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  • 1
    Electronic Resource
    Electronic Resource
    Inverse melting of metastable Nb–Cr solid solutions (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 80 (1996), S. 2156-2168 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Metastable body-centered-cubic (bcc) Nb–Cr solid solutions were produced by mechanical alloying over a wide concentration range, and investigated by x-ray diffraction, differential thermal analysis, and transmission electron microscopy. For comparison, metastable bcc and amorphous Nb–Cr alloys were fabricated by sputtering. Upon annealing, the solid solutions prepared by mechanical alloying undergo a transformation to an amorphous phase. The vitrification is polymorphous for a composition Nb45Cr55 whereas two-phase mixtures of amorphous and bcc phases are formed for other concentrations, indicating that metastable equilibria between these two phases develop upon heat treatment. These results, combined with a detailed analysis of the thermodynamic functions of the system, reveal that the amorphous phase has a lower enthalpy and entropy than the bcc phase near equiatomic composition. In consequence, the Gibbs-energy curves of bcc and liquid/amorphous Nb45Cr55 phases versus temperature exhibit two points of intersection, i.e., two melting points, with the amorphous phase being thermodynamically more stable than the bcc phase at low temperatures. Inverse melting is thus thermodynamically possible in the Nb–Cr system, so that the metastable crystalline phase would melt upon cooling. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.363048
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  • 2
    Electronic Resource
    Electronic Resource
    Determination of concentration depth profiles using total-reflection x-ray fluorescence spectrometry in combination with ion-beam etching (1996)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 67 (1996), S. 2332-2336 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A new method for quantitative concentration depth profiling is presented. It combines a sputter-etching technique with ex situ total-reflection x-ray fluorescence analysis. It allows quantitative depth-resolved analysis of alloys consisting of elements with atomic number ≥13. Basic requirements for a high depth resolution are reported. With this method, an experimental depth resolution of 1.4 (±0.4) nm is obtained that is comparable with the best achievable results from other depth profiling methods. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1146941
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  • 3
    Electronic Resource
    Electronic Resource
    Concentration-depth profiling using total-reflection x-ray fluorescence spectrometry in combination with ion-beam microsectioning techniques (1995)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 66 (1995), S. 20-23 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A new method for the determination of vertical concentration profiles and its application to bilayer diffusion couples are presented. The method combines sputter sectioning techniques with ex situ total-reflection x-ray fluorescence (TXRF) analysis. It allows quantitative depth-resolved analysis of alloys consisting of elements with atomic numbers ≥13, quantitatively and depth resolved. In contrast to other surface sensitive techniques like Auger electron spectrometry or secondary ion mass spectrometry, TXRF offers the advantage of determining the sampling depth with the same instrument, so that no assumptions about sputtering rates or auxiliary measurements are necessary. With this method a depth resolution of 2.5 nm is obtained, which is comparable with the best achievable results from other depth profiling methods. Moreover, an additional TXRF scan in a nondestructive mode of operation makes a mutual comparison with the results of the sectioning technique possible. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1145260
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  • 4
    Electronic Resource
    Electronic Resource
    Laser processing of Nb3Al(Ge, Si) high-field superconductors (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 74 (1993), S. 2686-2691 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The laser irradiation process is a promising method for the fabrication of metastable Al5–Nb3Al(Ge,Si) compounds. Thereby Nb-sheathed Nb-25 at. % Al(Ge,Si) microcomposite tapes are prepared by a conventional powder metallurgical method and heat treated by a high-intensity CO2-laser beam. The parameters selected during laser beam irradiation of the composite determine the relative amounts of molten, heat-affected or unreacted material. In order to obtain large overall superconducting currents, it is favorable to melt a large fraction of the microcomposite without melting the surrounding Nb sheath. The cooling rate after laser beam irradiation determines the structure and the formation of metastable phases in the resolidifying material. The sequences of the process are simulated by heat flow calculations, which consider laser power, beam diameter, tape velocity, thickness, and microstructure, as well as heat transfer coefficients. From these calculations we could deduce optimum conditions for the laser treatment, leading to Jc values of 105 A cm−2 at 10 T and 104 A cm−2 at 16 T.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.354662
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  • 5
    Electronic Resource
    Electronic Resource
    Amorphization of Ti/Cr multilayer films by solid-state reaction (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 2249-2253 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The solid-state reaction in Ti/Cr multilayered diffusion couples at temperatures between 450 and 650 °C was investigated by x-ray diffraction. It was found that an amorphous phase is formed at a concentration around 55 at. % Cr. The results are in excellent agreement with those obtained from mechanically alloyed powders, and with the corresponding metastable phase diagram calculated by the calculation of phase diagrams method. The observations strongly suggest that inverse melting is possible in the Ti-Cr system at around 55 at. % Cr.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.353129
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  • 6
    Electronic Resource
    Electronic Resource
