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Interaction of stress with the martensitic phase transition in A15 compounds

Description: Recently there has been a resurgence of interest in the effect of the martensitic phase transition which occurs in many A15 compounds on superconductivity and on elastic and anelastic behavior. Since in many practical applications, A15 compounds are subject to considerable stress and strain, it is of interest to examine the interaction of stress with the martensitic transition; this paper is an examination of the effects of stress predicted by a simple Landau model which successfully describes many features of the transition and the related temperature dependence of the elastic modulus (c/sub 11/-c/sub 12/)/2. The effect of stress on the temperature ranges of stability and metastability of various types of martensitic domain is discussed. The non-linearity of the stress-strain relation in a polycrystalline A15 is studied.
Date: January 1, 1981
Creator: Welch, D.O.
Partner: UNT Libraries Government Documents Department

Alteration of the superconducting properties of A15 compounds and elementary composite superconductors by non-hydrostatic elastic strain

Description: Elastic strains alter (usually, but not always, adversely) the critical temperatures, magnetic fields, and current densities of superconducting A15 compounds; non-hydrostatic strain states are particularly effective in this regard. This paper is a review of the experimental evidence, obtained by a variety of techniques, concerning the strain dependence of the critical properties of a number of A15 compounds and a discussion of theoretical models for describing such effects.
Date: January 1, 1979
Creator: Welch, D.O.
Partner: UNT Libraries Government Documents Department

Analysis of critical current-bend strain relationships in composite Nb/sub 3/Sn superconducting wires

Description: In order to be used successfully in fusion magnets, Nb/sub 3/Sn conductors must meet several mechanical strain criteria, including tolerance to bending strains encountered during magnet construction. Since Nb/sub 3/Sn is extremely brittle much information has been generated regarding the sensitivity of these conductros to tensile strain. A recent comparison of critical current-bend and tensile test data indicates that the strain required to initiate compound cracking during bending is significantly less than the strain required to do so by tensile of critical current on bending strains in monofilamentary Nb/sub 3/Sn wires is calculated and compared with experimental data. The calculation takes into account a shift in the composite's neutral axis which occurs during bending. The analysis correctly predicts the observed depdndence of the critical current on bending strains.
Date: January 1, 1979
Creator: Luhman, T. & Welch, D.O.
Partner: UNT Libraries Government Documents Department

A theoretical comparison of the effects of the shape of the pinning potential and a distribution of pinning energies on the apparent pinning energy as measured by magnetic flux creep

Description: In the analysis of magnetic flux creep experiments it is assumed that, at a given temperature, the pinning energy which must be overcome by thermal activation depends on the magnetic induction and its gradient by U(B,{nabla}B){congruent} U{sub p}(B)(1-{nabla}B/{nabla}B{sub max}){sup n} where U{sub p} is the pinning well depth and {nabla}B{sub max} corresponds to the critical current density with no thermal activation. Customarily, n is assumed to be unity and any unusual temperature dependence of U{sub p} is then ascribed to a distribution of well depths. However, realistic assumptions about the shape of the pinning potential yield 3/2 {approx lt} n {approx lt} 2, which yields an apparent distribution of well depths in the conventional analysis. Simple models will be used to illustrate the characteristics of these two quite different origins for the apparent temperature dependence of well depth obtained from magnetic flux creep rates. 13 refs., 8 figs.
Date: September 24, 1990
Creator: Welch, D.O.
Partner: UNT Libraries Government Documents Department

Bend strain tolerances of a Nb/sub 3/Sn conductor proposed for use in the magnetic fusion energy program

