E. D. Gibson

The resistivity and microstructure of heavily drawn Cu‐Nb alloys

A combined resistivity transmission electron microscopy (TEM) study has been done on heavily drawn Cu‐20 vol % Nb alloys (so‐called in situ alloys). The results show that electron scattering at Cu‐Nb interfaces makes the major contribution to resistivity in heavily drawn wire. The dislocation contribution is small and constant at deformation strains greater than around 4, apparently as a result of dynamic recovery/recrystallization of the Cu matrix which occurs during room‐temperature drawing. Results of this study and other recent TEM dislocation studies indicate that the dislocation density in heavily drawn Cu‐20 vol % Nb material does not exceed 1011 cm−2. It is demonstrated here that the 1013‐ cm−2 dislocation density predicted by the resistivity study of Karasek and Bevk [J. Appl. Phys. 52, 1370 (1981)] is high because the interface scattering contribution is more strongly reduced by coarsening than they assumed. It is shown that resistivity measurements provide a means of evaluating an average Cu ch...

The effect of composition and magnetic heat treatment on the magnetostriction of TbxDy1−xFey twinned single crystals

Twinned single crystals of the giant magnetostrictive intermetallic compound TbxDy1−xFey with a 〈112〉 axial alignment have been prepared by float‐zone solidification with Y varying from 1.90 to 1.98 and X being 0.32 or 0.28. The chemical compositions of the crystals were carefully analyzed. A very sharp jump in the magnetostrictive strain λ sometimes occurs in these alloys as the applied field H is increased to values of around 280 Oe under a compressive stress of 6.9 MPa. This large jump in λ is shown to depend strongly on the values of X and Y, and possible reasons for this result are discussed. A magnetic heat treatment has been developed which involves cooling the samples through the Curie temperature under a field applied transverse to the crystal axis. It is shown that this treatment is very effective at increasing the magnetostrictive properties at low applied compressive stresses for certain values of X and Y.

Development of deformation processed copper-refractory metal composite alloys

The strength, electrical conductivity, and substructure of deformation processed Cu-15vol%X alloys have been studied where X included Nb, Ta, and Cr. One alloy of Cu-15Nb was studied in which 2% Ag was added to examine solid solution strengthening of the Cu matrix. The alloys were prepared by consumable arc melting and ingot diameters of 7.6 and 15.2 cm were examined. Deformation was carried out at room temperature by rolling, press forging, and axisymmetric modes. The results show that the strength/conductivity properties of the Nb, Ta, and Cr alloys are essentially the same and are slightly better than the Cu-20vol%Nb alloys previously measured. The Ag alloy was found to be stronger at a given deformation, but the solid solution Ag decreased the conductivity more than it increased strength so that the net effect was to reduce the strength at a given conductivity. TEM studies showed that the substructures of all the alloys were similar to each other and to previous results on Cu-20vol%Nb alloys. Deformation by both press forging and rolling are not as effective at increasing strength as is axisymmetric deformation.

Strength and conductivity ofin situ Cu-Fe alloys

Alloys of Cu-Fe with iron contents from 10 to 30 wt % have been prepared by casting plus mechanical reduction. A series of heat treatments was done at various stages of the mechanical reduction to promote precipitation of the iron from the copper matrix with the hope of optimizing electrical conductivity at a given strength level. A curve of optimum tensile strength against electrical conductivity was determined. It was found to lie significantly below the available data for Cu-Nb alloys and it is suggested that further improvements may be possible in Cu-Fe alloys by improved thermal mechanism processing.

Superconducting properties of in situ prepared Nb‐Cu‐Sn alloys

Alloys of Cu–20 wt% Nb have been prepared by a chill casting technique. After drawing to fine wire, Nb3Sn was formed by plating and diffusion of Sn. The critical‐current properties and the resistance to degradation of Jc upon mechanical deformation both compare favorably to commercial Nb3Sn tape.

