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The thermodynamics and kinetics of Silicon (Si) segregation to grain boundaries in Iridium alloy DOP-26 with added trace levels of Si of 6, 11, 29, and 36 wppm was studied by Auger Electron Spectroscopy. The four alloys were annealed at 1500 or 1535 °C for 19 or 76 hours followed by cooling at three different rates. Si enrichment at the grain boundaries (GB) increased with increasing bulk Si content, with the grain boundary Si enrichment factors ranging from 62 to 344, depending on the bulk Si content and the cooling rate. Grain boundary Si contents increased with decreasing cooling rate in all alloys, indicating that Si GB segregation is influenced by both thermodynamic and kinetic factors in the alloys and temperature ranges of the study. A Langmuir-McLean isotherm-based model was successfully used to predict the temperature dependence of GB Si segregation in DOP-26 alloys with Si additions and estimate the temperature independent free energy of Si segregation to grain boundaries in DOP-26.

Minor amounts of boron additions have been found to greatly enhance the ductility of cast Ti–6Al–4V alloys, which was considered to be due to the grain-size refinement. In this paper, we report our interesting finding that the beneficial effect of boron on the ductility of the cast titanium alloys is due not only to the grain-size refinement but the enhancement of the prior- β grain-boundary cohesion by boron segregation at the grain boundaries, as evidenced by Auger electron microscopy.

The effect of B on the fracture behavior of Ni 3 Fe alloys (24 and 26 at.% Fe) was studied after cathodic charging with hydrogen. In contrast to its disordered state, ordered Ni 3 Fe underwent brittle intergranular fracture at room temperature. Boron addition changed its fracture mode to predominantly ductile transgranular. The grain boundary chemistry of ordered Ni 3 Fe was analyzed by Auger electron spectroscopy. Boron was found to segregate to the grain boundaries of both Ni–24Fe and Ni–26Fe and reduce the hydrogen-induced embrittlement of these alloys in the ordered state.

Mo–1.5 at.% Si alloys with additions of either Y2O3 or Zr were manufactured by mechanical alloying. The Y2O3 particles reduced the grain size and increased the room temperature strength, but did not alleviate the brittleness of previously investigated Mo–1.5 at.% Si without Y2O3. Additions of Zr, on the other hand, resulted not only in a fine grain size and an extremely high bend strength (∼2 GPa), but also in limited bend ductility at room temperature. Zr additions are seen to be beneficial for three reasons. First, Zr reduces the grain size. Second, Zr getters detrimental oxygen by forming ZrO2 particles (which in turn help to pin the grain boundaries). Third, in situ Auger analysis shows that Zr reduces the concentration of Si segregated at the grain boundaries. This is thought to enhance the grain boundary cohesive strength and thus leads to the observed ductility.

We have developed a simplified buffer layer architecture for the rolling-assisted biaxially textured substrates (RABiTS) template approach. The overall purpose of this research is to enable potentially a low-cost, high throughput and high yield manufacturing processes for buffer fabrication and to gain fundamental understanding of the growth of buffer layers for RABiTS templates. This understanding is critical to the development of a reliable, robust, and long-length manufacturing process for second generation wires. In this study, efforts are being made to either reduce the number of vacuum deposited layers or to replace one or more of these vacuum deposited layers with a solution based layer. We have chosen Gd2O3 - ZrO2 system as a potential barrier layer for this study. We have deposited epitaxial Gadolinia Stabilized Zirconia (GSZ) (Zr1-xGdxOy; x = 0-1) films on 30-nm thick e-beam evaporated Y2O3 seeded Ni-W (3 at. %) (Ni-3W) substrates using a metal-organic decomposition (MOD) process. Detailed X-ray studies indicated that all of the MOD-GSZ layers were cube textured and both in-plane and out-of-plane lattice parameters increases linearly with increasing the Gd content. We have also developed process conditions for reactively sputter deposited epitaxial Gd2Zr2O7 (Gadolinium Zirconium Oxide (GZO); Zr1-xGdxOy (x = 0.5)) films on the e-beam evaporated Y2O3/Ni - 3W substrates. YBCO films with a critical current density Jc of 2.14 times 106 A / cm2 and 1.4 times 106 A / cm2 at 77 K and self-field were grown on the newly developed architectures of Gd2Zr2O7/Y2O3/Ni - 3W and Zr1-xGdxOy (x = 0.2)/Y2O3/Ni - 3W substrates, respectively using a pulsed laser deposition (PLD) process. This work promises a route for producing cost-effective simplified buffer architectures for the RABiTS-based YBCO coated conductors.

The effects of alloying elements on dendritic segregation in Ir alloys were studied by Auger Electron Spectroscopy (AES). The addition of 10 at.% Nb induces significant segregation of carbon and thorium to dendritic interfaces. The addition of 5 at.% Zr to the Ir alloy leads to the formation of an Ir3Zr intermetallic phase, which results in less dendritic segregation of carbon and thorium. This dendritic segregation appears to be linked to the severe cracking observed in the Ir–Nb alloy, but not in the Ir–Zr alloy, after casting and heat treatment. The mechanism for the dendritic segregation was discussed by consideration of the solidification process and the change of carbon solubility in Ir by Zr addition.

