Study On High-Strength And High-Conductivity Copper Alloys

High-strength and high-conductivity copper alloys are widely used in microelectronic, transportation, astronavigation, aviation, metallurgy and mechanical-electronic industries. This paper has developed two new high-strength and high-conductivity copper alloys, Cu-Fe-P and Cu-Ni-Si, focusing on the electrified railway contact electric network and lead frames of super-large-scale integrated circuit materials respectively.The technical routine of this research is as follows: To calculate the phase equilibrium thermodynamics of alloys to be studied and predict the precipitates and their mass fractions at different aging temperatures by CALPHAD (Calculation of Phase Diagram) technique, then to guide the design for the composition of experimental alloys by CALPHAD results; To achieve microstructure and good properties by optimizing the composition of alloys and the heat-treatment process; To prepare alloys by combining aging strengthening, cold working hardening and fine-crystal strengthening, with a little influence on the conductivity of alloys; To achieve combination of the three strengthening technique by adding rare-earth and boron elements which possess special physical and chemical characteristics to raise the recrystallization starting temperature of alloys and refine grains.According to above technical routine, experimental alloys were designed and prepared. The phase equilibrium thermodynamics of Cu-Fe-P and Cu-Ni-Si alloy systems were calculated by Thermo-Calc software. The systematic investigation on effect of the composition, rare-earth and boron addition, heat treatment and cold working on the microstructures, mechanical properties and conductivity of experimental alloys were conducted by means of OM, SEM, EPMA, TEM, tensile test, Vickers hardness and resistivity measurements. The main results of the research are listed below:Based on the related experimental data, the thermodynamic parameters were optimized and the isothermal sections of Cu-Fe-P and Cu-Ni-Si phase diagrams were calculated by CALPHAD technique. Results show that there are several binary and ternary phases existing in the Cu-rich sectors of both diagrams at 300-700°C. The type and fraction of precipitates mainly depend on the ratios of Fe/P, Ni/Si and temperatures. Therefore, the preferable precipitates can be obtained by adjusting the alloy composition and aging temperature. The calculated results of phase equilibrium thermodynamics will provide the important theoretical basis for the design of composition and aging processe of high-strength and high-conductivity copper alloys.It was found that the recrystallization starting temperature of Cu-Fe-P and Cu-Ni-Si alloys was greatly raised by adding trace Ce and B, which resulted in that the recrystallization of alloys did not happen until the aging strengthening peak was reached. Thus, the good strengthening effect contributed by both precipitation and cold working was obtained, and not only the peak strength was obviously increased but also the strength peak and conductivity peak appeared at a same aging temperature. The mechanism of Ce and B addition affecting the recovery and recrystallization of experimental alloys is that B and Ce atoms have the tendency to exist at crystal defects and grain boundary, which prevents the spot defects from migration, the dislocations from glide and rearrangement, the recrystallized interfaces from migration in Cu matrix, and thus delays the recovery and inhibits the nucleation of recrystallization and the grain growth. Meanwhile, the grains were refined and alloy was purifyed obviously. The trace addition of Ce and B makes the alloys achieve an excellent combination of high strength and high conductivity.The effect of cold working on the structures and properties of the experimental alloys was studied. The results suggested that the deformation could greatly enhance the strength of alloys, while only exerted little influence on the conductivity. But the increasing tendency of strength would fall down when the deformation was over 50%.The interaction of the precipitation and recrystallization was analyzed. The theoretically analysed results showed that if the precipitation happened prior to the recrystallization, the precipitates would strongly inhibit the process of recrystallization. When the recrystallization started, the precipitates swept by the interface of recrystallized grains would coarsen quickly, which was coincident with the experimental results. So, it was suggested that the alloys should be avoid being aged at elevated temperaturea and lasting a long time.The microstructures of Cu-Fe-P alloys were analyzed, and the results showed that for the alloys with lower Fe/P ratio, there were mainly Fe₃P and Fe₂P precipitates, and for the alloy with higher Fe/P ratio, there were three precipitates, Fe₃P, Fe₂P andÉ‘-Fe. When aged at elevated temperatures, the alloys with larger amount of Fe and P would present a certain portion ofÉ‘-Fe precipitates besides Fe₃P and Fe₂P. The analyzed results approximately agreed with thermodynamic calculation results. The alloys with lower Fe/P ratio showed higher strength. However, decreasing the Fe/P ratio would result in an increase of P solid-solutioned in the matrix, which was harmful to conductivity. Therefore, the Fe/P ratio and the amount of added Fe and P should be appropriately chosen according to the required strength and conductivity for the alloys. The results in this research showed that when the addition of Fe and P was roughly at 0.28wt% and the Fe/P ratio was about 3.67, the alloy composition would locate in the two-phase region of FCC-A1 + Fe₃P and close to the ternary region of FCC-Al +É‘-Fe + Fe₃P, which was helpful for acquiring a combination of high-strength and high-conductivity .The microstructures of Cu-Ni-Si alloys were analyzed, and the results showed that when the Ni/Si ratio was about 4.2, there were mainlyδ-Ni2Si pricipitates and a fewγ-Ni₅Si₂ pricipitate, which agreed with thermodynamic calculation results. Theδ-Ni2Si precipitates were helpful for alloy's strength and conductivity.Based on the optimization of alloy composition and microstructure and the combined strengthening of cold working, aging and grain refinement, the Cu-0.22Fe-0.06P-0.02B-0.05Ce(wt%) and Cu-3.2Ni- 0.75Si-0.02B-0.05Ce(wt%) alloys have been developed in this research, which exhibit excellent properties: the tensile strength (σb) 525MPa and 755MPa, elongation 16% and 13%, and the conductivity 82%IACS and 43 %IACS, respectively, making the two alloys be of promising application prospect.