Description: Bend strain tolerances were studied on a 2869 filament bronze-processed Nb/sub 3/Sn wire conductor in magnetic fields to 8 T. Relative values of the wire's current transfer length to twist pitch were shown to influence the bend-strain tolerance. Low matrix resistivities, associated with Sn-depleted bronzes following heat-treatments of 48 h at 725/sup 0/C, produce current transfer lengths less than the twist pitch, 10 mm. The resulting bend-strain tolerances, at 10/sup -12/ ohm.cm, are improved over those found for shorter heat-treatment times. Results from bend-fatigue experiments were divided into two domains separated by the strain value required to produce compound cracking, epsilon/sub f//sup B/. Applied bending strains less than epsilon/sub f//sup B/ were found to increase zero strain critical current values and this increase was independent of the number of fatigue cycles. When applying strains large enough to produce cracking in the compounds critical currents decreased from their asreacted values tending to reach a minimum after several fatigue cycles. Evidence exists for a neutral axis shift during bending and slight differences between tensile and bend strain tolerances are accounted for in terms of such a shift.
Date: January 1, 1980
Creator: Luhman, T; Welch, D O & Suenaga, M
Partner: UNT Libraries Government Documents Department

Stress and strain effects on the properties of composite superconductors

Description: Practical superconductors for use in the production of high magnetic fields are generally in the form of composites of filaments of superconducting material embedded in a matrix of normally conducting material. Lorentz forces which arise during magnet operation are examples of sources of external stress, while internal stresses can arise during the fabrication of the composite superconductor, primarily due to differential thermal contraction between different materials in the composite. The properties of superconducting compounds are often sensitive functions of the elastic strain state in the compound; consequently there is a strong coupling between the mechanical and electrical properties of composite superconductors. The basic features of this phenomenon will be illustrated by a discussion of the properties of simple composite superconductors.
Date: January 1, 1982
Creator: Welch, D.O.
Partner: UNT Libraries Government Documents Department

Theoretical models of flux pinning and flux motion in high- Tc superconducting oxides

Description: Various issues involved in the development of phenomenological models of flux pinning and motion in high-{Tc} oxides are discussed. A simplified model is presented for the critical current density and is used to examine the question of whether flux flow results from an instability due to plasticity of the flux-line array or from pin breaking.
Date: January 1, 1991
Creator: Welch, D.O.
Partner: UNT Libraries Government Documents Department

Internal friction due to domain-wall motion in martensitically transformed A15 compounds

Description: A lattice instability in A15 materials in some cases leads to a cubic-to-tetragonal martensitic transformation at low temperatures. The transformed material orients in lamellae with c axes alternately aligned along the <100> directions producing domain walls between the lamellae. An internal-friction (delta) feature below T/sub m/ is attributed to stress-induced domain-wall motion. The magnitude of the friction increases as temperature is lowered below T/sub m/ as (1-c/a) increases, and behaves as (1-c/a)/sup 2/ from T/sub m/ down to the superconducting critical temperature where the increasing tetragonality is inhibited. The effect of strain in the lattice is to decrease the domain-wall internal friction, but not affect T/sub m/. Neutron-induced disorder and the addition of some third-elements in alloying decrease both delta and T/sub m/, with some elements reducing only the former. Less than 1 at. % H is seen to completely suppress both delta and T/sub m. Martensitically transformed V/sub 2/Zr demonstrates low-temperature internal-friction and modulus behavior consists with easy ..beta../m wall motion relative to the easy m/m motion of the A15's. For the V/sub 2/Zr, a peak in delta is observed, qualitatively in agreement with expected ..beta../m wall motion.
Date: January 1, 1985
Creator: Snead, C.L. Jr. & Welch, D.O.
Partner: UNT Libraries Government Documents Department

Theoretical models of flux pinning and flux motion in high-{Tc} superconducting oxides

Description: Various issues involved in the development of phenomenological models of flux pinning and motion in high-{Tc} oxides are discussed. A simplified model is presented for the critical current density and is used to examine the question of whether flux flow results from an instability due to plasticity of the flux-line array or from pin breaking.
Date: December 31, 1991
Creator: Welch, D. O.
Partner: UNT Libraries Government Documents Department