Crystallization studies were performed of epitaxial La{sub 2}Zr{sub 2}O{sub 7} (LZO) films on biaxially textured Ni-3at.%W substrates having thin Y{sub 2}O{sub 3} (10 nm) seed layers. LZO films were deposited under controlled humid atmosphere using reel-to-reel slot-die coating of chemical solution precursors. Controlled crystallization under various processing conditions has revealed a broad phase space for obtaining high-quality, epitaxial LZO films without microcracks, with no degradation of crystallographic texture and with high surface crystallinity. Crack-free and strong c-axis aligned LZO films with no random orientation were obtained even at relatively low annealing temperatures of 850-950 C in flowing one atmosphere gas mixtures of Ar-4% H2 with an effective oxygen partial pressure of P(O2){approx}10{sup -22} atm. Texture and reflection high-energy electron diffraction analyses reveal that low-temperature-annealed samples have strong cube-on-cube epitaxy and high surface crystallinity, comparable to those of LZO film annealed at high temperature of 1100 C. In addition, these samples have a smoother surface morphology than films annealed at higher temperatures. Ni diffusion rate into the LZO buffer film is also expected to be significantly reduced at the lower annealing temperatures.

NdBa{sub 2}Cu{sub 3}O{sub 7-{delta}} (NdBCO) films were grown on rolling-assisted biaxially textured substrates (RABiTS) via pulsed laser deposition. c-Axis-oriented epitaxial NdBCO films with high performance were obtained under optimal deposition conditions. Transmission electron microscopy analysis shows that the NdBCO film grown on RABiTS has a clear interface with a CeO{sub 2} cap layer and a nearly perfect lattice structure. The NdBCO film exhibits higher {Tc} of 93.7 K and better in-field J c in magnetic fields and at all field orientations, compared to pure YBCO films.

We report significant enhancement in the flux-pinning of epitaxial NdBa2Cu3O7−δ (NdBCO) films on rolling-assisted biaxially textured substrates (RABiTS) via the incorporation of a two-dimensional array of columnar defects comprised of self-assembled BaZrO3 (BZO) nanodots. Pure NdBCO films and NdBCO films incorporating BZO nanodots, both with the same thickness of 0.6 µm, were epitaxially grown on RABiTS by pulsed laser deposition. A very dense and homogeneous distribution of BZO nanodot columns with typical intercolumn spacing of around 15–20 nm oriented along the c-axis of the film was observed in the NdBCO+BZO film. Compared to the pure NBCO film on RABiTS, the NdBCO+BZO film on RABiTS has an in-field critical current density (Jc) of a factor of more than 2 in 0.1–1.5 T and a smaller power-law exponent α~0.21 in the field regime where Jc~H−α.

RABiTS based metal-organic deposition (MOD) buffer/YBa 2 Cu 3 O 7− δ (YBCO) approach has been considered as one of the potential, low-cost approaches to fabricate high performance second generation coated conductors. The most commonly used RABiTS architectures consisting of a starting template of biaxially textured Ni–W (5 at.%) substrate with a seed layer of Y 2 O 3 , a barrier layer of YSZ, and a CeO 2 cap. In this three layer architecture, all the buffers are deposited using physical vapor deposition (PVD) techniques. Using these PVD deposited templates, 0.8-μm thick MOD-YBCO films with an I c (critical current) of 250 A/cm have been achieved routinely in short lengths. We have developed a low-cost, non-vacuum, MOD process to grow epitaxial buffer layers on textured Ni–5W substrates. The main challenge in this effort is to match the performance of MOD templates to that of PVD templates. We have recently shown that the properties of MOD-La 2 Zr 2 O 7 (LZO) layers can be improved by inserting a thin Y 2 O 3 seed layer. Using MOD-CeO 2 cap layers, we have demonstrated the growth of high performance MOD-YBCO films with an I c of 200 A/cm-width on MOD-La 2 Zr 2 O 7 /Y 2 O 3 /Ni–5W substrates. This approach could potentially decrease the overall cost of the coated conductor fabrication.

Epitaxial NdBa2Cu3O7−δ (NdBCO) films with and without the incorporation of self-assembled nanodots of BaZrO3 (BZO) were grown on rolling-assisted-biaxially-textured substrates (RABiTS) by pulsed laser deposition. Compared to epitaxial YBa2Cu3O7−δ (YBCO) films on RABiTS, NdBCO films are found to have superior performance in applied fields at all field orientations, suggesting the presence of an increased density of pinning centres. The incorporation of columns of self-assembled nanodots of BZO within the NdBCO films results in further enhancement of the in-field Jc at all field orientations. Jc varies as Jc~H−α at intermediate fields, with α values for NdBCO and BZO-doped NdBCO films being ~0.37 and ~0.43, respectively, which are significantly lower than α~0.52 for YBCO film.

We report the epitaxial growth and superconducting properties for NdBa2Cu3O7−δ (NdBCO) films grown on rolling-assisted, biaxially-textured substrates (RABiTS) by pulsed laser deposition. At the optimum deposition temperature of 760 °C, the critical current densities, Jc, at 77 K, self-field, of NdBCO films ranging in thickness from 0.13 to 0.25 µm were found to be in the range 2.2–3.4 MA cm−2, with a lower thickness corresponding to a higher Jc. Compared to epitaxial YBa2Cu3O7−δ (YBCO) films, these NdBCO samples are found to have superior field and angular dependences of Jc. In an applied field of 1 T, NdBCO of the same thickness has more than twice the Jc of YBCO films with a significantly enhanced Jc peak for a field applied parallel to the c-axis. These results suggest the presence of an increased density of c-axis correlated pinning centres within NdBCO films compared to YBCO films.

