 GPa) and relatively high strength (σf = 618 MPa). The technology has a high degree of readiness for direct industrial application.

Effect of the Rate, Temperature, and Magnitude of Prestraining on the Parameters of Phase Transformations and the Thermomechanical Characteristics of the 45% Ti–45% Ni–10% Nb Shape Memory Alloy in Press-Formed Conditions

This work investigates the effect of the rate, temperature, and magnitude of the strain induced preliminarily to heating for developing shape memory effect on the phase composition, martensite transformation, phase transition temperature, lattice parameters, substructure parameters, and thermomechanic characteristics of the shape memory (SM) 45Ti–45Ni–10Nb (at %) alloy in the press-formed state. The relation between structural and thermomechanical parameters of the alloy is identified. The conditions of prestraining the SM alloy for achieving the best thermomechanic characteristics are determined. The results of this study have been used to develop a fire-protection device for nuclear power facilities.

Parameter optimization of microwave sintering porous Ti-23%Nb shape memory alloys for biomedical applications

Abstract Porous Ti-23%Nb (mole fraction) shape memory alloys (SMAs) were prepared successfully by microwave sintering with excellent outer finishing (without space holder). The effects of microwave-sintering on the microstructure, phase composition, phase-transformation temperature, mechanical properties and shape-memory effect were investigated. The results show that the density and size of porosity vary based on the sintering time and temperature, in which the smallest size and the most uniform pore shape are exhibited with Ti-23%Nb SMA after being sintered at 900 °C for 30 min. The microstructure of porous Ti-Nb SMA consists of predominant α″, α, and β phases in needle-like and plate-like morphologies, and their volume fractions vary based on the sintering time and temperature. The β phase represents the largest phase due to the higher content of β stabilizer element with little intensities of α and α″ phases. The highest ultimate strength and its strain are indicated for the sample sintered at 900 °C for 30 min, while the best superelasticity is for the sample sintered at 1200 °C for 30 min. The low-elastic modulus enables these alloys to avoid the problem of “stress shielding”. Therefore, microwave heating can be employed to sinter Ti-alloys for biomedical applications and improve the mechanical properties of these alloys.

Plastic deformation behaviour of single-crystalline martensite of Ti-Nb shape memory alloy

β-Ti alloys have attracted considerable attention as new biomedical shape memory alloys. Given the critical importance of the plastic deformation in the martensite phase for the shape memory effect and superelasticity, we investigated here the plastic deformation behaviour of a single crystal of α″ (orthorhombic) martensite of Ti-27 mol%Nb shape memory alloy obtained by the stress-induced martensitic transformation of a single crystal of the parent β phase. Four operative plastic deformation modes were observed, including two dislocation slips and two twinnings. To the best of our knowledge, two of these plastic deformation modes (one dislocation slip and one twinning) were discovered for the first time in this study. The identified slip and twinning systems in the martensite phase have corresponding slip and twinning systems in the parent β phase with which they share many similarities. Therefore, we believe that the plastic deformation of the α″ martensite is inherited from that of the parent β phase.

Microstructure and Shape Memory Behavior of Ti–Nb Shape Memory Alloy Thin Film

Ti–Nb shape memory alloy (SMA) thin film is a promising candidate applied as microactuator in biomedical field. In this study, the microstructure and shape memory behavior of Ti–Nb SMA thin films in different heat treatment conditions have been investigated. Fine ω phases embedded in the β phase matrix suppress the martensitic transformation of the films. As a result, the as-deposited and most of the annealed films consist of the β and α″ dual phases. The annealed Ti–Nb thin film shows excellent superelasticity effect when deformed above the reverse martensitic transformation temperature, that is 3.5% total recovery strain can be obtained when 4% pre-strain is loaded.

