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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.

High-temperature shape memory alloy (HTSMA) thin films are candidates for development of microactuators with operating temperatures exceeding 100 °C. This article reviews recent advances and developments in the field of HTSMA thin films during the past decade, with focus on the systems Ti–Ni–X (X = Hf, Zr, Pd, Pt and Au), Ti–Ta, and Au–Cu–Al. These actuator films offer a wide range of transformation temperatures, thermal hysteresis, and recoverable strains suitable for high-temperature applications. Promising alloy compositions in the systems Ti–Ni–Hf, Ti–Ni–Pd, Ti–Ni–Au, and Au–Cu–Al are highlighted for further upscaling and development. The remaining challenges as well as prospects for development of HTSMA thin films are also discussed.

Ti–Ta thin films exhibit properties that are of interest for applications as microactuators and as biomedical implants. A Ti–Ta thin film materials library was deposited at T = 25 °C by magnetron sputtering employing the combinatorial approach, which led to a compositional range of Ti87Ta13 to Ti14Ta86. Subsequent high-throughput characterization methods permitted a quick and comprehensive study of the crystallographic, microstructural, and morphological properties, which strongly depend on the chemical composition. SEM investigation revealed a columnar morphology having pyramidal, sharp tips with coarser columns in the Ti-rich and finer columns in the Ta-rich region. By grazing incidence X-ray diffraction four phases were identified, from Ta-lean to Ta-rich: ω phase, α″ martensite, β phase, and a tetragonal Ta-rich phase (Ta(tetr)). The crystal structure and microstructure were analyzed by Rietveld refinement and clear trends could be determined as a function of Ta-content. The lattice correspondences be...

Ti-Ta based alloys are an interesting class of high-temperature shape memory materials. When fabricated as thin films, they can be used as high-temperature micro-actuators with operation temperatures exceeding 100 degrees C. In this study, microstructure, shape memory effect and thermal cycling stability of room-temperature sputter deposited Ti67Ta33 thin films are investigated. A disordered alpha ‘’ martensite (orthorhombic) phase is formed in the as-deposited Ti67Ta33 films. The films show a columnar morphology with the columns beingoriented perpendicular to the substrate surface. They are approximately 200 nm in width. XRD texture analysis reveals a martensite fiber texture with 51206 and 51026 fiber axes. The XRD results are confirmed by TEM analysis, which also shows columnar grains with long axes perpendicular to the 51206 and 51026 planes of alpha ‘’ martensite. The shape memory effect is analyzed in the temperature range of -10 to 240 degrees C using the cantilever deflection method, with special emphasis placed on cyclic stability. Ti67Ta33 thin films undergo a forward martensitic transformation at M-s approximate to 165 degrees C, with a stress relaxation of approximately 33 MPa during the transformation. The actuation response of the film actuators degrades significantly during thermal cycling. TEM analysis shows that this degradation is related to the formation of nanoscale omega precipitates (5-13 nm) which form above the austenite finish temperature. These precipitates suppress the martensitic transformation, as they act as obstacles for the growth of martensite variants.

Due to their high work output and good mechanical properties, actuators made from shape memory alloys (SMAs) are used in numerous applications. Unfortunately, SMAs such as nickel–titanium (Ni–Ti) can only be employed at temperatures up to about 100 °C. Lately, high-temperature shape memory alloys (HT SMAs) have been introduced to overcome this limitation. Ternary systems based on Ni–Ti have been intensively characterized and alloys are available that can operate at elevated temperatures. However, these alloys either contain substantial amounts of expensive noble elements like platinum and palladium, or the materials are brittle. The titanium–tantalum (Ti–Ta) system has been developed to overcome these issues. Binary Ti–Ta provides relatively high M S temperature combined with excellent workability, but it suffers from fast cyclic degradation. By alloying with third elements this drawback can be overcome: The ternary Ti–Ta–Al alloy shows overall promising properties as will be shown in the present work. In-situ thermo-mechanical cycling experiments were conducted and allowed for evaluation of the factors affecting the functional and structural fatigue of this alloy. Functional fatigue is dominated by ω-phase evolution, while structural fatigue is triggered by an interplay of ω-phase induced embrittlement and deformation constraints imposed by unsuitable texture. In addition, a concept for fatigue life extension proposed very recently for binary Ti–Ta, is demonstrated to be also applicable for the ternary Ti–Ta–Al.

