J. D. Verhoeven

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 growth of single crystal Terfenol-D crystals

Rods of the highly magnetostrictive material, Terfenol-D, Tb0.3Dy0.7Fe2, have been prepared by both Bridgman and float zone techniques. It is found that the dendritic growth front consists of parallel sheets of dendrites growing with a primary direction of <112> and sheet planes of {111}. Unseeded rods show a strong preference for <112> alignment of the grains. Seeding experiments have been successful for <112> orientations but not <111>, and the cause of this difference is discussed. The <112> single crystal rods are seen to contain parallel {111} twin boundaries throughout their volume, growing near the central plane of the dendrite sheets. These results are analogous to those of Ge and Si. The role of the twin planes on domain wall motion and methods of eliminating the twins are discussed.

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.

Primary dendrite spacing of lead dendrites in Pb-Sn and Pb-Au Alloys

Directional solidification experiments have been carried out on dendritic alloys of Pb-Sn and Pb-Au as a function of temperature gradient,G, solidification rate,R, and composition. Variation of temperature gradient was carried out at a moderate growth rate, 20 µm/s, and variation of growth rate was carried out at high gradients, around 350 °C/cm. Based on these experiments the primary dendrite spacings, λ, are found to follow the correlation λ =kG−aR-b only at rates above 45 µm/s in Pb-Sn alloys and above 10µm/s in Pb-Au alloys. Below these rates λ becomes essentially constant with rate. Possible causes of this break in the rate data are discussed. The values of the exponentsa andb at the higher rates are found to be significantly different from the values predicted by the recent theoretical model of Hunt.

Measurement of filament spacing in deformation processed CuNb alloys

Abstract Previous studies of Cu-20 vol.% Nb alloys used optical and SEM techniques to evaluate the thickness and spacing of the Nb filaments. It had been known that the filament sizes were smaller than the resolution limit of the SEM at high deformation strain but it was assumed that this problem would not significantly change the observed trends. Results of this TEM study using dark-field techniques on the same materials show that the previous SEM measurements significantly overestimated the spacing at the higher deformations. Analysis of resistivity data on the samples also confirms this result. The strength dependence on Cu channel spacing, t Cu , was found to display a large transition region, with the dependence changing from a t Cu −0.5 at low strains to a t Cu −0.38 at high strains. The significance of these results is discussed. It is also pointed out that the filament sizes observed at high strains in these materials range to below 20 nm and they exhibit strength/filament size characteristics similar to vapor deposited nanocrystalline materials.

A Review of Microsegregation Induced Banding Phenomena in Steels

A review is presented of banding in hypoeutectoid and hypereutectoid steels. The data available on hypereutectoid steels are quite limited and therefore a study is presented on banding in a 52100 steel. Similarities and differences are identified in the banding that occurs in commercial hypoeutectoid versus hypereutectoid steels. The distinct surface patterns of Damascus steels, which are nearly pure hypereutectoid steels, have recently been shown to be due to carbide banding. It is shown that the carbide banding in the 52100 steel occurs by a distinctly different mechanism than the carbide banding of the Damascus composition steels.

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.

Strength and electrical conductivity of deformation-processed Cu-15 Vol Pct Fe alloys produced by powder metallurgy techniques

Powder metallurgical techniques have been employed to prepare the precursor billets in the preparation of Cu-15 vol pct Fe alloys by deformation processing. It has been demonstrated that by (1) using high-purity gas-atomized Cu powders blended with commercial high-purity Fe powders and (2) controlling the time/temperature processing conditions within specific limits, it is possible to produce Cu-Fe deformation-processed alloys with strength/conductivity properties matching those of Cu-Nb, Cu-Ta, and Cu-Cr alloys. These properties are significantly superior to the best commercial alloys.