L.S. Chumbley

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...

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.

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.

Déformation processed métal-métal composites

Deformation processed metal–metal composites (DMMCs) are composites in which both the matrix and the reinforcing phase are ductile metals. The ductility of the metals permits the composite to be heavily deformed by drawing or rolling to reduce the thickness and spacing of the two phases to as small as 10 nm. Their nano-scale microstructures are exceptionally strong, with strengths approaching whisker strength in some cases. And since the materials are comprised of two essentially pure metals, their electrical conductivities are quite high for current flow in directions parallel to the composite’s filaments or lamellae, leading to numerous potential applications. During the 1990s, continuing research has produced DMMCs in a wide range of matrix metals, including Al, Au, Mg, Sc, and Ti. Methods are also being developed to produce DMMCs with much larger dimensions in rod and sheet forms to permit their use in a wider variety of engineering applications.

Effect of temperature on the strength and conductivity of a deformation processed Cu-20%Fe composite

The high temperature (22–600 °C) properties were evaluated for a Cu-20%Fe composite deformation processed from a powder metallurgy compact. The ultimate tensile strengths decreased with increasing temperature but were appreciably better than those of similarly processed Cu at temperatures up to 450 °C. At 600 °C, the strength of Cu-20%Fe was only slightly better than that of Cu as a result of the pronounced coarsening of the Fe filaments. However, at temperatures of 300 and 450 °C, the strength of Cu-20%Fe is about seven and six times greater, respectively, than that of Cu, as compared to about a two fold advantage at room temperature. Therefore, Cu-20%Fe composites made by deformation processing of powder metallurgy compacts have mechanical properties much superior to those of similarly processed Cu at room temperature and at temperatures up to 450 °C. The pronounced decrease in electrical conductivity of deformation processed Cu-20%Fe as compared to Cu is attributed to the appreciable dissolution of Fe into the Cu matrix which occurred during the fabrication of the starting compacts where temperatures up to 675 °C were used. While the powder metallurgy compacts used for the starting material for deformation processing in this study did not lead to a high conductivity composite, the powder metallurgy approach should still be a viable one if processing temperatures can be reduced further to prevent the dissolution of Fe into the Cu matrix.

Problems in evaluating the dislocation densities in heavily deformed Cu-Nb composites

Abstract The dislocation density of the Cu matrix in a heavily cold-rolled Cu-20%Nb composite is evaluated as a function of deformation strain. The methods and problems involved in this analysis are described and discussed in detail. The maximum dislocation density was found to be 10 10 -10 11 cm -2 at all strains investigated. Dislocations were not uniformly distributed but rather had a low energy dislocation structure. The effects of sample preparation, specifically ion-thinning, were evaluated and found to have little effect on the dislocation population of worked Cu but actually introduced defects into annealed Cu.

Characterization of strength and microstructure in deformation processed Al-Mg composites

The microstructures, mechanical properties and electrical resistivity have been evaluated for deformation processed Al-20 vol % Mg and Al-13 vol % Mg composites. The Mg second phase adopts a convoluted, ribbon shape filamentary morphology after deformation. Both the size and spacing of these filaments decreases with deformation. The strength of these composites increases exponentially with reduced spacing of Mg filaments. The electrical resistivity of these Al-Mg composites is slightly higher than that of pure Al.

In situ strengthening of titanium with yttrium

In situ processing consists of heavily deforming a two-phase alloy of mutually immiscible elements to produce composite sheet or wire. In the well-studied Cu(fcc)-Nb(bcc) system, severe deformation by swaging and drawing reduces niobium filament phase thicknesses from 1–5 μm (as-cast) to 0.007–0.030 μm (after deformation). Cu-20% (vol.) Nb ultimate tensile strengths exceed 2000 MPa for material deformed to a true strain of η=12, where η=In (areaoriginal/areafinal). In a study on in situ strengthening in immiscible hexagonal close-packed metals, Ti-50 wt % Y and Ti-20 wt % Y alloys were deformed by hot extrusion, hot swaging, and cold swaging. As deformation progressed, samples were taken for tensile testing and examination by SEM and TEM. Ti-Y alloys deformed to final true strains of 6.6 (Ti-50Y) and 7.6 (Ti-20Y) contain nanofilaments (100 nm phase spacing) similar to those of deformation-processed Cu-20Nb at comparable strains. The ultimate tensile strengths of the alloys approximately tripled as deformation progressed from the as-cast condition to these final true strains, although the exponential strength increase seen in Cu-Nb alloys was not observed.

Effect of Temperature on the Mechanical Properties and Microstructures of In Situ Formed Cu-Nb and Cu-Ta Composites

The effect of temperature on the mechanical properties and microstructures has been evaluated for heavily cold drawn Cu-20% Nb and Cu-20% Ta composites. The strengths of the composites decrease with increasing temperature, with the decrease becoming most pronounced at temperatures above about 300°C and at larger draw ratios. Cu-20% Ta composites are stronger than Cu-20% Nb composites throughout the temperature range studied (22–600°C) with the improvement increasing with increasing temperature. Resistivity measurements and substructure analyses showed that at temperatures where softening accelerated, resistivity decreased indicating a substructural change which was observed to be coarsening of the Nb and Ta filaments in the composites.

Synthesis and characterization of hexagonal Cd51Yb14 single crystals

Synthesis of large, bulk single grains of the hexagonal crystalline phase Cd51Yb14 has been accomplished using the Bridgman method. Growth was carried out under controlled solidification conditions in sealed Ta crucibles, which maintain compositional integrity by eliminating evaporative losses of Cd and reaction with the containment material. Compositional analysis of the as-grown phase indicates no evidence of macrosegregation along the length of the crystal. High-resolution transmission electron microscopy (TEM) characterization of the hexagonal phase is hampered by the enhanced atmospheric reactivity of the higher Yb content of the hexagonal phase. Such a high sensitivity to ambient exposure was not observed during the recent synthesis and TEM studies of the quasicrystalline Cd5.7Yb or cubic approximate phase Cd6Yb. TEM and X-ray transmission diffraction results taken at the Advanced Photon Source at Argonne National Laboratory show the presence of a fine-structured second phase, which is randomly oriented throughout the matrix. The precipitates have been identified as pure Cd by d-spacing comparisons and likely form by selective atmospheric oxidation of Yb from the Cd51Yb14 phase.