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To reveal mechanisms of martensite stabilization by prestrain in shape memory alloys, details of reverse martensitic transformation (MT) in prestrained Ni45Ti46Nb9 alloy were studied using calorimetry, dilatometry and resistivity. The first reverse MT is burst-like and is shifted to higher temperatures by ~120 K as compared to the nominal MT. During thermal cycling approaching the first reverse MT, stabilized martensite shows “forbidden” behaviours like burst-like reverse MT not during heating, but during cooling and under isothermal conditions. These unusual effects are attributed to non-thermoelastic nature of the first reverse MT in stabilized martensite under high chemical driving force.

Ti75Ta25 high-temperature shape memory alloys exhibit a number of features which make it difficult to use them as spring actuators. These include the high melting point of Ta (close to 3000 °C), the affinity of Ti to oxygen which leads to the formation of brittle α-case layers and the tendency to precipitate the ω-phase, which suppresses the martensitic transformation. The present work represents a case study which shows how one can overcome these issues and manufacture high quality Ti75Ta25 tensile spring actuators. The work focusses on processing (arc melting, arc welding, wire drawing, surface treatments and actuator spring geometry setting) and on cyclic actuator testing. It is shown how one can minimize the detrimental effect of ω-phase formation and ensure stable high-temperature actuation by fast heating and cooling and by intermediate rejuvenation anneals. The results are discussed on the basis of fundamental Ti–Ta metallurgy and in the light of Ni–Ti spring actuator performance.

Ti–Ta alloys represent candidate materials for high-temperature shape-memory alloys (HTSMAs). They outperform several other types of HTSMAs in terms of cost, ductility, and cold workability. However, Ti–Ta alloys are characterized by a relatively fast microstructural degradation during exposure to elevated temperatures, which gives rise to functional fatigue. In the present study, we investigate how isothermal aging affects the martensitic transformation behavior and microstructures in Ti70Ta30 HTSMAs. Ti–Ta sheets with fully recrystallized grain structures were obtained from a processing route involving arc melting, heat treatments, and rolling. The final Ti–Ta sheets were subjected to an extensive aging heat treatment program. Differential scanning calorimetry and various microstructural characterization techniques such as scanning electron microscopy, transmission electron microscopy, conventional X-ray, and synchrotron diffraction were used for the characterization of resulting material states. We identify different types of microstructural evolution processes and their effects on the martensitic and reverse transformation. Based on these results, an isothermal time temperature transformation (TTT) diagram for Ti70Ta30 was established. This TTT plot rationalizes the dominating microstructural evolution processes and related kinetics. In the present work, we also discuss possible options to slow down microstructural and functional degradation in Ti–Ta HTSMAs.

The structural characterization of the various phases that occur in Ti–Ta-based high-temperature shape-memory alloys is complicated by the presence of many competing phases as a function of composition. In this study, we resolve apparent inconsistencies between experimental data and theoretical calculations by suggesting that phase separation and segregation of undesired phases are not negligible in these alloys, and that finite temperature effects should be taken into account in the modeling of these materials. Specifically, we propose that the formation of the ω phase at low Ta content and of the σ phase at high Ta content implies a difference between the nominal alloy composition and the actual composition of the martensitic and austenitic phases. In addition, we show that temperature affects strongly the calculated values of the order parameters of the martensitic transformation occurring in Ti–Ta.

The rapid degradation of the functional properties of many Ti-based alloys is due to the precipitation of the $\omega$ phase. In the conventional high-temperature shape memory alloy Ti-Ta the formation of this phase compromises completely the shape memory effect and high (>100{\deg}C) transformation temperatures cannot be mantained during cycling. A solution to this problem is the addition of other elements to form Ti-Ta-X alloys, which often modifies the transformation temperatures; due to the largely unexplored space of possible compositions, very few elements are known to stabilize the shape memory effect without decreasing the transformation temperatures below 100{\deg}C. In this study we use transparent descriptors derived from first principles calculations to search for new ternary Ti-Ta-X alloys that combine stability and high temperatures. We suggest four new alloys with these properties, namely Ti-Ta-Sb, Ti-Ta-Bi, Ti-Ta-In, and Ti-Ta-Sc. Our predictions for the most promising of these alloys, Ti-Ta-Sc, are subsequently fully validated by experimental investigations, the new alloy Ti-Ta-Sc showing no traces of $\omega$ phase after cycling. Our computational strategy is immediately transferable to other materials and may contribute to suppress $\omega$ phase formation in a large class of alloys.

In this study, different composite coatings with 20 wt.% silicon and 1 wt.% multi-walled carbon nanotubes of hydroxyapatite were developed on NiTi substrate using a combination of electrophoretic deposition and reactive bonding during the sintering. Silicon was used as reactive bonding agent. During electrophoretic deposition, the constant voltage of 30 V was applied for 60 s. After deposition, samples were dried and then sintered at 850 °C for 1 h in a vacuum furnace. SEM, XRD and EDX were used to characterize the microstructure, phase and elemental identification of coatings, respectively. The SEM images of the coatings reveal a uniform and compact structure for HA–Si and HA–Si–MWCNTs. The presence of silicon as a reactive bonding agent as well as formation of new phases such as SiO2, CaSiO3 and Ca3SiO5 during the sintering process results in compact coatings and consumes produced phases from HA decomposition. Formation of the mentioned phases was confirmed using XRD analysis. The EDX elemental maps show a homogeneous distribution of silicon all over the composite coatings. Also, the bonding strength of HA–Si–MWCNTs coating is found to be 27.47 ± 1 MPa.