Crystallographic texture determinations from inverse susceptibility measurements

Description: Determination of the quantitative relationship between crystallographic texture and magnetic properties in advanced permanent magnets may be hampered by complex microstructures, which complicate methods that rely on diffraction, or by interparticulate interactions, which adversely affect methods based on magnetic remanence measurements. To this end, new techniques in the determination of texture of bulk permanent magnets are being explored to overcome these inherent experimental difficulties. The analysis of inverse paramagnetic susceptibility measurements constitutes a new method to investigate crystallographic texture. Such measurements also provide Curie temperature data, which is sensitive to chemical changes that may have occurred in the magnetic phase during processing. The mathematical formalism underlying the analysis of inverse susceptibility measurements is outlined, and is used to evaluate magnetic measurements taken from a series of Nd{sub 2}Fe{sub 14}B magnets that have been processed by different means, and thus contain different degrees of texture. While this method does provide qualitative information concerning the relative crystallographic alignment of magnet samples, it needs calibration to obtain an explicit value for a texture order parameter.
Date: October 1, 1996
Creator: Lewis, L.H. & Welch, D.O.
Partner: UNT Libraries Government Documents Department

On the theory of critical currents and flux flow in superconductors by the mechanism of plastic deformation of the flux-line lattice

Description: In this paper the author will discuss how the nature of the stress state in the flux-line lattice (FLL) of superconductors arises from the distribution, density, geometry, and strength of pinning centers. Under certain conditions this stress causes the onset of plastic deformation in the FLL for values of the current density below that required for flux-flow by general depinning. He will describe an analytic framework, based on a theory of plasticity of the FLL, which describes the flux-flow characteristics, including the possibility of thermally-activated flow and flux creep.
Date: July 1, 1999
Creator: Welch, D.O.
Partner: UNT Libraries Government Documents Department

Theory of point-defects, non-stoichiometry, and solute additions in SmCo{sub 5+x}-Sm{sub 2}Co{sub 17{minus}y} and related compounds

Description: There is considerable interest in the possibility of producing Sm-Co-based nanocomposite magnets by rapid solidification and other far-from-equilibrium processing methods. Thermodynamic and kinetic models are quite valuable in understanding and optimizing such methods. This paper describes a method of estimation, utilizing tight-binding-based bond-order interatomic interaction potentials, of the thermodynamic properties of point defects such as vacancies, interstitials, antisite defects, and solute additions in the SmCo{sub 5+x} and Sm{sub 2}Co{sub 17{minus}y} phases and related rare-earth-transition metal compounds. Illustrative calculations for point defects in SmCo{sub 5} will be presented. The results suggest a unified model of the thermodynamic properties of the SmCo{sub 5+x}-Sm{sub 2}Co{sub 17{minus}y} region of the phase diagram, based on the 1-5 structure and the replacement of Sm by interacting dumb-bell interstitials to form the 2-17 structure; the model is similar in nature to theories of the thermodynamics of metal hydrides.
Date: August 1, 1998
Creator: Welch, D.O.
Partner: UNT Libraries Government Documents Department

Theory of point-defects, non-stoichiometry, and solute additions in SmCo{sub 5+x}-Sm{sub 2}Co{sub 17{minus}y} and related compounds

Description: There is considerable interest in the possibility of producing Sm-Co-based nanocomposite magnets by rapid solidification and other far-from-equilibrium processing methods. Thermodynamic and kinetic models are quite valuable in understanding and optimizing such methods. This paper describes a method of estimation, utilizing tight-binding-based bond-order interatomic interaction potentials, of the thermodynamic properties of point defects such as vacancies, interstitials, antisite defects, and solute additions in the SmCo{sub 5+x} and Sm{sub 2}Co {sub 17}-y phases and related rare-earth-transition metal compounds. Illustrative calculations for point defects in SmCo{sub 5} will be presented. The results suggest a unified model of the thermodynamic properties of the SmCo{sub 5+x} -- Sm{sub 2} Co{sub 17{minus}y} region of the phase diagram, based on the 1-5 structure and the replacement of Sm by interacting dumb-bell interstitials to form the 2-17 structure; the model is similar in nature to theories of the thermodynamics of metal hydrides.
Date: September 3, 1998
Creator: WELCH,D.O.
Partner: UNT Libraries Government Documents Department

