D. S. Easton

A prediction of the stress state in Nb3Sn superconducting composites

The critical current Ic as a function of applied tensile strain depends upon the residual stress state that exists in Nb3Sn bronze‐process composite conductors after cooling from the reaction temperature to 4.2 K. This paper presents a simple analytical method to predict the residual stress state and the stress‐strain curves of Nb3Sn composites at 4.2 K. The method requires only a prior knowledge of the volume fraction of the composite components and the percentage of Nb converted to Nb3Sn. Experimental results and analytically derived values are compared and good agreement is generally found.

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...

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.)

A new metastable phase near 60 at% Zr from amorphous Ni-Zr

Metallic glass alloys of Ni-Zr were prepared by vapour and liquid quenching. For glass compositions at and near 60 at% Zr, crystallization proceeded by the sequence: amorphous → metastable crystalline phase → NiZr2 + NiZr. The metastable Ni40Zr60 phase exhibited a very distinctive X-ray diffraction pattern and was present in both liquid- and vapour-quenched samples. Long-term anneals of samples with the metastable structure produced the equilibrium phases NiZr2 and NiZr. The crystallization of the amorphous structure directly to a single metastable phase shows a correspondence between the compositions of the amorphous and crystalline phases. These results thus suggest a connection between the short-range structure of the glassy phase and the crystalline phases to which they transform. An observation of peaks and valleys in a plot of the X-ray scattering vector,Qp, against glass composition is noted.

Crystallization kinetics of Mo-N

Thin film samples of molybdenum were electron-beam evaporated at 80 K in a 6.6×10−4 Pa partial pressure of nitrogen and subsequently analysed for structure, superconductivity and crystallization behaviour. X-ray diffraction of as-deposited samples showed a typical amorphous structure and fcc and bcc phases upon heating. A superconducting transformation at 7.0 to 7.2 K was detected by the resistivity technique. Isothermal DSC scans obtained for a range of temperatures were calibrated and integrated to produce transformed volume (x)-time data. All plots of In [-In (1−x)] against Int have two linear regions with slopes ofn1≅3.6 for 0<x<0.5 andn2≅ 2.0 for 0.7<x<1; wheren is the principal parameter in the Johnson-Mehl-Avrami theory of the crystallization transformation. The relationship of this binary pattern of crystallization to the observation of two apparent crystallization peaks in dynamic DSC scans of several samples of electron-beam deposited Mo-N and to the appearance of both Mo-bcc and Mo2N-fcc phases in the X-ray patterns of the crystallized samples is discussed. Possible patterns of simultaneous or successive modes of nucleation and growth, including combinations of varying nucleation rates, different dimensionals of growth, and interfacial and diffusion controlled growth, are considered.

Some properties of electron beam evaporated amorphous Mo-N films*

A series of amorphous Mo-N films was prepared by electron beam evaporation of molybdenum in varying partial pressures of nitrogen and deposited onto substrates cooled to about 80 K. The alloy films were characterized by X-ray diffraction, the superconducting transition temperature,Tc, and the crystallization temperature,Tx. The maximumTc (8.3 K) and sharpest transition occurred with the minimum nitrogen pressure necessary to form the amorphous structure, as revealed by X-ray diffraction. After annealing the as-deposited films, both bcc and fcc phases were found with the bcc/fcc ratio decreasing with increasing nitrogen partial pressure. Differential scanning calorimetry (DSC) measurements showed significant differences in the shape of peaks associated with either bcc, bcc+fcc, or fcc phases. The temperature,Tx, associated with the fcc crystallization increased with nitrogen content. Heats of crystallization had an average value of about 63 J g−1. Changes in position of the first amorphous X-ray diffraction peak showed that the amorphous structure was expanded by increasing nitrogen content.

Evidence for Microstructural Effects Under Strain in Bronze Process Nb3Sn

The possible effects of externally applied strain on the flux pinning process can be divided into two groups according to whether the significant property changes produced by strain are associated with the crystalline defects which act as flux pinners, or with the bulk superconducting material which carries the supercurrent Changes in the latter are reflected in the equilibrium properties such as the upper critical field, BC2, and the Ginzburg-Landau parameter, κ. To first approximation, changes in the former are not seen in the equilibrium properties, but may affect the bulk pinning force, Fp, by changing the number of pinning centers or their strength. For want of a better term, we have called changes associated with the flux pins microstructural effects, even though they may or may not involve gross changes in structure such as the martensitic transformation which is known to occur in Nb3Sn at low temperature.