a b s t r a c t The interaction of human mesenchymal stem cells (hMSCs) with Ti and TiO2 nano-columnar surfaces fabricated using glancing angle sputter deposition was investigated. The adherence and prolifera- tion of hMSCs on different nano-columnar surfaces, including vertical columns, slanted columns and chevrons, were examined with calcein-acetoxymethyl ester fluorescence staining and scanning electron microscopy. For comparison, adherence of hMSCs on compact, dense films was also studied. After 24 h and 7 days, adherent and viable cells were observed on both, Ti nano-columns as well as dense Ti films, which confirms the biocompatibility of these nanostructures. Very small pseudopodia with width of approx- imately 20-35 nm and length varying from 20 to 200 nm were observed between the nano-columns, independent of the type of the nano-columnar morphology. Large inter-column spacing and effectively increased surface area make these nanostructures promising candidates for bio-functionalization or drug loading on the surface of Ti-based implants.

A Lia(NixMnyCoz)Or cathode materials library was fabricated by combinatorial magnetron sputtering. The compositional analysis of the library was performed by a new high-throughput approach for Li-content measurement in thin films, which combines automated energy-dispersive X-ray spectroscopy, Deuteron-induced gamma emission, and Rutherford backscattering measurements. Furthermore, combining this approach with thickness measurements allows the mapping of density values of samples from the materials library. By correlating the obtained compositional data with structural data from high-throughput X-ray diffraction measurements, those compositions which show a layered (R3̅m) structure and are therefore most interesting for Li-battery applications (for cathode (positive) electrodes) can be rapidly identified. This structure was identified as being most pronounced in the compositions Li0.6(Ni0.16Mn0.35Co0.48)O2, Li0.7(Ni0.10Mn0.37Co0.51)O2, Li0.6(Ni0.23Mn0.33Co0.43)O2, Li0.3(Ni0.65Mn0.08Co0.26)O2, Li0.3(Ni0.63Mn0.08Co0.29)O2, Li0.4(Ni0.56Mn0.09Co0.34)O2, Li0.5(Ni0.45Mn0.13Co0.42)O2, and Li0.6(Ni0.34Mn0.14Co0.52)O2.

Five different Ag dots arrays (16 to 400dots/mm2) were fabricated on a continuous platinum, palladium, or iridium thin film and for comparison also on titanium film by sputter deposition and photolithographic patterning. To analyze the antibacterial activity of these microstructured films Staphylococcus aureus (S. aureus) were placed onto the array surfaces and cultivated overnight. To analyze the viability of planktonic as well as surface adherent bacteria, the applied bacterial fluid was subsequently aspirated, plated on blood agar plates and adherent bacteria were detected by fluorescence microscopy. A particular antibacterial effect towards S. aureus was induced by Ag dot arrays on each of the platinum group thin film (sacrificial anode system for Ag) in contrast to Ag dot arrays fabricated on the Ti thin films (non-sacrificial anode system for Ag). Among platinum group elements the Ir-Ag system exerted the highest antibacterial activity which was accompanied by most advanced dissolution of the Ag dots and Ag ion release compared to Ag dots on Pt or Pd.

The martensitic transformation in Ti–Ni–Hf thin films with ultra-fine grain structure has been analyzed as a function of composition using a high-throughput array of nanocalorimeters. The martensite–austenite transformation temperature is significantly lower than in bulk Ti–Ni–Hf, but increases linearly with Hf content at a rate comparable to bulk Ti–Ni–Hf. The response to high-temperature cycling (22 °C

The structural, surfacial and nanoscale mechanical properties evolution of Ti–Ni–Cu thin films, prepared by the co-sputtering of TiNi and Cu targets during rapid thermal annealing (RTA) were investigated. Crystallization took place in a few seconds at 480 °C. With increasing annealing time (up to 180 s), roughness increased dramatically, and was far more prominent than in films crystallized by conventional thermal annealing (CTA). Although RTA is energy efficient due to the lower annealing time, the film roughness is less ideal than CTA, which may prove limiting in specific applications. The surface and subsurface chemical states of Ti, Ni and Cu was similar for RTA and CTA processed materials, demonstrating they are exposed to comparable redox potentials during annealing. Using X-ray absorption spectroscopy (XAS), it was found that the RTA (180 s) and CTA (1 h) films possessed longer range order. The evolution of nanoscale mechanical properties of the RTA films during rapid thermal annealing was also studied.

