Xiaowei Zuo

Studies of electrical resistivity of an annealed Cu-Fe composite

Cu-15 vol. % Fe composites produced by cold deformation were annealed at various temperatures for 1 h. The electrical resistivity, tensile strength, and microstructure were investigated by four-probe technique, electronic universal testing machine, and scanning electron microscopy (SEM), respectively. The Fe solubility in Cu-matrix was estimated from the saturation magnetization of the composites. The results reveal that the resistivity of impurity scattering ρimp is the main contributor to the resistivity of the Cu-Fe composites if the phonon and dislocation scattering contributions to the electrical resistivity are considered to be constants. At annealing temperatures below 500 °C, an increase of filament spacing and reduction of interface area in unit volume result in a marginal decrease of the resistivity with temperatures. Above 500 °C, the Fe solubility in Cu-matrix rapidly increases with temperature, which directly causes the increase of composites resistivity.

Influence of grain boundary characteristics on thermal stability in nanotwinned copper

High density grain boundaries provide high strength, but may introduce undesirable features, such as high Fermi levels and instability. We investigated the kinetics of recovery and recrystallization of Cu that was manufactured to include both nanotwins (NT) and high-angle columnar boundaries. We used the isothermal Johnson-Mehl-Avrami-Kolmogorov (JMAK) model to estimate activation energy values for recovery and recrystallization and compared those to values derived using the non-isothermal Kissinger equation. The JMAK model hinges on an exponent that expresses the growth mechanism of a material. The exponent for this Cu was close to 0.5, indicating low-dimensional microstructure evolution, which is associated with anisotropic twin coarsening, heterogeneous recrystallization, and high stability. Since this Cu was of high purity, there was a negligible impurity-drag-effect on boundaries. The twin coarsening and heterogeneous recrystallization resulted from migration of high-angle columnar boundaries with their triple junctions in one direction, assisted by the presence of high concentration vacancies at boundaries. Analyses performed by electron energy loss spectroscopy of atomic columns at twin boundaries (TBs) and in the interior showed similar plasma peak shapes and L3 edge positions. This implies that values for conductivity and Fermi level are equal for atoms at TBs and in the interior.

Strength of Cu-28 Wt%Ag Composite Solidified Under High Magnetic Field Followed by Cold Drawing

Cu-Ag composite is one of the best conductors for high-field magnets. Increasing its strength is crucial for designing newer high-field magnets. Cu-28 wt%Ag samples were solidified with and without a 12-T high magnetic field (HMF), and then cold-drawn. We investigated the influence of HMF on microstructure, hardness and strength of Cu-Ag samples both before and after cold-drawing. The introduction of external HMF during solidification increased both the dendrite arm spacing and the dissolved Ag in Cu, and it reduced the spacing between both the Ag precipitates in proeutectic Cu and the eutectic lamellae. The transversal microstructure after cold-drawing inherited the network solidification structure, but at a refined scale. The Cu dendrite spacing in the 12-T HMF samples at all deformation strain was larger than that without HMF. HMF slightly increased the intensity of <111> fiber texture of Cu, which strengthened proeutectic Cu at the level of 3.5 deformation strain. In samples deformed to strain of 3.5, refined Ag precipitation spacing, increased Ag solubility in Cu matrix, and refined eutectic lamellar spacing by 12-T HMF increased the strength by 5% in the sample compared with that without HMF.

Influence of Growth Rate and Magnetic Field on Microstructure and Properties of Directionally Solidified Ag-Cu Eutectic Alloy

We report the influence of growth rate and external magnetic field on the eutectic lamellar spacing and properties of directionally-solidified Ag-Cu eutectic alloys. The results indicated that the relationship between the lamellar spacing of directionally-solidified Ag-Cu alloys and the growth rate matched the prediction of the Jackson-Hunt model, and the constant was 5.8 µm3/s. The increasing external magnetic field during solidification tilted the growth direction of the lamellar eutectics, and coarsened the eutectic lamellar spacing. These decreased the microhardness and strength of Ag-Cu alloys, but increased their electrical conductivity. The competitive strengthening contributions between the refinement of the eutectic lamellar spacing and the change in growth direction of the eutectics resulted in higher strength in the as-rolled sample with a 0.8 T magnetic field than with other samples, which was confirmed from higher relieved deformation energy using differential scanning calorimetry.

Microstructural Evolution and Performance of In-situ Ag-Ni Composite after Solidification under Electromagnetic Stirring and Deformation

The effect of electromagnetic stirring (EMS) on microstructure and performance of Ag-8 mass% Ni composite was investigated under both solidified and deformed conditions. Without EMS, the Ag matrix formed short, thick dendrites in the ingot; whereas with EMS, dendrites were long and slim. Ni phase mainly formed particles or ribbons, distributed along boundaries between dendrite arms. Cold drawing of the solidified Ag-Ni ingots, both with and without EMS, produced high strength in-situ metal-matrix composite (MMC) consisting of Ag matrix reinforced by Ni ribbons. EMS improved the ductility of the composite, consequently enhancing its drawability and strength. EMS also increased the electrical conductivity in both solidified ingots and deformed in-situ composite wires. In both cases, hardness and tensile strength remained high. A model based on a combination of the modified linear rule of mixtures and the Hall-Petch relationship was used to rationalize the tensile strength and hardness with respect to its fabrication parameters and the microstructure of Ag-Ni in-situ composite.