Ti-Ta-X (X = Al, Sn, Zr) compounds are emerging candidates as high-temperature shape memory alloys (HTSMAs). The stability of the one-way shape memory effect (1WE), the exploitable pseudoelastic (PE) strain intervals as well as the transformation temperature in these alloys depend strongly on composition, resulting in a trade-off between a stable shape memory effect and a high transformation temperature. In this work, experimental measurements and first-principles calculations are combined to rationalize the effect of alloying a third component to Ti-Ta based HTSMAs. Most notably, an $\textit{increase}$ in the transformation temperature with increasing Al content is detected experimentally in Ti-Ta-Al for low Ta concentrations, in contrast to the generally observed dependence of the transformation temperature on composition in Ti-Ta-X. This inversion of trend is confirmed by the $\textit{ab-initio}$ calculations. Furthermore, a simple analytical model based on the $\textit{ab-initio}$ data is derived. The model can not only explain the unusual composition dependence of the transformation temperature in Ti-Ta-Al, but also provide a fast and elegant tool for a qualitative evaluation of other ternary systems. This is exemplified by predicting the trend of the transformation temperature of Ti-Ta-Sn and Ti-Ta-Zr alloys, yielding a remarkable agreement with available experimental data.

Titanium–tantalum based alloys can demonstrate a martensitic transformation well above 100 °C, which makes them attractive for shape memory applications at elevated temperatures. In addition, they provide for good workability and contain only reasonably priced constituents. The current study presents results from functional fatigue experiments on a binary Ti–25Ta high-temperature shape memory alloy. This material shows a martensitic transformation at about 350 °C along with a transformation strain of 2 pct at a bias stress of 100 MPa. The success of most of the envisaged applications will, however, hinge on the microstructural stability under thermomechanical loading. Thus, light and electron optical microscopy as well X-ray diffraction were used to uncover the mechanisms that dominate functional degradation in different temperature regimes. It is demonstrated the maximum test temperature is the key parameter that governs functional degradation in the thermomechanical fatigue tests. Specifically, ω-phase formation and local decomposition in Ti-rich and Ta-rich areas dominate when T max does not exceed ≈430 °C. As T max is increased, the detrimental phases start to dissolve and functional fatigue can be suppressed. However, when T max reaches ≈620 °C, structural fatigue sets in, and fatigue life is again deteriorated by oxygen-induced crack formation. Copyright © Materials Research Society 2017

Release of Ni(1+) ions from NiTi alloy into tissue environment, biological response on the surface of NiTi and the allergic reaction of atopic people towards Ni are challengeable issues for biomedical application. In this study, composite coatings of hydroxyapatite-silicon multi walled carbon nano-tubes with 20wt% Silicon and 1wt% multi walled carbon nano-tubes of HA were deposited on a NiTi substrate using electrophoretic methods. The SEM images of coated samples exhibit a continuous and compact morphology for hydroxyapatite-silicon and hydroxyapatite-silicon-multi walled carbon nano-tubes coatings. Nano-indentation analysis on different locations of coatings represents the highest elastic modulus (45.8GPa) for HA-Si-MWCNTs which is between the elastic modulus of NiTi substrate (66.5GPa) and bone tissue (≈30GPa). This results in decrease of stress gradient on coating-substrate-bone interfaces during performance. The results of nano-scratch analysis show the highest critical distance of delamination (2.5mm) and normal load before failure (837mN) as well as highest critical contact pressure for hydroxyapatite-silicon-multi walled carbon nano-tubes coating. The cell culture results show that human mesenchymal stem cells are able to adhere and proliferate on the pure hydroxyapatite and composite coatings. The presence of both silicon and multi walled carbon nano-tubes (CS3) in the hydroxyapatite coating induce more adherence of viable human mesenchymal stem cells in contrast to the HA coated samples with only silicon (CS2). These results make hydroxyapatite-silicon-multi walled carbon nano-tubes a promising composite coating for future bone implant application.

Ti-Ta alloys are attractive materials for applications in actuators as well as biomedical implants. When fabricated as thin films, these alloys can potentially be employed as microactuators, components for micro-implantable devices and coatings on surgical implants. In this study, Ti100-x Ta x (x = 21, 30) nanocolumnar thin films are fabricated by glancing angle deposition (GLAD) at room temperature using Ti73Ta27 and Ta sputter targets. Crystal structure, morphology and microstructure of the nanostructured thin films are systematically investigated by XRD, SEM and TEM, respectively. Nanocolumns of ∼150-160 nm in width are oriented perpendicular to the substrate for both Ti79Ta21 and Ti70Ta30 compositions. The disordered α″ martensite phase with orthorhombic structure is formed in room temperature as-deposited thin films. The columns are found to be elongated small single crystals which are aligned perpendicular to the [Formula: see text] and [Formula: see text] planes of α″ martensite, indicating that the films' growth orientation is mainly dominated by these crystallographic planes. Laser pre-patterned substrates are utilized to obtain periodic nanocolumnar arrays. The differences in seed pattern, and inter-seed distances lead to growth of multi-level porous nanostructures. Using a unique sputter deposition geometry consisting of Ti73Ta27 and Ta sputter sources, a nanocolumnar Ti-Ta materials library was fabricated on a static substrate by a co-deposition process (combinatorial-GLAD approach). In this library, a composition spread developed between Ti72.8Ta27.2 and Ti64.4Ta35.6, as confirmed by high-throughput EDX analysis. The morphology over the materials library varies from well-isolated nanocolumns to fan-like nanocolumnar structures. The influence of two sputter sources is investigated by studying the resulting column angle on the materials library. The presented nanostructuring methods including the use of the GLAD technique along with pre-patterning and a combinatorial materials library fabrication strategy offer a promising technological approach for investigating Ti-Ta thin films for a range of applications. The proposed approaches can be similarly implemented for other materials systems which can benefit from the formation of a nanocolumnar morphology.