This paper reports the rapid thermal annealing (RTA) of Ti-rich TiNi thin films, synthesized by the co-sputtering of TiNi and Ti targets. Long-range order of aperiodic alloy could be achieved in a few seconds with the optimum temperature of 773 K. Longer annealing (773 K/240 s), transformed the film to a poorly ordered vitreous phase, suggesting a novel method for solid state amorphization. Reitveld refinement analyses showed significant differences in structural parameters of the films crystallized by rapid and conventional thermal annealing. Dependence of the elastic modulus on the valence electron density (VED) of the crystallized films was studied. It is suggested that RTA provides a new approach to fabricate patterned shape memory thin films.

TiNi alloy is a well-known shape memory alloy and has been widely used for bio-medical, mechanical and electrical applications. In this study, a Ni-rich NiTi alloy was prepared by vacuum arc melting in a water-cooled copper crucible. Three samples of this alloy were heated to 1000 °C and cooled in three media: furnace, water, and dry-ice bath. Differential scanning calorimetry (DSC), X-ray diffraction (XRD), hardness measurement and tensile test were carried out to investigate the effect of cooling rate on transformation temperature and mechanical properties. The results show that Ni3Ti intermetallic compounds have a great influence on martensitic phase transformation temperature. These tests clearly showed the correlation between cooling rate and properties of the alloy.

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.

Ten different Ag dot arrays (16 to 625 microstructured dots per square mm) were fabricated on a continuous Au thin film and for comparison also on Ti film by sputter deposition and photolithographic patterning. To analyze the antibacterial activity of these microstructured films Escherichia coli and Staphylococcus aureus were placed onto the array surfaces and cultivated overnight. To analyze the viability of planktonic as well as surface adherent bacteria, the applied bacterial fluid was subsequently aspirated, plated on blood agar plates and adherent bacteria were detected by fluorescence microscopy. A particular antibacterial effect towards both bacterial strains was induced by Ag dot arrays on fabricated Au thin film (sacrificial anode system for Ag), due to the release of Ag ions from dissolution of Ag dots in contrast to Ag dot arrays fabricated on the Ti thin films (non-sacrificial anode system for Ag) which remained intact to the original dot shape. The required number of Ag dots on gold film to achieve complete bactericidal effects for both bacterial strains was seven times lower than that observed with Ag dot arrays on Ti film.

The aim of this study was to reproduce the physico-mechanical antibacterial effect of the nanocolumnar cicada wing surface for metallic biomaterials by fabrication of titanium (Ti) nanocolumnar surfaces using glancing angle sputter deposition (GLAD). Nanocolumnar Ti thin films were fabricated by GLAD on silicon substrates. S. aureus as well as E. coli were incubated with nanostructured or reference dense Ti thin film test samples for one or three hours at 37 °C. Bacterial adherence, morphology, and viability were analyzed by fluorescence staining and scanning electron microscopy and compared to human mesenchymal stem cells (hMSCs).Bacterial adherence was not significantly different after short (1 h) incubation on the dense or the nanostructured Ti surface. In contrast to S. aureus the viability of E. coli was significantly decreased after 3 h on the nanostructured film compared to the dense film and was accompanied by an irregular morphology and a cell wall deformation. Cell adherence, spreading and viability of hMSCs were not altered on the nanostructured surface. The results show that the selective antibacterial effect of the cicada wing could be transferred to a nanostructured metallic biomaterial by mimicking the natural nanocolumnar topography.

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.