ON THE THEORY OF CRITICAL CURRENTS AND FLUX FLOW IN SUPERCONDUCTORS BY THE MECHANISM OF PLASTIC DEFORMATION OF THE FLUX-LINE LATTICE.

Description: In this paper I will discuss how the nature of the stress state in the flux-line lattice (FLL) of superconductors arises from the distribution, density, geometry, and strength of pinning centers. Under certain conditions this stress causes the onset of plastic deformation in the FLL for values of the current density below that required for flux-flow by general depinning. I will describe an analytic framework, based on a theory of plasticity of the FLL, which describes the flux-flow characteristics, including the possibility of thermally-activated flow and flux creep.
Date: July 7, 1999
Creator: WELCH,D.O.
Partner: UNT Libraries Government Documents Department

Hydrogen absorption in the niobium--vanadium system

Description: The effect of dilute vanadium additions to niobium on hydrogen absorption has been studied by measuring equilibrium pressure--composition--temperature data for a Nb + 6 at.% V alloy and by constructing a theoretical model of the effect of hydrogen trapping on phase equilibria in such systems. The results indicate the H--V binding energy is substantially smaller than thought previously. A depression of the ..cap alpha..--..cap alpha..&#x27; critical temperature is observed which, if interpreted in terms of trapping, leads to a H--V binding energy of approx. 0.04 eV.
Date: January 1, 1979
Creator: Pick, M A & Welch, D O
Partner: UNT Libraries Government Documents Department

Studies of the strain-dependent properties of A15 filamentary conductors at Brookhaven National Laboratory

Description: Work at BNL pertaining to the strain response of filamentary bronze-processed superconductors is reviewed. This work includes the intrinsic strain dependence of the critical properties of A15 structure compounds, the nature of the initial internal strain state of composite conductors, and the interplay between these residual strains and applied strains which governs the response of the conductor to external strain. Some factors which can enhance the strain tolerances of filamentary conductors are briefly discussed. (GHT)
Date: January 1, 1980
Creator: Luhman, T & Welch, D O
Partner: UNT Libraries Government Documents Department

Flux pinning in bronze-processed Nb/sub 3/Sn wires

Description: The scaling law derived by Kramer for magnetic flux pinning in high magnetic fields was examined for its applicability to the magnetic field dependence of critical-current densities in the bronze processed monofilamentary Nb/sub 3/Sn wires. From this it was concluded that: (1) its prediction for the form of the dependence of critical current on magnetic field and grain size (/J vector /sub c/ x H vector/ approx. h/sup 1/2/(1-h)/sup 2/(1-a/sub 0/..sqrt..rho)/sup -2/) was found to be very good in most cases including wires with very small Nb/sub 3/Sn grains (approx. 400 A). It was found very useful in comparison of J/sub c/ for different wires and in extrapolating to obtain H/sub c2/ for these wires. (2) However, it could not account consistently for the anisotropy in critical current of a tape which was measured with H applied perpendicular and parallel to the tape face. (3) The values of kappa/sub 1/ which were determined with the scaling law were too small by a factor of 2 to 3, and the trend in the variation with heat-treating time was opposite to that which is reasonably to be expected. That the behavior of kappa/sub 1/ is thus seriously in contradiction with the expected behavior for Nb/sub 3/Sn suggests basic faults in the derivation of the scaling equation for critical currents at high magnetic fields.
Date: January 1, 1980
Creator: Suenaga, M & Welch, D O
Partner: UNT Libraries Government Documents Department