High-quality lanthanum zirconium oxide (La2Zr2O7 or LZO) films have been deposited and processed on Ni–W substrates using a sol-gel processing approach. It has been demonstrated that crack-free coatings with thicknesses up to 100 nm can be processed in a single step, while thicker coatings (200–225 nm) were processed using a multiple coating and annealing process. Using simulated metalorganic deposition (MOD)-YBa2Cu3O7−δ (YBCO) processing conditions, the barrier properties of the sol-gel LZO coating with a thickness of 120 nm were found to be comparable to that of the standard 3-layer buffer stack deposited using physical vapor deposition. Secondary ion mass spectroscopy depth profile analysis of LZO films annealed in oxygen-18 shows that LZO effectively stops the diffusion of Ni within the first 80–100 nm. Using MOD processes, a CeO2 cap layer and superconducting YBCO layer were deposited on sol-gel LZO/Ni–W. For the first time, using such an all-solution conductor architecture, a critical current (Ic) of 140 A/cm with a corresponding critical current density (Jc) of 1.75 MA/cm2 has been demonstrated. Using a very thin Y2O3 seed layer (∼10 nm) deposited by electron beam evaporation; improved texture quality in the LZO layers has been demonstrated. The performance of the LZO deposited on these samples was evaluated using a sputtered CeO2 cap layer and MOD YBCO layer. Critical currents of up to 255 A/cm (3.2 MA/cm2) with 0.8-μm-thick YBCO films have been demonstrated, comparable to the performance of films grown using physical vapor deposited yttria stabilized zirconia as a barrier layer. Similar experiments using an MOD-CeO2 cap layer and MOD-YBCO layer yielded critical currents of 200 A/cm (2.5 MA/cm2) with 0.8-μm-thick YBCO films.

Epitaxial films of rare-earth (RE = La, Ce, Eu, and Gd) tantalates, RE3TaO7 with pyrochlore structures were grown on biaxially textured nickel-3 at.% tungsten (Ni-W) substrates using chemical solution deposition (CSD) process. Precursor solution of 0.3∼0.4 M concentration of total cations were spin coated on to short samples of Ni-W substrates and the films were crystallized at 1050∼1100 °C in a gas mixture of Ar- 4% H2 for 15 to 60 min. X-ray studies show that the films of pyrochlore RE tantalate films are highly textured with cube-on-cube epitaxy. Improved texture was observed in case of lanthanum tantalate (La3TaO7) film grown on Ni-W substrates. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) investigations of RE3TaO7 films reveal a fairly dense and smooth microstructure without cracks and porosity. The rare-earth tantalate layers may be potentially used as buffer layers for YBa2Cu3O7-δ (YBCO) coated conductors.

Pulsed electron deposition (PED), an ablation-based film growth technique similar to pulsed-laser deposition, is a relatively new method for the physical vapor deposition (PVD) of films. In the area of PVD ex situ superconductor precursor deposition, PED is potentially simpler and more versatile than its traditional e-beam co-evaporation counterpart. We have deposited near-stoichiometric YF 3 –BaF 2 –CuO PED precursors of various thicknesses in moving configuration on RABiTS substrates. Critical current densities ( J c ) as high as 1.6 MA/cm 2 have been achieved on these precursors converted at standard slow rates of roughly 1 A/s, which has previously been necessary for high performance PVD precursors. More importantly, these as-deposited PED precursors were found to be able to tolerate aggressive conversion conditions. Consequently, simultaneous attainment of high conversion rate of nearly 8 A/s and high J c of 1 MA/cm 2 has been realized.

Deposition of (Y2BaCuO5–0.5 nm/YBa2Cu3O7−x∼15 nm) × N multilayer films on rolling-assisted biaxially textured Ni-alloy (RABiTSTM) substrates was investigated, as a new candidate coated conductor architecture for improved flux pinning. Significant enhancements of critical current density (Jc) > 6-fold were measured for applied magnetic fields up to 7 T at 77 K, for multilayer films compared to YBa2Cu3O7−x—only films. By comparing Jc(H)/Jc(0 T) plots of films deposited on RABiTS and single-crystal substrates, the relative increase of Jc(H) from pinning was the same as measured on both substrates. This indicates the varying microstructural properties of the RABiTS templates were, on average, not adversely affecting the pinning enhancements.

Pulsed electron deposition (PED), an ablation-based film growth technique similar to pulsed-laser deposition, is a relatively new method for the physical vapour deposition (PVD) of thin films. This paper describes the implementation of PED in a reel-to-reel apparatus for the room-temperature deposition of fluorine-based precursors onto buffered and textured Ni?W tapes (RABiTS). These precursors have been converted into superconducting YBa2Cu3O7?x films with high critical currents. The influence of the PED parameters, the background gas pressure, and the target composition on the resulting material is analysed. Ion-probe measurements are used to monitor the deposition process and aid in the determination of the optimum deposition conditions. Special emphasis is placed on issues related to throughput and reliability. The strengths of the PED approach are discussed together with a careful analysis of the open issues and limiting factors that determine whether this method can be used for the commercial fabrication of coated conductors.

Adhesion of thin films of epitaxial oxides to nickel-based metallic substrates is important for the successful development of high-temperature superconductor coated conductors. Detachment of epitaxial oxide buffer layers at the oxide/metal interface during either oxide growth or subsequent processing renders the conductor useless. In this study, thermal desorption spectroscopy (TDS) has been used to identify and understand one of the causes of buffer layer detachment, oxidation of carbon at the oxide–metal interface to form carbon monoxide. Results of TDS indicate that on the surface of a bare nickel-based alloy substrate, the rate of carbon oxidation depends on both the supply of carbon from the substrate and the supply of oxygen from the vapor. Sulfur at the surface of the alloy substrate reduces the rate of carbon oxidation. The effectiveness of various treatments of the bare substrate to eliminate CO formation and epitaxial oxide detachment has been demonstrated. TDS provides both a means to evaluate the kinetics of the oxidation reaction and a tool to assess the need and effectiveness of a substrate oxidation treatment.

