W. A. Spitzig

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.

Strengthening in deformation-processed Cu-20% Fe composites

Three Cu-20% Fe composites with different iron powder sizes were fabricated using powder metallurgy processes. The strengths of these composites after extensive deformation processing by rod swaging and wire drawing were shown to be anomalously higher than those predicted by rule of mixtures equations. However, the strengths obey a Hall-Petch type relationship with the iron filament spacings. The strengths of the Cu-20% Fe composites after equivalent deformation processing increased with decreasing initial iron powder size. Comparison of a Cu-20% Fe composite with a similar Cu-20% Nb composite showed that Cu-20% Fe was stronger after an identical degree of deformation processing. This increase in strength of a Cu-20% Fe composite over that of a Cu-20% Nb composite correlated with the greater shear modulus of iron compared to niobium using a barrier model for hardening.

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).

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.

DEFORMATION PROCESSED Cu-REFRACTORY METAL COMPOSITES

ABSTRACT Recent studies have shown that heavily drawn Cu-Nb alloys can achieve strengths above 2000 MPa and outstanding combinations of strength plus conductivity (both electrical and thermal). The properties result from an aligned composite structure formed by a mechanical reduction which produces ribbon shaped Nb filaments In the Cu matrix. These metal-metal matrix composites are referred to here as deformation processed composites (DPC). A whole series of metal-metal matrix Cu-X alloys may be prepared by deformation processing, where X may be any of the BCC metals: V, Nb, Ta, Cr, Mo, W or Fe. Processing consists of two primary steps: preparation of the Cu-X billet with the X phase uniformly dispersed as small particles, and then a very large mechanical reduction. The billet may be prepared by either solidification or powder processing. Existing experimental results are reviewed, some new data on powder processed Cu-W are presented, and the general applicability of solidification processed and powder pr...

Effect of hydrogen on the mechanical properties of a deformation processed Cu-20% Nb composite

The influence of H on the mechanical properties of a deformation processed Cu-20% Nb composite was analysed and compared with the results on similarly processed pure Cu and pure Nb. In this composite the matrix phase (Cu) is relatively resistant to H while the filamentary phase (Nb) is highly susceptible to H embrittlement. The results show that H, in an amount equivalent to that which causes embrittlement of pure Nb, causes no significant deleterious influence on the mechanical properties of Cu-20% Nb. Apparently the ductile Cu matrix creates a favourable stress state for the hydrogenated Nb filaments so that they are constrained from fracturing and continue to deform in a ductile manner.

Comparison of strengthening in wire-drawn or rolled Cu-20% Nb with a dislocation accumulation model

Strengthening after large deformations by wire-drawing or rolling of Cu, Nb and Cu-20% Nb was compared with the predictions of a proposed modified substructural strengthening model for ductile two-phase alloys. The comparisons indicate that the more extensive and refined model of Funkenbusch and Courtney offers no improvement over the original model of Ashby in predicting the strengthening with increased deformation processing or the dislocation densities necessary to produce the observed strengthening in Cu-20% Nb. Both models can predict the strengthening behaviour of Cu-20% Nb. However, neither model is in accord with the observations that the dislocation density in the Cu matrix is essentially independent of the degree of deformation processing, and that the magnitudes of the dislocation density are much the same in the Cu in Cu-20% Nb and pure Cu identically deformation-processed. In addition, there is no experimental support for the Funkenbusch and Courtney model prediction of an order of magnitude greater dislocation density in the Nb filaments than in the Cu matrix in Cu-20% Nb. It appears that a mechanism that does not require an accumulation of dislocations for strengthening, such as the difficulty in propagating dislocations between closely spaced barriers, is more likely to be responsible for strengthening in Cu-Nb-type deformation-processed composites.