Influence of Fe addition on microstructure and properties of Cu-Ag composite

We investigated the effects of Fe content on microstructure and properties in as-cast and as-drawn Cu-(5.1-x) vol%Ag-x vol%Fe alloys. In microscale, increasing Fe content first refined and then coarsened Cu dendrites. In nanoscale, the size and length of Ag precipitates in Fe-doped alloys were smaller than the size and length of Ag precipitates in Fe-free alloy, and the γ-Fe precipitates in Cu-2.9 vol%Ag-2.4 vol%Fe alloy were finer than the γ-Fe precipitates in Cu-5.1 vol%Fe alloy. The maximum hardness in as-cast Cu-Ag-Fe alloys was found in the Cu-2.9 vol%Ag-2.4 vol%Fe alloy. With increasing drawing strain, both ultimate tensile strength and hardness of Cu-Ag-Fe composites were increased. Simulation data among the relative volume fractions of Fe, hardness and electrical conductivity showed that, as the relative value approached 40%, the Cu-Ag-Fe composite displayed greater hardness than other samples. As a small amount of Ag was replaced by Fe, the electrical conductivity decreased significantly with a descending slope of approximately 3%IACS (International Annealed Copper Standard) per vol% Fe. As 47 vol%Ag was replaced by Fe, however, the electrical conductivity decreased by 51% and remained almost invariable with further increasing Fe content. After annealing at 450 °C for 4 h, the electrical conductivity of the Cu-2.9 vol%Ag-2.4 vol%Fe composite was elevated up to 68.3%IACS from 38.5%IACS.

Numerical Simulation of Droplets Behavior of Cu-Pb Immiscible Alloys Solidifying under Magnetic Field

A model has been presented for the coarsening of the dispersed phase of liquid-liquid two-phase mixtures in Cu-Pb alloys under the effect of a high magnetic field (HMF). The numerical results show that the evolution of size distribution is the result of several factors and the diffusional growth, the collision-coagulation of the Cu-rich droplets (gravity sedimentation and Marangoni migration), and melt flow also have obvious effects on the movement of droplets and coarsening process. The effect of the HMF in the coarsening process of Cu-Pb alloy is studied in this work both by simulation and experiment. The analysis shows that the HMF leads to a decrease in the melt flow velocity, and can also lead to a decrease in the moving velocity of Cu-rich droplets. The HMF significantly reduces the coarsening rate of droplets as compared by the distribution evolutions. Finally, it is shown that droplet collision and coagulation can be dramatically retarded by the HMF. The results of the simulation are compared with the experiments performed with immiscible Cu-Pb alloys, and the discrepancy between theory and experiment is discussed.

Thermal Stability of Fe Filaments in Deformed Cu-Fe Composites

The Cu-12.8wt%Fe alloys are prepared in a vacuum induction furnace and then drawn to Cu-Fe composite wires with the drawing ratio of 8.2. The thermal stability of Fe filaments in the deformed Cu-12.8wt% Fe composite wires under different annealed temperature is investigated. The results show that the instability of the Fe filaments in the Cu-Fe composites is controlled by the longitudinal boundary splitting, then the splitting Fe filaments subsequently evolve into the cylinders. The thermal instability of the cylindrical Fe filaments is controlled by the two instability modes of Rayleigh perturbation and two dimensional Ostwald coarsening. The model calculations of two modes indicate that the perturbation breakup of cylindrical Fe filaments firstly occurs at the ones with smaller diameter. The breakup time of cylindrical Fe filaments decreases with the increasing of the annealing temperature. The coarsening diameters of cylindrical Fe filaments increase in linear proportion with the holding time. The smaller is the diameter of cylindrical Fe filaments, the larger is the coarsening rate.

Strength and Conductivity of Deformed Cu-Fe Composites after Solidified with a Horizontal Magnetic Field

In this paper, the Cu-Fe alloys are fabricated by solidifying with and without a 1.0 Tesla horizontal magnetic field and they are drawn to composite wires under different drawing ratios, then their further strength and conductivity are investigated. The results show that, when the drawing ratio is lower, the strength of the Cu-Fe composites pre-solidified with a horizontal magnetic field is lower, which is caused by the coarser solidification microstructures induced by the injected magnetic field. However, the increase ratio in the strength of the Cu-Fe composites, which is plotted as a function of the Fe content and the drawing ratio, is higher in the case of the imposed magnetic field. It indicates that it is more efficient applying the magnetic field to fabricate Cu-Fe composites with high Fe contents and larger drawing ratio. A quantitative relationship is fitted to predict the influence of the Fe content on the strength of the Cu-Fe composites wires. On the other hand, the conductivity of the Cu-Fe composite wires is decreased with increasing Fe content. The injected magnetic field has no effect on the conductivity.

Co-Precipitation, Strength and Electrical Resistivity of Cu–26 wt % Ag–0.1 wt % Fe Alloy

Both a Cu–26 wt % Ag (Fe-free) alloy and Cu–26 wt % Ag–0.1 wt % Fe (Fe-doping) alloy were subjected to different heat treatments. We studied the precipitation kinetics of Ag and Cu, microstructure evolution, magnetization, hardness, strength, and electrical resistivity of the two alloys. Fe addition was incapable of changing the precipitation kinetics of Ag and Cu; however, it decreased the size and spacing of rod-shaped Ag precipitates within a Cu matrix, because Fe might affect the elastic strain field and diffusion field, suppressing the nucleation of Ag precipitates. Magnetization curves showed that γ-Fe precipitates were precipitated out of the Cu matrix, along with Ag precipitates in Fe-doping alloy after heat treatments. The yield strength of the Fe-doping alloy was higher than that of the Fe-free alloy, and the maximum increment was about 41.3%. The electrical conductivity in the aged Fe-doping alloy was up to about 67% IACS (International Annealed Copper Standard). Hardness, strength, and electrical resistivity were intensively discussed, based on the microstructural characterization and solute contributions of both alloys. Our results demonstrated that an increasing fraction of nanoscale γ-Fe precipitates and decreasing spacing between Ag precipitates resulted in the increasing strength of the Fe-doping alloy.