We have developed a simple alternative buffer layer architecture for the rolling-assisted biaxially textured substrates (RABiTS) approach. Buffer layers with both oxygen and metal diffusion barrier properties are of interest. Cube textured magnesium oxide MgO buffers were grown directly on biaxially textured Ni and Ni–W3 at.% substrates using electron beam evaporation. We have also grown epitaxial MgO layers on 2 m long textured Ni–W3 at.% tapes in a reel-to-reel e-beam evaporation. Highly textured LaMnO3 (LMO) buffers were grown on MgO-buffered Ni substrates using rf sputtering. MgO and LMO buffers have been proved to be good oxygen diffusion barriers and Ni diffusion barriers, respectively. YBCO films with a Jc of 1 × 106 A cm−2 at 77 K and self-field were grown on this newly developed architecture of LMO/MgO/Ni using pulsed laser deposition.

The growth of 0.9–1.0 µm thick Y Ba2Cu3O7−δ (YBCO) films on biaxially textured Ni–3 at.% W (NiW) substrates using the BaF2 ex situ process was investigated at reduced pressures. By varying the water vapour pressure (PH2O), Y–BaF2–Cu–O (YBFCO) precursor films deposited by e-beam co-evaporation were converted at a reduced total pressure (Ptotal) of 50–55 Torr and conversion temperature (TS) of 740 °C for a wet conversion time (tW) of 60 min. Critical current density (JC) values greater than 1 MA cm−2 for the thick YBCO films were obtained under the condition of varying PH2O from low pressure to 10 Torr. The transition temperatures (TC) of the samples were over 90 K with ΔTC = 1.8–2.5 K. Pre-heat treatment of the precursor films on CeO2/Y SZ/Y2O3/Ni/NiW substrates under an O2 atmosphere condition before the conversion resulted in smooth surfaces without large secondary phase particles embedded in the films.

The effects of conversion parameters on transport properties of YBa2Cu3O7-δ (YBCO) films on rolling assisted biaxially textured substrates (RABiTS) in the BaF2 ex situ process were investigated for total pressures Ptotal between 0.1 and 1.3 atm, water vapor pressures PH2O between approximately 7 and 70 Torr and processing temperatures TS between 700 and 790 °C. For this study, a 0.3-μm-thick Y–BaF2–Cu–O precursor film was deposited on a 1-cm-wide Ni=3 at.% W RABiTS with a buffer layer architecture of CeO2/YSZ/Y2O3/Ni deposited in single passes in various reel-to-reel systems for each layer. Under the conditions of Ptotal = 0.1 atm, TS = 740 °C and PO2 approximately 150 mTorr, JC > 2 MA/cm2 was obtained at 77 K and self field for PH2O ≤ 20 Torr. At higher PH2O (=70 Torr), however, the maximum attainable JC decreased. In addition, the JC at these higher PH2O dropped rapidly with increased dwell time. The highest JC, 2.5 MA/cm2, was achieved at 730 °C with Ptotal = 0.1 atm and PH2O approximately 7 Torr. Finally, the variation of IC with wet conversion time was performed at each processing temperature.

Reel-to-reel x-ray diffraction (XRD) and Raman micro-spectroscopy are being evaluated as potential diagnostic tools for on-line feedback in the manufacturing of long-length coated conductors. To facilitate this evaluation, a procedure based on differentially heat-treated Y–BaF2–Cu precursors exposed to time-synchronized phase composition gradients has been developed. Two time-gradient-processed Y Ba2Cu3O7−x (YBCO) tapes of different thicknesses were fabricated using this procedure. The two techniques (used in combination) provided detailed phase and microstructure information as a function of temperature and heat treatment time and identified the same optimum processing time domain windows. More importantly, these deduced optimum times were found to be in close agreement with transport Jc measurements on replicate tapes. In addition, Raman data provided unambiguous identification of key intermediate phases such as BaF2, CuO, Y2Cu2O5, and barium cuprates. Using these results, a hypothetical Y–BaF2–Cu to YBCO reaction mechanism is proposed.

Use of many Ni-based and Cu-based alloys requires deposition of a Ni overlayer prior to the deposition of the seed layer in order to obtain better epitaxy. In this work, Ni overlayers have been deposited by DC sputtering on biaxially textured NiW and Cu substrates. Films were characterized by x-ray diffraction and atomic force microscopy as a function of temperature, film thickness and deposition rate. The optimum temperature range for deposition was found to be around 400?500??C, which results in relatively smooth films with a good cube texture on both substrates.

