Nano-Ag Precipitation And Properties Of Cu-Ag Alloys

Cu-based alloy with high-strength and high-conductivity is one of important functional materials in modern electric information,electrical,energy and mechanical industries.Among Cu-based alloys,Cu-Ag material with an excellent combination of high-strength and high-conductivity is paid an attention and applied as conductors in both DC resistive and pulsed high-field magnets.With increasing requirement of high-field magnets,the improvement of properties in Cu-Ag material becomes much more important.In this dissertation,we established relationships of strengthening and resistivity with microstructure,and thought that nano-Ag precipitation played an important roles in the optimization of properties.Three methods,which include the controlling of solidification structure,the optimization of heat treatments,and the addition of a third element,were used to improve the combination properties of strength and conductivity.Ag precipitation in Cu matrix and its effect on the strength and electrical conductivity were emphatically discussed.Cu-28wt%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 the Ag precipitates in proeutectic Cu.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 were larger than that without HMF.HMF slightly increased the intensity of<111>fiber texture of Cu.In the samples deformed to strain of 3.5,refined Ag precipitation spacing and increased Ag solubility in Cu matrix by 12 T HMF increased the strength by 5%compared with that without HMF.The application of HMF slightly increased the electrical resistivity at both 293 K and 77 K because of the increasing Ag solubility in Cu.The hardness,mechanical strength,ductility,and electrical conductivity of Cu-7.9wt%Ag alloy were investigated after heat treatment with different time and temperature.Discontinuous and continuous precipitation phenomena were shown by differential scanning calorimetry(DSC)to correspond to two distinct exothermic reactions,in this alloy.Activation energy was found to be 63.7±0.1 kJ/mol for discontinuous and 68.7±2.3 kJ/mol for continuous precipitation.At higher temperatures(450?-580?),both continuous and discontinuous precipitation occurred,but at lower temperatures(410?),only discontinuous precipitation was observed.Cube-on-cube relationship was observed between Ag precipitates and Cu matrix.Two kinds of interface between Ag precipitates and Cu matrix were observed.One is(1-1 1)cu/(1-11)Ag flat interface,and another one is step-like interface which was consisted of(1-11)and(11-1).Coherent{111} twin boundaries were found in some of Ag precipitates.Because of higher density and very fine continuous precipitates,the microhardness and tensile strength of the sample aged at 475? was up to 22%higher than those in samples aged at other temperatures(e.g.170?,410?,580?,and 680?).Optimized microstructure also enhances the ductility.Because the formation of precipitates reduces Ag dissolved in Cu matrix,the electrical resistivity of aged samples began to decrease above 410?.The size,volume fraction,and spacing of continuous Ag precipitates apparently play important roles in maximization of total strength and electrical resistivity.The microstructure,precipitation kinetics,electrical conductivity and mechanical properties of Cu-5.8wt%Ag-0.6wt%Nb alloy were investigated.As-cast microstructure and DSC data revealed both discontinuous and continuous precipitation of Ag in Cu matrix.As-cast Cu-5.8wt%Ag-0.6wt%Nb alloy showed more continuous Ag precipitates than Cu-7.9wt%Ag alloy.Spherical micro-size and nano-size Nb particles were observed.In isothermal ageing at 170? and 475?,addition of Nb suppressed the growth of Ag discontinuous precipitates because of the pinning force caused by Nb nanoparticles and the suppressed grain-boundary diffusion caused by spherical Nb particles.Both hardness and strength of isothermal-aged Cu-5.8wt%Ag-0.6wt%Nb alloys were higher than that of Cu-7.9wt%Ag alloys because of the refined spacing between Ag precipitates.As-cast Cu-7.9wt%Ag and Cu-5.8wt%Ag-0.6wt%Nb alloys were cold-rolled without any heat treatment.The strength of Cu-5.8wt%Ag-0.6wt%Nb sheet was higher than that of Cu-7.9wt%Ag sheet by 6%,which was mainly because of the higher percentage of continuous precipitates in Cu-5.8wt%Ag-0.6wt%Nb alloy.The ductility of as-rolled Cu-5.8wt%Ag-0.6wt%Nb alloy maintained high despite of high strength because of the improved work-hardening rate.