Titanium-nickel based thin films can be used as actuators in micro-electro-mechanical systems (MEMS) in such applications as valves, pumps, grippers and sensors. These films are typically prepared using magnetron sputtering. The resultant films are amorphous if deposited at room temperature and annealing is necessary as actuation occurs in the crystalline state by a martensitic phase transformation. Consequently, understanding the nature of the annealing process is critical for microstructural control and optimization of MEMS actuators. The first part of the study is concerned with describing and correlating the evolution of structural, surfacial and nano-scale mechanical properties of amorphous Ti-Ni-Cu thin films crystallized by rapid thermal annealing (RTA), where the treatment takes place under a protected atmosphere by electromagnetic irradiation with fast heating rate (< 100 oC/sec). Long range order of amorphous Ti-Ni-Cu thin films was achieved in a few seconds (> 60 sec), with the optimum crystallization temperature of 480oC. The fast crystallization process allows the precise control of microstructure in a lower thermal budget (product of processing temperature and time). With increasing the annealing time (up to 180 sec), roughness increased dramatically, and was far more prominent than in films prepared by conventional thermal annealing (CTA). Although RTA is energy efficient due to the shorter annealing time, the film roughness is less ideal than CTA, which may prove limiting in specific applications. Using X-ray absorption spectroscopy (XAS), it was found that the RTA (180 sec) and CTA (1 hr) films possessed the longest range order. The evolution of the nano-scale mechanical properties of the RTA films during annealing was also studied, vii and demonstrated the suitability of nanoindentation as a simple and reliable tool to study crystalline domain formation in Ti-Ni-Cu thin films. In the second part of this study, the martensitic transformation, crystallization and thermal cycling behavior of Ti-Ni-Hf thin films were analyzed as a function of composition using a high-throughput array of nanocalorimeters. Amorphous as-deposited thin-film samples were crystallized by local heating at a rate of approximately 1.6×104 oC/sec, resulting in an ultra-fine (18 ± 5 nm) grain structure. The (Ti,Hf)-rich Ti-Ni-Hf alloy compositions with Ni contents below ~ 49.3 at.% exhibited the martensite-austenite transformation. The transformation temperature increases linearly with increasing Hf content in the range 14-20 at.% at a rate comparable to that observed for bulk materials, but with reduced martensite-austenite transformation temperature. The decrease in transformation temperature is caused by the fine nano-scale structure in Ti-Ni-Hf thin samples. The response of Ti-Ni-Hf thin films to high-temperature cycling (22ºC< T< 850ºC) changes with Ni concentration. For Ni ≤ 47 at.%, the transformation temperature increases during high-temperature cycling due to precipitation of (Ti1-x,Hfx)2Ni and the resulting enrichment of Hf in the surrounding matrix, while for Ni ≥ 47.7 at.% the transformation is gradually suppressed due to the lack of undercooling as a result of a reduction in transformation temperature. Low-temperature cycling (22ºC< T< 450ºC) resulted in a decrease and stabilization of the transformation temperature. Relaxation of internal stresses by dislocations generated during thermal cycling is suggested as the active mechanism. Thermal cycling stability of the films was improved compared to previous studies on bulk Ti-Ni-Hf. This was attributed to the very small grain size (18 ± 5 nm) of the samples. Alloys with superior thermal cycling stability; i.e. Ti39.6Ni46.1Hf14.3 and Ti42.3Ni42.5Hf15.2 are viii identified. The ability to control the transformation temperature through multiple thermal cycling has been demonstrated. A significantly improved level of time efficiency is achieved using an ultra-fast analysis of Ti-Ni-Hf sample library in a high-throughput manner. DOCTOR OF PHILOSOPHY (MAE)

Ti-Ta based alloys are potential high-temperature shape memory materials with operation temperatures above 100 °C. In this study, the room temperature fabrication of Ti-Ta thin films showing a reversible martensitic transformation and a high temperature shape memory effect above 200 °C is reported. In contrast to other shape memory thin films, no further heat treatment is necessary to obtain the functional properties. A disordered α″ martensite (orthorhombic) phase is formed in the as-deposited co-sputtered Ti70Ta30, Ti68Ta32 and Ti67Ta33 films, independent of the substrate. A Ti70Ta30 free-standing film shows a reversible martensitic transformation, as confirmed by temperature–dependent XRD measurements during thermal cycling between 125 °C to 275 °C. Furthermore, a one-way shape memory effect is qualitatively confirmed in this film. The observed properties of the Ti-Ta thin films make them promising for applications on polymer substrates and especially in microsystem technologies.