A reel-to-reel furnace equipped with seven transverse flow modules has been developed to continuously process ex situ BaF2 precursor coated conductors. Experimental results have revealed that as the precursor thickness increases, the processing parameters need to be adjusted in order to obtain high quality YBCO films with Jc' s greater than 1 MA cm−2 at 77 K and self-field. For thin precursors, a relatively aggressive conversion environment of high P(H2O) can be used to obtain epitaxial YBCO with an extremely small amount of undesirable random or a-axis grains. Thin 0.3 µm YBCO was converted on a 2.3 m long by 1 cm wide Ni RABiTS substrate, yielding an end-to-end Ic of 18.9 A (Jc = 0.63 MA cm−2). 16 cm sectional measurements showed that the tape performance is uniform, with a mean Ic of 19.1 A (0.64 MA cm−2) and a 5.4% SD. In thick precursors, however, a less aggressive environment in the form of low P(H2O) is better suited for epitaxial YBCO nucleation during the incubation period. By using a low initial P(H2O) with progressively higher values during later stages of conversion, an end-to-end Ic of 92 A (1.02 MA cm−2) was obtained in 1.1 m long by 1 cm wide 0.9 µm YBCO on better-textured Ni–W RABiTS. This conductor possesses uniform Ic according to 1 cm sectional measurements with a mean Ic of 94.4 A (1.05 MA cm−2) and only 4.3% SD. Further adjustment in the P(H2O) schedule resulted in a 45 cm long sample with end-to-end Ic of 111.8 A (1.24 MA cm−2), a mean Ic of 114.8 A (1.28 MA cm−2) and 4.8% SD. The highest Ic of 138 A (1.53 MA cm−2) was obtained in a short 0.9 µm YBCO sample on a pre-treated Ni–W RABiTS substrate.

In situ X-ray diffraction has been used to study the conversion of ∼3000 A thick, evaporated barium fluoride precursor films to YBCO on buffered metal substrates for a range of conversion time, ramp rate, water vapor pressure, oxygen pressure, and temperature. All films were taken from the same continuously processed tape. Critical current densities as high as 1.34 MA/cm 2 have been obtained for a precursor reaction rate of 2.0 A/s. Higher reaction rates (up to 17 A/s) lead to lower critical current densities and incompletely developed YBCO. The reaction rate increases with increasing P H 2 O , increasing T , and decreasing P O 2 . The reaction rate increases linearly with increasing P H 2 O for the range of pressures used here (0.0125–5.0 mTorr). The formation of non-epitaxial YBCO is generally associated with lower T and higher reaction rate, whereas the formation of barium cerate, which results from a reaction of the precursor and the CeO 2 -capped substrate, is generally associated with higher T and lower reaction rate. General trends in the formation of crystalline phases during conversion may serve as a baseline for development of higher rate conversion processing for thicker, higher performance YBCO.

The current carrying capabilities of RABiTS are connected to the crystalline quality of the seed buffer layer and the stability of the metal/seed layer interface. Our study shows that the epitaxial growth of commonly used seed layers on textured Ni is mediated by a sulfur superstructure present on the metal surface. Many structural defects generated during seed layer growth (secondary orientations, in-plane rotation, incomplete cube texture) can be related to the S surface concentration and superstructure coverage. More generally, our results indicate that the epitaxial deposition of several classes of oxides (fluorite, perovskite, RE/sub 2/O/sub 3/) on several {100} fcc metals depends, in addition to chemical stability and lattice match, on the existence and optimization of S superstructures on the metal surface. On these bases, we discuss issues related to the growth of different oxides on Ni, Ni-alloys, and Pd surfaces having different chemical and structural properties.

Mo3Si single crystals were tested in compression at 1325 °C in vacuum. The 〈1 1 0〉 and 〈1 1 1〉 specimens exhibited pronounced yield points. Slip trace analysis showed the active slip planes to be {0 0 1} and {0 1 2}, respectively, with similar critical resolved shear stresses. The hard 〈1 0 0〉 orientation exhibited neither slip traces nor an anomalous yield point.

We investigated the influence of a chemisorbed S template with c(2 × 2) structure on the epitaxial growth of different oxide buffer layers on {100}〈100〉 Ni. The sulfur superstructure spontaneously forms on the Ni surface during the texturing anneal as a consequence of segregation. However, depending on the initial S bulk concentration and/or specific annealing conditions, the S layer can cover less than the entire substrate's surface. We show that an incomplete c(2 × 2) coverage causes degradation of the seed buffer layer texture as compared to the substrate texture. A simple step consisting of an H2S predeposition anneal can be used to control the superstructure coverage and optimize the seed layer texture.

Low-cost, non-vacuum bulk solution technique has been developed to deposit epitaxial Gd 2 O 3 and La 2 Zr 2 O 7 (LZO) buffer layers directly onto both textured Ni and mechanically strengthened biaxially textured Ni–W(3 at.%)–Fe(1.7 at.%) (hereafter referred to as Ni–W) substrates. A reel-to-reel continuous dip-coating unit was used to produce meter lengths of highly textured, crack-free Gd 2 O 3 and LZO buffers on Ni and Ni–W tapes. Auger electron spectroscopy analysis of the textured Ni–W substrates indicated the presence of sulfur segregation to the surface, which possibly facilitated the growth of solution seed layers directly on Ni–W substrates. On dip-coated seed layers, a reel-to-reel sputtering unit was used to deposit epitaxially 200 nm thick YSZ layers followed by 10 nm thick CeO 2 to complete the RABiTS architecture. Pulsed laser deposition was used to grow YBa 2 Cu 3 O 7− δ (YBCO) films on these tapes in short lengths. YBCO films with J c values of 1.2×10 6 and 1.9×10 6 A/cm 2 at 77 K were obtained on Gd 2 O 3 and LZO buffered Ni–W substrates, respectively. YBCO films grown by ex situ BaF 2 precursor process on 80 cm long, 1 cm wide CeO 2 /YSZ/Gd 2 O 3 /Ni tapes exhibited end-to-end J c of 6.25×10 5 A/cm 2 at 77 K and self-field. This demonstrates that the solution layers provide a very good crystallographic template for the growth of high current density YBCO films, with performance approaching that of vacuum-deposited seed layers.