Softening of Young's modulus of polycrystalline Nb/sub 3/Sn

Description: Ultrasonic measurements show that the shear modulus (C/sub 11/-C/sub 12/)/2 softens dramatically as single crystals of Nb/sub 3/Sn approach the martensitic transition near 50K. It is expected that Young's modulus of polycrystals will olso soften, but previous ultrasonic measurements, which suffer from severe damping, fail to show the expected effect. We have measured Young's modulus of polycrystalline Nb/sub 3/Sn between 4.2K and 300K by static beam deflection methods, and observe marked softening. A value of 14.3 +- .5 x 10/sup 11/ dyne cm/sup -2/ was obtained at 300K by the deflection of thin Nb/sub 3/Sn-Nb-Nb/sub 3/Sn composite strips by external stress. The variation of the modulus with T was obtained from the change in the radius of curvature of internally stressed Nb/sub 3/Sn-Nb composite strips. (This method is made possible by the near perfect match between the thermal expansion coefficients of Nb and Nb/sub 3/Sn.) The modulus is found to be proportional to 1nT between 50K and 300K and is temperature independent below the superconducting T/sub c/, resulting in a decrease by a factor of approx. 2 between 300K and 18K. The observed softening is somewhat less than that predicted by a polycrystalline average of the experimental single crystal elastic constants, but is much larger than that observed with ultrasonic measurements.
Date: June 1, 1979
Creator: Welch, D.O.; Bussiere, J.F. & Suenaga, M.
Partner: UNT Libraries Government Documents Department

Recovery by annealing of T/sub c/ in disordered Nb/sub 3/Pt

Description: Nb/sub 3/Pt has been disordered by rapid splat quenching, which results in the superconducting critical temperature falling from 11K to below 7K. The kinetics of reordering, as evidence by the recovery of T/sub c/, were followed by isothermal and isochronal anneals. The results were analyzed according to Welch's model of reordering in A15 compounds and were found to follow 2nd order kinetics, as had been assumed in an earlier study of recovery in several neutron-irradiated A15 compounds. The activation energy for the reordering process is 3.5 +- 0.2eV. These results will be compared to those on the recovery of samples of Nb/sub 3/Pt irradiated to a dose of 7.5 x 10/sup 18/nvt fast (E > 1MeV) neutrons with a T/sub c/ = 6.15K. The implications of the nature of radiation damage in A15 compounds is discussed.
Date: January 1, 1979
Creator: Dew-Hughes, D.; Jones, R.; Welch, D.O. & Caton, R.
Partner: UNT Libraries Government Documents Department

Degradation mechanism of Nb/sub 3/Sn composite wires under tensile strain at 4. 2 K

Description: Bronze-processed Nb/sub 3/Sn composite wire conductors exhibit changes in their superconducting parameters when strained in tension. This paper describes a detailed study of the effect of strain on critical current and an analysis by optical and SEM techniques of crack formation in the Nb/sub 3/Sn layer under strain. The effect of strain history on both reversible and irreversible changes in critical current and the roles of differential thermal contraction induced residual strains and of Nb/sub 3/Sn cracking are discussed.
Date: September 28, 1978
Creator: Luhman, T.; Suenaga, M.; Welch, D.O. & Kaiho, K.
Partner: UNT Libraries Government Documents Department

Chemical compositions of grain boundaries in bronze-processed pure and alloyed Nb3Sn

Description: Chemical compositions of grain boundaries in pure and alloyed Nb3Sn (with Ti and Ta) wires produced by the bronze process wire were measured using Auger electron spectroscopy. The amount of Ti at the grain boundaries was found to be approximately twice as much as that in the bulk. However, the amount of Ta at the grain boundaries was less than that in the bulk.
Date: January 1, 1985
Creator: Suenaga, M.; Corderman, R.R. & Welch, D.O.
Partner: UNT Libraries Government Documents Department