Study On Microstructure And Property Of High-strength And High-conductivity Cu-Cr-Zr Alloys

High-strength and high-conductivity Cu-Cr-Zr alloys are widely used in integrated circuit lead frames, high-speed railway electricity contact wires and aerospace industries. With the development of modern industry technique, copper alloys with higher mechanical and electrical properties are required. This places an urgent need to exploit new Cu-Cr-Zr alloys and related processing technique, as well as to clarify the underlying mechanisms behind the microstructure formation and performances of the alloys.Alloy ingots of Cu-0.81wt%Cr-0.12wt%Zr（weight percent） containing minor La and Y were produced by using the vacuum induction method. The as-cast ingots were subjected to homogening annealing, hot rolling, solid solution, cold rolling, and aging in proper sequence. The microstructures of the alloys at every stage were observed by an optical microscope and a scanning electronic microscope（SEM）. The phase constitution was analyzed using an X-ray diffractometer（XRD）. The precipitated phase was identified by a high resolution transmission electron microscope（HRTEM）. Microhardness was measured in a digital-type hardness tester. Strength was measured in a universal mechanical testing machine. Electrical resistance was measured using a microhmmeter. Thin ribbons of the supersaturated solid solution were prepared by rapid quenching. Their microhardness and electrical resistance before and after aging treatment were also measured. Besides, Cu-0.81 Cr hypoeutectic alloy was directionally solidified by liquid-metal cooling method, and the microstructure, mechanical and electrical properties of the sample were also examined. The main results are as follows:Addition of 0.05wt% La or 0.05wt% Y to Cu-0.81Cr-0.12 Zr alloy does not change the phase constitution. The alloy ingot is still composed of three phases: Cu matrix, Cr and Cu5 Zr. The majority of Cr phase distributes at the grain boundaries of Cu as（Cr+Cu） eutectic or small grains, while a small quantity of Cr particles distribute in the Cu matrix, and Cu5 Zr phase exists only at the Cu grain boundaries. But addition of rare-earth elements to the alloy can obviously refine the microstructure of the alloy ingot.Alloy ingots of Cu-0.81Cr-0.12Zr-0.05La-0.05 Y were homogenized at 1193 K for 60 min and then hot rolled into sheets. After solid solution treatment at 1223 K for 60 min, the sheets were quenched into water for cold rolling at room temperature. the samples cold rolled to different strains were aged at various temperatures for different time, and their properties are investigated in details. It is found that an excellent comprehensive property can be obtained in the 60% strain sample when it is further aged at 773 K for 60 min. The microhardness reaches 186 HV while the electrical conductivity maintains 81% IACS. Letting the sample undergo another cold rolling 40% strain, the microhardness and electrical conductivity raise to 203 HV and 81.9% IACS, respectively. Meanwhile, the tensile strength can reach to 604 MPa and the elongation percentage reaches 8.5%.When the Cu-0.81Cr-0.12Zr-0.05La-0.05 Y alloy cold rolled by 60% strain was heated from room temperature at a rate of 20 K/min, a precipitation and a recrystallization reaction take place in the temperature range from 653 K to 698 K and 743 K to 823 K, respectively. The microstrain in the cold rolled Cu-0.81Cr-0.12Zr-0.05La-0.05 Y alloy is slightly higher than that in pure copper. The（111）Cu intensity in the XRD patterns of the cold rolled alloy decreases with the increasing aging temperature, but the（220）Cu intensity increases instantly.The phases precipitated during aging the solid solution treated Cu-0.81Cr-0.12Zr-0.05La-0.05 Y alloy are composed of bcc-Cr and fcc- Cu5 Zr. When the alloy aged to state with the best comprehensive properties, part of the precipitates keep a coherent relationship with the copper matrix. The so-called Nishiyama-Wassermann orientation relationship exists between the Cr precipitate and the matrix:（111） Cu //（110） Cr;[01₁] Cu // [001] Cr;[2₁1] Cu // [1₁0] Cr。As a supersaturated solid solution Cu-0.81Cr-0.12Zr- 0.05La-0.05 Y ribbon exhibits an the exothermic peak with an onset temperature of 655 K and an ending temperature of 688 K due to the precipitation reaction. When it is heated at a rate of 20 K/min. An optimal combination of the mechanical and electrical properties is obtained in the ribbon aged at 773 K for 15 min: a microhardness of 215 HV and an electrical conductivity of 77.6% IACS. The microhardness of the ribbon is 29 HV more than that of the 60% cold rolled sample, indicating that the strengthening effect of rapid quenching is better than the conventional solution treatment.The microstructure of the directionally solidified Cu-0.81 Cr sample consists of primary α-Cu dendrites（cells） surrounded by（Cu+Cr） eutectics. Cr fibers randomly distribute in the Cu matrix in the eutectics structure. However, such an case that the eutectics grow along the longitudinal boundaries of the primary α-Cu dendrite still significantly elevate the strength, plasticity and conductivity. With the increasing temperature gradient, the directional solidification structure is refined, and a primary arm can extend a longer distance in the longitudinal direction, leading to better mechanical properties and electrical conductivity. When the drawing rate rises, the strength and conductivity increase first and then decrease, but the plasticity changes inversely.