Study On Microstructure Evolution Of Cu-Cr-Zr System Alloys And Interaction Mechanism Between Alloying Elements

Age-hardened Cu-Cr-Zr system alloys have excellent mechanical properties, high thermal and electrical conductivity, which can be widely used in the field of aviation and navigation, electric power and transportation. The resluts show that the addition of Zr to Cu-Cr alloy can refine the Cr and Zr-rich phase and increase the tensile strength in the aging process, due to the interaction between Cr and Zr elements. Although the microstructure and properties of Cu-Cr-Zr system alloys have been investigated and some conclusions on aging behavior of alloys have been obtained, there are still some ambiguities about precipitation sequence of Cr precipitates and crystallographic structure of Zr-rich precipitates. Besides, the interaction mechanism between Cr and Zr elements is not clear. Thus, in the study, the interaction mechanism between Cr and Zr elements is studied by combining the experimental result, data analysis and theoretical research methods. The tensile strength and electrical conductivity of a series of Cu-Cr, Cu-Zr and Cu-Cr-Zr alloy are tested and the interaction of Cr and Zr is verified according to two-way analysis of variance. In addition, the mechanism of the interaction between Cr and Zr is analyzed by investigated the aging sequence of Cr and Zr-rich phase of Cu-Cr, Cu-Zr and Cu-Cr-Zr alloy.The tensile strength and electrical conductivity of a series of Cu-Cr, Cu-Zr and Cu-Cr-Zr alloy were analyzed by mathematical statistics and two-way analysis of variance. The results show that the added strength caused by addition of0.1at.%Cr and Zr is respectively16.27MPa and48.09MPa, corresponding equation ocr=233.0+162.7XCr and ozr=233.0+480.9Yzr, and decreased electrical conductivity caused by addition of0.1at.%Cr and Zr is respectively0.81%IACS and7.15%IACS, corresponding equation λcr=100-8.1Xcr and λzr-100-71.5Yzr. Additionally, since there is a positive interaction between Cr and Zr in the Cu-Cr-Zr alloy, the ultimate strength of Cu-Cr-Zr alloy is greater than superposition of strength increment caused by corresponding Cr and Zr and strength of pure copper, the electrical conductivity of Cu-Cr-Zr alloy has same change trend as the strength. The difference between before and later in ultimate strength and electrical conductivity are respectively5-25MPa and0.5-3%IACS. Meanwhile, the ultimate strength and electrical conductivity of a Cu-Xcr Cr-Yzr Zr can be respectively estimated by the equation of o=233.0+162.7Xcr+480.9YZr+64.9XCrYzr1/3MPa and λ=100-8.1Xcr-71.5Yzr+8.94XCrYzr1/3%IACS.The microstructure evolution and physical properties change of Cu-Cr-Zr system alloys were investigated in the homogenization process by OM, SEM and TEM, and the as-cast microstructure of Cu-Cr-Zr system alloys were studied as well. The results show that the as-cast microstructure of Cu-Cr alloy is mainly composed of Cu and a network Cr dendrite. The as-cast microstructure of Cu-Zr alloy is mainly comprised of Cu and eutectic structure which is made of Cu and CusZr phase with a fine lamellar structure. While the as-cast microstructure of Cu-Cr-Zr is comprehensive of as-cast Cu-Cr and Cu-Zr alloy, no Cr2Zr phase could be found. Dissolution of Zr-rich phases and precipitation of Cr phases are simultaneously found in the alloy during the homogenization. With the increasing of homogenization temperature and time, the volume fraction of the eutectic structure and Cr phase decrease gradually. The hardness of the alloy exhibite first a gradual decrease, then a rapid increase and finally an almost saturated value. While the electrical conductivity of the alloy showes a contrary tendency. The proper homogenizing process is900℃x12h.The aging behavior and strengthening mechanism of Cu-Zr alloy were investigated. The results showed that hardness-time curve of Cu-Zr alloy exhibites a single peak aged at450℃and500℃, while electrical conductivity-time curve showes a rapid increase first and trendes a stable value finally. The precipitation sequence of Cu-Zr alloy aged at450℃is supersaturated solid solutionâ†'Zr atomic clusterâ†'semi-coherent CusZr phase. The orientation relationship with [112]Cu‖[011]Cu5Zr,(111)Cu‖(111)Cu5Zr existes between CusZr phase and matrix. Furthermore, the morphology of CusZr phase is identified as a kind of disk-precipitate with a certain thickness, whose habit plane is parallel to the{111}cu plane of the matrix. The yield strength aged at450℃for8h is calculated to be167.4MPa according to the Orowan strengthening, which is quite consistent with the experimental data (169.1MPa). The aging behavior of the Cu-Cr system alloys and the orientation relationship between Cr phase and matrix were studied. The results showed that the tensile strength-time curve of Cu-Cr system alloy exhibite a peak aged at450℃and500℃. For Cu-Cr alloy, the peak strength effect occurres after aging at450℃for8h. The addition of Zr to Cu-Cr alloy can shorten the time to the peak and increased the tensile strength in the aging process. The electrical conductivity of Cu-Cr system alloys exhibite first a gradual increase, then a rapid increase and finally an almost saturated value. The addition of Zr ato Cu-Cr alloy showe the same change rule of electrical conductivity as the Cu-Cr alloy. The precipitation sequence of Cr phase in the Cu-Cr and Cu-Cr-Zr alloys is supersaturated solid solutionâ†'G.P zonesâ†'f.c.c Cr phaseâ†'order f.c.c Cr phaseâ†'b.c.c Cr phase. The precipitation process of Zr-rich phase in the Cu-Cr-Zr alloy is supersaturated solid solutionâ†'Zr atomic clusterâ†'metastable CuCrZr phaseâ†'CusZr phase. In the evolution of decomposition, the change process of the orientation relationship between Cr phase and matrix is cube-on-cubeâ†'Nishiyama-Wassermanâ†'Kurdjumov-Sachs.The interaction mechanism between Cr and Zr on the aging process of Cr precipitates was investigated in detail in the Cu-Cr-Zr alloy. The results show that the addition of Zr to Cu-Cr alloy could accelerate the segregation of Cr elements and enhance the precipitation of Cr phase and the ordering process in the early aging stage. The Zr addition also shortens time to reach the peak, restrained the growth of Cr precipitates and decreases the width of precipitate free zone (PFZ) to increase the stability of Cr phases and the comprehensive properties of the alloy effectively. While the Cr, Zr and Cu elements forme a metastable CuCrZr phase, decomposed into Cu5Zr phase and Cr phase in the aging process, which postpones the formation of CusZr and restrained the growth of this phase.