Directional solidification and heat treatment of terfenol-D magnetostrictive materials

Directional solidification techniques have been developed to produce grain-aligned rods of the highly magnetostrictive material Terfenol-D, TbxDy1-xFey The magnetostrictive properties have been compared to those of single-crystal, bicrystal, and polycrystal rod prepared by a float zone melting (ZM) technique. It was found that the properties of the grain-aligned material were comparable to three-and four-grain ZM material and the magnetostriction for both materials was less than the ZM single crystal and the better bicrystal rods by around 20 pct. It was demonstrated that a heat treatment consisting of a 1-hour hold at 950 °C was very effective at improving the magnetostrictive properties of all of the materials. Experiments have demonstrated that, to be effective, the heat-treatment temperature must be above somewhere between 850 °C and 900 °C. It is postulated that the mechanism for the improved properties involves a relief or redistribution of residual stresses produced by localized melting near second-phase particles at the predicted eutectic temperature of around 875 °C in the Terfenol-D material.

Processing to optimize the strength of heavily drawn Cu-Nb alloys

Heavily drawn Cu-Nb alloys display quite high ultimate tensile strengths. A modification to the consumable arc-casting technique used to prepare these alloys is shown to decrease the as-cast niobium dendrite diameter,t0, and also increase strength. Evaluation of strength, niobium filament spacing and thickness data show that strength varies with as-cast niobium dendrite size as somewhere betweento−0.36 toto−0.50. Splat-cooling techniques demonstrate that minimum niobium dendrite sizes as small as 0.22μm are possible. These sizes are over a factor of 10 smaller than has been achieved by consumable arc casting, and it is therefore suggested that processing rapidly solidified powders of Cu-Nb alloys should have significant advantages for preparing high-strength heavily drawn Cu-Nb alloys.

Strength and electrical conductivity of a deformation-processed Cu-5 Pct Nb composite

A Cu-5 pct Nb alloy was deformation processed by wire drawing to very large reductions (99.9993 pct) and the strength and electrical conductivity properties compared with similarly deformation processed Cu-20 pct Nb. The results showed that the Cu-5 pct Nb alloy was transformed into a composite material with the original Nb dendrites becoming ribbonlike filaments in a similar fashion to higher Nb-containing Cu-Nb alloys. The degree of strength increase with increasing deformation processing greatly exceeds rule-of-mixtures expectations at higher degrees of deformation processing, where the Nb becomes highly aligned with the wire axis. A 5 pct Nb addition appears to contain close to the minimum amount of Nb phase necessary to produce appreciable strengthening during deformation processing of Cu-Nb alloys. The strength-conductivity properties of the deformation-processed Cu-5 pct Nb alloy show significant improvements in strength over the best commercial alloys in the conductivity range of 80 to 90 pct international annealed copper standard (IACS).

Auger study of grain boundaries in large‐grained YBa2Cu3Ox

A scanning Auger microscopy study has been carried out on fractured surfaces of YBa2Cu3Ox samples prepared by standard sintering procedures from powders. It is concluded that in the large‐grained samples examined, a majority of the grain boundaries are sufficiently ‘‘open’’ that standard metallographic preparation causes them to become contaminated with C. This suggests that an actual loss of contact at the grain boundaries during sample preparation is responsible for the low Jc values observed in bulk‐sized large‐grained YBa2Cu3Ox.

Casting of dendritic Cu‐Nb alloys for superconducting wire

Consumable electrode‐arc‐casting techniques have been developed for the preparation of large billets of dendritic Cu‐Nb alloys which are suitable for the fabrication of multifilamentary superconducting wire. The dendrite structure is somewhat more coarse than chill cast material but metallographic and chemical analyses show acceptably small radial and longitudinal segregation. The billets can be drawn to wire with no intermediate anneals. Both external diffusion after tin plating and internal diffusion of wire with a tin core can be used to transform the Nb filaments to Nb3Sn. The arc cast wire displays Jc values equivalent to previously reported values on in situ wire at high fields, but somewhat lower values at low fields.