Assessment of Pipe Coupling by Using the Recovery of Stress-Induced Martensites in Superelastic Cu-11.8Al-0.6Be-0.5Nb Alloy

Superelastic Cu-11.8wt.%Al-0.6wt.%Be-0.5wt.%Nb shape memory alloy was tested under loading and unloading tensile tests at different temperatures and different strain rates of 5 × 10−4 s−1 and 1 × 10−5 s−1. The results of tests performed at 243 and 298 K demonstrate that this Cu-Al-Be alloy exhibits an appreciable amount of residual strain at room temperature, reaching strain levels as high as high as 4% when tested at 243 K for an imposed deformation of 7%. The residual stress associated with retained martensite is reverted by heating at the new austenite start (A s) temperature, which is 60 °C higher than that of the initial alloy. In addition, with increasing temperature, the critical stress for the austenite → martensite transformation increases. Tube joining experiments show that the reversion of retained martensite is very effective to make pipe coupler fittings. The reversion of stress-induced martensite results in a coupling with a pull-out force equivalent to that obtainable by conventional shape memory effect.

Effect of deformation on the microstructure, transformation temperature and superelasticity of Ti–23 at% Nb shape-memory alloys

Extensive research on the effect of spark-plasma sintering and free forging on the microstructure, transformation temperature, and superelasticity of Ti-23at% Nb shape-memory alloys (SMAs) was undertaken. Based on the obtained results, it was found that the specimen showed significant results after deformation than the homogenization step, from which it can be concluded that the deformation has considerable effects on the mechanical properties and shape-memory characteristics of Ti-based SMAs. The microstructure of homogenized and deformed Ti-23at% Nb SMAs consists of β, α, α′ and ω phases, while the sintered sample shows only β and α phases. Endothermic and exothermic peaks of the austenite↔martensite transformation are seen in the homogenized sample, then after being deformed, the exothermic peak disappeared. The highest austenite start and finish temperatures were observed with the deformed sample due to disappearance of TiNb4 intermetallic compounds and a high percentage of β phase. The ductility of the deformed sample shows the highest value compared with the homogenized sample due the elimination of coarse α precipitates and brittle intermetallic compounds of TiNb4, along with the suppression of the brittle ω phase and retaining β matrix.

Nd:YAG laser welding of Ti-27 at.% Nb shape memory alloys

A Nd:YAG laser welding technique with varying pulse widths was employed to weld Ti-27 at.% Nb shape memory alloy plates, and subsequent phase transformation characteristics and microhardness of welds were systematically investigated. From microstructure observation, it is apparent that weld seams contain a columnar growth with the low pulse and transferred into a combined growth of columnar and planar with the high pulse width. Moreover, grains in the weld zone exhibited a finer size when compared to the unwelded zone. A small amount of NbTi4 was also found in the welds. In the forward transformation, transformation temperatures did not exhibit peaks, indicating the same behaviour as the original TiNb alloy. Microhardness of the TiNb welds increased as the pulse width increased when compared to the original TiNb alloy. However, little reduction in hardness occurred after a pulse width of 5 ms.

Corrosion and bioactivity performance of graphene oxide coating on Ti[sbnd]Nb shape memory alloys in simulated body fluid

In the present work, the microstructure, corrosion, and bioactivity of graphene oxide (GO) coating on the laser-modified and -unmodified surfaces of TiNb shape memory alloys (SMAs) were investigated. The surface morphology and chemical composition was examined using field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD). The surface modification was carried out via a femtosecond laser with the aim to increase the surface roughness, and thus increase the adhesion property. FE-SEM analysis of the laser-treated Ti-30at.% Nb revealed the increase in surface roughness and oxygen/nitrogen containing groups on the Ti-30at.% Nb surface after being surface modified via a femtosecond laser. Furthermore, the thickness of GO was increased from 35μm to 45μm after the surface was modified. Potentiodynamic polarisation and electrochemical impedance spectroscopy studies revealed that both the GO and laser/GO-coated samples exhibited higher corrosion resistance than that of the uncoated TiNb SMA sample. However, the laser/GO-coated sample presented the highest corrosion resistance in SBF at 37°C. In addition, during soaking in the simulated body fluid (SBF), both the GO and laser/GO coating improved the formation of apatite layer. Based on the bioactivity results, the GO coating exhibited a remarkable antibacterial activity against gram-negative bacteria compared with the uncoated. In conclusion, the present results indicate that Ti-30at.% Nb SMAs may be promising alternatives to NiTi for certain biomedical applications.