The microhardness, and tensile and fracture-toughness properties of drop-cast and directionally-solidified Cr-9.25 at.% (atomic percent) Ta alloys have been investigated. Directional solidification was found to soften the alloy, which could be related to the development of equilibrium and aligned microstructures. It was observed that the tensile properties of the Cr–Ta alloys at room and elevated temperatures could be improved by obtaining aligned microstructures. The directionally-solidified alloy also showed increased fracture toughness at room temperature. This trend is mainly associated with crack deflection and the formation of shear ribs in the samples with aligned microstructures. The sample with better-aligned lamellae exhibits greater fracture toughness.

Tensile tests were performed in air and vacuum on Fe–40Al single crystals oriented so that predominately edge or screw dislocations accommodated the plastic strain. It was found for both orientations that ductility was less in air than in vacuum. However, no discernible correlation was found between the character of the dislocations, and the effect of environment.

HASTELLOY B-2 alloy was found to exhibit environmental embrittlement when tested in air and hydrogen at ambient temperature after atomic ordering introduced by a heat treatment at 700 °C for 24 h. Molybdenum in the HASTELLOY B-2 alloy was probably reactive enough to dissociate water vapor in air to generate atomic hydrogen, resulting in hydrogen embrittlement. The percentage of transgranular fracture increased as the test environment changed from hydrogen gas to moist air or water vapor, and vacuum or oxygen. With the addition of 100 wt ppm B, the environmental embrittlement was completely eliminated, with the tensile properties independent of both test environment and strain rate. The fracture mode remained the same, i.e. ductile dimpling, after B-doping when tested in different environments. The immunity of the B-doped B2 alloy to environmental sensitivity remained even after long-term heat treatment. Auger analysis does not detect any boron segregation at the grain boundaries. The mechanism of boron doping in eliminating the environmental embrittlement in the Ni–Mo alloy is apparently different from that in many L1 2 -type alloys such as Ni 3 Al and Ni 3 Si.

Impurity effects were investigated in an Ir alloy of nominal composition Ir–0.3 W–0.006 Th–0.005 Al1. The impurities that were added included Fe, Ni, Cr, Al, and Si, at levels ranging from 50 to 5000 ppm. Of the elements investigated (impurities as well as alloying additions), only Si and Th were found to segregate to the grain boundaries. In alloys doped with low levels of Si (

The current study was undertaken to determine what effects, if any, larger amounts of certain impurities (Al,Cr,Fe,Ni, and Si) might have on the physical metallurgy and mechanical properties of the DOP-26 iridium alloy. This report summarizes the effects of these impurities on grain growth behavior and high-temperature high-strain-rate tensile ductility. Comparisons are made to the grain growth behavior and high-strain-rate tensile properties of the DOP-26 alloy without intentional impurity additions.

Ni 3 (Si,Ti) alloys containing different levels of boron were tensile tested in air at room temperature at two different strain rates. The grain boundary compositions and fracture modes of these alloys were determined by Auger spectroscopy and scanning electron microscopy, respectively. Tensile elongation and fracture mode depend upon the fabrication procedure, heat treatment, and strain rate. Widely different boron concentrations were observed at the grain boundaries, depending on the fabrication procedure and heat treatment. In addition, silicon and titanium were depleted while nickel was enriched at the grain boundaries in all specimens examined. Tensile elongations correlated well with the grain boundary concentration of boron and also with an embrittlement parameter defined as (Si+Ti−B)/Ti. A sharp brittle-ductile transition was found to occur with increasing grain boundary concentration of boron and with decreasing values of the embrittlement parameter (Si+Ti−B)/Ti. The critical grain boundary concentrations corresponding to this transition were found to be sensitive to the strain rate. All the results can be explained in terms of the effect of grain-boundary composition on moisture-induced environmental embrittlement.

Chemical vapor deposited diamond films grown in a hot filament reactor using three filament metals (tungsten, tantalum, and rhenium) have been analyzed for their metal impurity content. This is the first report wherein all three common CVD filament metals have been examined and a single technique used for diamond film analysis. Tungsten carbide filaments yielded the lowest impurity level (few ppm by mass), whereas rhenium yielded the highest (parts per thousand). The effects of filament temperature and addition of ammonia or oxygen to the reactant gas mixture were examined. A correlation was observed between the metal content of the product films and their quality, as judged using Raman spectroscopy. Films with the highest metal content yielded Raman spectra showing the lowest fluorescence background, the smallest sp2 carbon contribution, and the narrowest 1332 cm−1 diamond line.

Critical current densities (Jc) over 2 MA/cm2 have been obtained for epitaxial NdBa2Cu3O7-delta (NdBCO) films deposited via pulsed laser deposition on RABiTS having the configuration of Ni - 3at.%W /Y2O3/YSZ/CeO2. Compared to YBa2Cu3O7-delta (YBCO) films on RABiTS, enhanced Jc in applied magnetic fields is observed for the NdBCO films. Further improvement in in-field Jc is achieved by incorporation of self-assembled BaZrO3 (BZO) nanodots within the NdBCO film. At intermediate fields Jc varies as Jc ~ H-alpha, with alpha values for un-doped and BZO-doped films being ~0.43 and ~0.37 respectively, significantly lower than alpha ~ 0.52 for pure YBCO films. Combined effects of better pinning in NdBCO films over YBCO films as well as creation of columnar defects via incorporation of self-assembled BZO nanodots, result in significant improvement over pure YBCO films on RABiTS.

