J. O. Scarbrough

Oxidation induced decomposition of YBa2Cu3O7−x

At low oxygen potentials, YBa2Cu3O7−x decomposes by chemical reactions involving reduction. The results of this study show that the compound also decomposes at higher oxygen potentials. The initial decomposition products were found to be Y2BaCuO5 and a Ba‐Cu oxide phase. The Ba‐Cu phase was found to be a peroxide‐type and the decomposition can be described by a chemical reaction involving oxidation: 4(YBa2Cu3O7−x)+(1/2 − 3/2δ +2x)O2⇄2Y2BaCuO5+3Ba2Cu3O6−δ+CuO. At 800 °C the equilibrium pressure for this reaction is slightly <1 bar, and the enthalpy change is ∼80‐kJ/mol Y123. This result is not consistent with the conclusions drawn from some other studies of the stability of the superconducting Y‐Ba‐Cu‐O compounds, and the differences are discussed.

A study of cooling rates during metallic glass formation in a hammer and anvil apparatus

A model is presented of the simultaneous spreading and cooling of the liquid drop in a hammer and anvil apparatus for rapid quenching of liquid metals. The viscosity of the melt is permitted to vary with temperature, and to avoid mathematical complications which would be associated with spatial variation of the viscosity, Newtonian cooling is assumed. From an expression for the force required to spread the specimen, coupled equations for the mechanical energy balance for the system and the heat transfer from the sample to the hearth and hammer were obtained, and solved numerically. The sample reaches its final thickness when the force required to deform it becomes greater than the force exerted on it by the decelerating hammer. The model was fit to measurements of sample thickness versus hammer speed, using the interface heat transfer coefficient, h, as an adjustable parameter. The values of h so obtained vary somewhat with the melt alloy/substrate metal combination. From predicted cooling curves, the eff...

Some observations of the effects of high pressures and temperatures on the stability of YBa2Cu3O7−x

Hot isostatic pressing and heat treatments in high‐pressure oxygen were used to study the effect of pressure on the stability of YBa2Cu3O7−x. At 875 °C the compound decomposes at an oxygen pressure of about 7 bars. The decomposition involves formation of Y2BaCu3O5 and a Ba‐Cu oxide phase. Formation of the latter phase involves additional oxidation because this phase can contain as many as 1.5 oxygen atoms per barium atom. The decomposition can most simply be visualized by considering the chemical reaction 2(YBa2Cu3O7−x)+( (1)/(4) +x−y/2)O2→Y2BaCuO5+Ba3Cu5O9.5−y. Encapsulated samples also decompose during hot isostatic pressing, but the process appears to be more complex. The compound YBa2Cu4O8 was detected in addition to the two products observed in oxygen‐treated samples. The reaction above, with YBa2Cu3O7−x serving as an oxygen source, aids in understanding the microstructural observations for encapsulated samples. Decomposition can be completely reversed by annealing in air for 20 h at 875 °C.

Crystallization of Zr-Ni metallic glasses

The crystallization of Zr-Ni metallic glasses of composition between 55 and 70 at % Zr has been investigated using differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The samples were prepared by splat quenching in an arc-hammer device. Transformation temperatures, effective activation energies, and enthalpy changes are reported as a function of composition. Results of XRD patterns obtained as a function of annealing temperature in the DSC are presented. A high temperature exothermic DSC peak and XRD patterns indicate the presence of a metastable phase which occurs between 57 and 63.5 at % Zr. The results tend to support suggestions of a connection between the short range structure of the glass and the crystalline phase to which it transforms. It was found that the metastable phase, whose presence is strongest at 57 to 59 at % Zr, and the process of phase separation around the eutectic composition (63.5 at % Zr) play important roles in the crystallization process.

Retardation of annealing embrittlement in iron-based glasses by microaddition of cerium

Abstract The effects of microadditions of cerium on the rate of annealing embrittlement, stress relief, and as-quenched magnetic domain structure in three iron-based metallic glasses have been investigated. Ribbons melt-spun from optimally doped castings of composition Fe 80 B 16 Si 2 C 2 and Fe 80 B 20 did not embrittle during annealing until the onset of crystallization, whereas Fe 78 B 13 Si 9 ribbons were unaffected by cerium additions. In the first two glasses optimal doping resulted in the disappearance of quenched-in stresses in the ribbons, as evidenced by an absence of maze domains, and, for the Fe 80 B 20 alloy, a slight enhancement of stress relief rate for small times and temperatures. The effects of doping were qualitatively different in the Fe 78 B 13 Si 9 glass. We conclude that annealing embrittlement in the first two glasses is an impurity effect, and that the embrittling impurities are oxygen and/or sulfur, the dissolved concentrations of which are strongly reduced by cerium additions. That low concentrations (50–100 at. ppm) of these impurities are sufficient to cause annealing embrittlement suggests that impurity segregation may be involved in the embrittlement process.

The effect of strain upon the scaling law for flux pinning in bronze process Nb3Sn

The semiempirical scaling law for flux pinning, Fp=ABnc2bl (1−b)m has been tested for bronze process Nb3Sn conductors under uniaxial tension. The dependences of the bulk pinning force Fp on the upper critical field Bc2 and the reduced field, b=B/Bc2, were weakly affected by strain. However, the factor A, which depends on the number and strength of the pinning centers, and on the Ginzburg‐Landau parameter κ decreased by more than a factor of 20 when the compressive prestrain on the Nb3Sn was removed by application of external stress. The variations of Tc and Bc2 with strain suggest that the change in κ is not sufficient to account for the change in A. Therefore, it is probable that strain induces microstructural changes which affect the number and/or strength of the pinning centers.

Superconductivity in Heat‐Treated Nb‐Zr Alloys

A tenfold increase in critical current density was achieved by short heat treatments of Nb- Zr superconductors subjected to cold deformation. The two effects are additive, and the highest critical current densities are attained by the proper combination. Critical current density data for 72% Nb-28% Zr at 4.2 deg K are presented in graphical form. (L. N. N.)

The thermal and mechanical stability of Fe90−xAlxZr10 metallic glasses☆

Amorphous Fe90−xAlxZr10 alloys were prepared by rapid solidification in either an arc hammer apparatus or by melt-spinning for the composition range 0 25. An annealing embrittlement phenomenon was observed for the alloys with x > 5 in the ductile melt-spun ribbons. The thicker foils produced in the arc hammer apparatus were generally brittle as-cast even though the structure was completely amorphous. The annealing embrittlement was observed to correlate with increases in microhardness. The composition dependences of the crystallization behavior and the annealing embrittlement are consistent with recent suggestions that viscosity increases caused by structural relaxation during annealing are involved in embrittlement.

The effect of chemical short range ordering on crystallization of Zr-Ni glasses

Abstract Previous measurements of low temperature specific heat (ref. 1) demonstrated the presence of phase separation and chemical short range ordering in Zr-Ni glasses with zirconium contents from 60 to 66 at. % Zr. This ordering is found to play a significant role in the crystallization process.

Ambient temperature thermal conductivity of YBa2Cu3O7−x

A comparative method was used to measure the thermal conductivity λ of a YBa2Cu3O7−x sample over the temperature range 305–360 K. Using one sample, data were obtained at two different x levels. Surprisingly, the smaller x value (higher Tc) produced a lower thermal conductivity. This observation is discussed in terms of scattering of phonons by point defects and electrons.