    Metastable phase formation in mechanically alloyed and ball milled Ti–Si (1992)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 72 (1992), S. 2478-2487 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The Ti–Si system is chosen as a model system to study the mechanical alloying of elemental powder blends in which one of the components is brittle. In addition, the stability of intermetallic compounds upon milling is investigated, in order to interpret the metastable phase formation during the milling process. Detailed x-ray investigations show that during mechanical alloying, a solution of Si in Ti preceeds the formation of amorphous or intermetallic phases. The selection of phases formed, as well as the final state, are influenced by the overall composition of the powder blends, and by the milling procedure applied. However, predictions can be made by considering the free energy curves of the stable and metastable phases in the Ti–Si system, which have been calculated by the CALPHAD method. Milling of the intermetallic compounds generally leads to the same final state as the mechanical alloying of elemental powder blends with corresponding compositions. These results demonstrate that the intermetallic compounds are energetically destabilized upon milling, allowing for phase transformations into metastable phases, such as amorphous or metastable crystalline alloys. The energetic destabilization of intermetallic compounds mainly originates from the chemical disorder caused by the milling process.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.351594
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  • 7
    Electronic Resource
    Electronic Resource
    The thermodynamics of amorphous phases in immiscible systems: The example of sputter-deposited Nb–Cu alloys (1997)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 81 (1997), S. 6024-6030 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Amorphous metallic alloys, frequently observed to occur in systems with large negative heats of mixing, are much less common in systems which are immiscible in the equilibrium solid state, such as Nb–Cu. However, amorphous Nb–Cu alloys can be produced over a wide composition range by sputtering. Using isothermal and nonisothermal differential scanning calorimetry, both the kinetics and the thermodynamics of these amorphous Nb–Cu alloys were characterized quantitatively. It was found that the formation enthalpies of the amorphous alloys amounted to only 4.5–7.6 kJ/g atom. These data were combined with a modeling of the thermodynamic functions of the system. The unexpected low enthalpies and Gibbs energies of the amorphous phase demonstrate the thermodynamic stabilization of the liquid phase which develops with undercooling. This is connected with a change of sign in the heat of mixing of the liquid phase, which is positive at high temperatures and negative at low temperatures. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.364451
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  • 8
    Electronic Resource
    Electronic Resource
    Superconducting properties of powder metallurgically processed Ti-alloyed Nb3Sn microcomposites (1987)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 62 (1987), S. 1937-1942 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: CuNb3Sn microcomposite wires with Ti additions have been produced by the hot-powder-metallurgical preparation method. The influence of the Nb3Sn filament diameter, heat treatment, and Ti addition on the superconducting properties are discussed on the basis of critical current densities (up to fields of 22 T), critical temperatures, as well as x-ray microprobe analyses. Critical current measurements of Nb3Sn multifilamentary tapes indicate that filament/matrix interfaces are dominant flux pinning centers in these composites. When Ti is incorporated into the A15 phase, the upper critical field is enhanced by about 2 T with respect to unalloyed superconductors. A drastic improvement of the superconducting properties can be achieved by the utilization of larger Nb powder particles. In microcomposites prepared from larger Nb powder (100–300 μm), Bc2 is enhanced by about 3–4 T with respect to finer Nb powder (20–45 μm). Critical fields up to 21.4 T and critical current values of 1.2×104 A/cm2 at a magnetic field of 16 T and 4.5×103 A/cm2 at 18 T are obtained for microcomposites prepared from coarser Nb powder with an addition of 2 wt. % Ti.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.339530
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  • 9
    Electronic Resource
    Electronic Resource
    Stability limits of the perovskite structure in the Y-Ba-Cu-O system (1989)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 54 (1989), S. 2148-2150 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The equilibrium (molecular) oxygen partial pressure of the Y1Ba2Cu3O7−x phase has been measured by an electrochemical method. The investigations are focused on the low-temperature range between 560 and 750 °C and on oxygen stoichiometries close to the oxygen-poor phase boundary. In particular, the oxygen partial pressure has been determined below which the perovskite structure becomes thermodynamically unstable. This critical value should be considered during the preparation of high Tc Y1Ba2Cu3O7−x superconductors, especially in the case of an in situ preparation of thin films.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.101513
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  • 10
    Electronic Resource
    Electronic Resource
    Phase formation in mechanically alloyed Nb-Al powders (1988)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 53 (1988), S. 1399-1401 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Nb-Al alloys have been prepared by mechanical alloying of elemental crystalline powders and investigated with x-ray diffraction and transmission electron microscopy. Crystalline equilibrium phases form for Al-rich alloys. Amorphous phase formation is observed for the Nb50Al50 composition. Mechanically alloyed powders with more than 70 at. % Nb exhibit the metastable bcc (A2) solid solution which transforms after thermal annealing into the A15 phase. Formation of the different phases can be understood by considering the free-energy curves of the amorphous and bcc phases as calculated from the phase diagram and additional thermodynamic data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.99989
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