Slot die coating and conversion of La2Zr2O7(LZO) buffer layers were performed on rolling assisted biaxially textured Ni-W with and without thin Y2O3 seed layers. The slot die coating, drying and processing were all performed using reel to-reel systems. The slot die coated LZO precursor films were then reacted under a variety of conditions and the texture and morphology development were characterized using optical microscopy, scanning and transmission electron microscopy and x-ray diffraction. The conversion conditions were optimized to produce the highest degree of texture in the LZO films. This paper will describe the equipment used for depositing the LZO films and describe the texture and morphology of LZO films optimized for supporting the growth of high Ic (>300 amps per centimeter width) YBCO superconducting films.

The utility of polysynthetically-twinned (PST) TiAl, which contains a high density of parallel, atomically-flat interfaces within a set of identical crystallographic orientations, as a potential model system for a detailed investigation of interface diffusion is explored. Macroscopic PST crystals were grown in an optical float zone fumace. Thin films were cut from oriented crystals and polished with and directions normal to the film. After sputter cleaning, Ag was deposited on one side of the TiAl thin films. Auger spectra were obtained from these films over a wide range of sputter/anneal conditions. The Al and Ti concentrations were analyzed as well as the important impurity elements, S, Ar, C, N and O. Ag diffusion on the interfaces was confirmed. Using the present data and existing knowledge of the microstructure and crystallography of PST TiAl, the potential of this material for providing a detailed understanding of the atomistic mechanisms of interface diffusion is analyzed.

The effect on the growth morphology of changing methane concentration and substrate temperature has been studied in a hot filament chemical vapor deposition reactor. Methane concentration and substrate temperatures were varied from 0.5 to 1.5% and 536 to 1140 °C, respectively. The morphology produced has been quantified as the ratio of the rate of growth in the [100] and [111] crystallographic directions, and the dependence of this ratio on growth parameters has been used to produce a morphology map for the hot filament system. The morphology map generated shows trends similar to those observed for microwave plasma depositions. In addition, the apparent growth rates in the [100] and [111] crystallographic directions have been determined in the region of 1

The mechanical properties of two Zr-base bulk amorphous alloys (BAA), Zr-10Al-30Cu-5Ni (BAA-10) and Zr-10Al-5Ti-17.9Cu-14.6Ni (BAA-11), were studied by both tensile and compressive tests at room temperature in various test environments. The BAA ingots up to 7 mm in diameter were successfully produced by both arc melting and drop casting and induction melting and injection casting. The BAA specimens deformed mainly elastically, followed by catastrophic failure along shear bands. Examination of the fracture region revealed ductile fracture features resulting from a substantial increase in temperature, which was attributable to the conversion of the stored elastic strain energy to heat. Surprisingly, “liquid droplets” located at major shear-band cracks adjacent to the fracture section were observed, indicating the occurrence of local melting during fracture. The angle orientation of shear bands, shear-band cracks, and fracture surfaces relative to the stress axis is quite different for BAA specimens tested in tension and compression. This suggests that both shear stress and normal stress may play a role in developing shear bands during plastic deformation. The tensile properties of BAAs were found to be insensitive to the test environment at room temperature. However, the reaction of BAAs with distilled water and heavy water was detected by laser desorption mass spectrometry (LDMS). These results suggest that moisture-induced hydrogen embrittlement in BAAs may be masked by catastrophic fracture following shear bands.

Three different Ni3(Si,Ti) alloys were cathodically charged with hydrogen and fractured under high vacuum conditions. Scanning Auger microscopy (SAM) and scanning electron microscopy (SEM) were used to determine the chemical compositions of the fracture surfaces and the mode of fracture. The compositions of the transgranular regions were compared with the intergranular regions in order to determine the amount of grain-boundary segregation or depletion of the various constituents. In all the alloys studied, Si and Ti were depleted on the grain boundaries, while Ni was enriched relative to the bulk. Correlations were made between the grain-boundary compositions and the previously reported environmental embrittlement of these alloys. An embrittlement factor defined as [(Si+Ti)/Ti] on the grain boundaries is suggested to be related to the degree of environmental embrittlement at ambient temperatures. Some possible reasons for this correlation are discussed.

This is a reply to Schulson`s comment on the authors` recently published paper. Schulson makes three points in his paper: (a) grain boundaries in Ni-rich Ni{sub 3}Al are enriched in Ni, whereas those in stoichiometric Ni{sub 3}Al are not; (b) boron increases this Ni-enrichment, in some cases leading to a detectable disordered grain-boundary solid-solution; and (c) the embrittlement reported by the authors may be related more to an attack by hydrogen on Ni-enriched grain boundaries, made richer by the addition of B, than to an attack on Ni{sub 3}Al per se. The authors examine all three points again.

Samples of two polysynthetically twinned (PST) TiAl alloys (Ti–49at.%Al and Ti–49at.%Al-0.6at.%Cr) were fractured in different environments at room temperature and the fracture surfaces were analyzed using scanning Auger and scanning electron microscopy. These techniques were employed to determine the fracture mode and the chemical composition of the fracture surfaces of the TiAl alloys. Our study clearly indicates that all of the fracture surfaces exposed by bend testing were α2, except when fracture occurred across the lamellae. No chromium enrichment was observed on any of the surfaces exposed during the fracture. From these observations, it was concluded that the fracture mode for these materials is cleavage through the α2-phase.

Ni 3 Al alloys, of nominal composition Ni-24 at.% Al and three different B concentrations (50, 100 and 500 wppm), were tensile tested at room temperature in high-purity H 2 gas at pressures ranging from ∼6 × 10 −8 to 7 × 10 3 Pa. The highest elongations to fracture were obtained in ultrahigh vacuum: ∼36, 45 and 60% for the 50, 100 and 500 wppm B alloys, respectively. With increasing H 2 pressure, the ductility of all three alloys dropped precipitously. Accompanying this drop in ductility was a change in the fracture mode from predominantly transgranular to predominantly intergranular. An intriguing result of our present study is that, at the higher H 2 pressures employed, B-doped Ni 3 Al (50 or 100 wppm) is brittle and fractures predominantly intergranularly, whereas B-free Ni 3 Al is ductile and fractures predominantly transgranularly. This result indicates that B—by possibly promoting the dissociation of molecular H 2 into atomic H— embrittles Ni 3 Al in a dry H 2 environment, unlike in H 2 O-containing environments, where B suppresses grain boundary fracture and improves the ductility of Ni 3 Al. © 1997 Acta Metallurgica Inc .

Addition of a small amount of oxygen to the CH{sub 4} and H{sub 2} feed gas permits hot filament assisted chemical vapor deposition (HFCVD) of diamond at significantly lower filament and substrate temperatures. The former can be reduced to as low as 1400{degree}C and the latter to 450{degree}C. The amount of oxygen required is much lower than what has been used in most studies of the oxygen effect. For each CH{sub 4}{percent}, there is a narrow window in the O/C ratio, where diamond can be deposited at low temperature. This window shifts to higher O/C ratios as the CH{sub 4}{percent} increases and expands with increases in filament temperature. The effects of changing substrate and filament temperatures on growth rate and film quality are often not consistent with previous experiences with HFCVD of diamond. Increasing the filament temperature does not always improve the growth rate and film quality, and the non-diamond carbon content in the film is dramatically reduced at lower substrate temperatures. Optimum conditions were found that gave reasonable growth rates ({approximately}0.5{mu}m/h) with high film quality at filament temperatures below 1750{degree}C and substrate temperatures below 600{degree}C. With these reductions in operating temperatures, power consumption can be significantly reduced and themore » filament lifetime extended indefinitely. {copyright} {ital 1997 Materials Research Society.}« less

The addition of a small amount of oxygen to a hot filament assisted chemical vapor deposition reactor allows diamond to be deposited at significantly lower filament and substrate temperatures. Scanning electron microscopy and Raman spectroscopy are used to compare films grown with and without oxygen addition as a function of substrate temperature at high and low filament temperatures. Oxygen addition is found to favor growth of high quality diamond at low substrate temperatures (

Samples of two polysynthetically twinned (PST) TiAl alloys (Ti-49 at% Al and Ti-49 at%Al-0.6 at%Cr) were fractured under different environmental conditions at room temperature, and the fracture surfaces were analyzed using scanning Auger and scanning electron microscopy. These techniques were employed to determine the fracture mode and the chemical composition of the fracture surfaces of the TiAl alloys. Our study clearly indicates that all of the fracture surfaces exposed by bend testing were α2, except when fractures occurred across the lamellae. No chromium enrichment was observed on any of the exposed surfaces. From these observations, it was concluded that the fracture mode for these materials is cleavage through the α2-phase.

High performance YBCO films have been realized through MOD approach by hybrid of Ba-TFA with TMAP (trimethylacetates with propionic). High I/sub c/ over 140 A/cm-w and high J/sub c/ up to 2.4 MA/cm/sup 2/ at 77 K for 0.6 /spl mu/m YBCO films on RABiTS have been shown on un-bridged and bridged samples. Texture characterizations performed in XRD four-circle diffractometer confirmed well-aligned YBCO grains with a volume fraction of cubic phase /spl sim/95%. T/sub c/ measurements showed sharp transitions in less than 1 K and zero resistance above 90 K. The formation of YBCO phase follows the mechanism of interface island nucleation and epitaxial growth. The liquid phase in the precursor enhances Y123 epi-growth and accelerates the conversion process.

The rate at which thin film YBCO precursor converts to high J/sub c/ superconductor depends on many factors including the method by which the precursor is prepared, the precursor film thickness, and the conditions during precursor deposition and conversion. Using in-situ X-ray diffraction and reduced pressures, we have examined and compared conversion rates for precursors prepared by physical vapor deposition (PVD) and metal-organic deposition (MOD). For conversion conditions found to be optimal for obtaining high J/sub c/ and high precursor conversion rate (G/sub p/) for PVD precursors, the G/sub p/ is nearly identical (/spl sim/0.2 nm/sec) for the both PVD and MOD precursors. The development of crystalline BaF/sub 2/ is, however, distinctly different for the two precursors. This may suggest that conversions of these PVD and MOD precursors follow different chemical pathways. For both precursors, higher G/sub p/ is realized using higher water pressure and ramp rate. Although G/sub p/ is higher, YBCO is randomly oriented and J/sub c/ is zero for the PVD precursor. The MOD precursor seems to be generally more tolerant of higher reaction rates than the PVD precursor. The reaction rate versus P/sub H2O/ determined for both precursor types shows a square root of P/sub H2O/ dependence of G/sub p/. This behavior is consistent with a reaction rate limited by gas-phase HF removal.