Study On Action Of Microalloying Elements And Structure Activity Relationship In Cu-Cr-Zr-Ti Alloys

Cu-Cr-Zr alloys with high strength,high electrical,thermal conductivities and corrosion resistance have been widely used in aerospace,lead frames,high speed railway contact network cable,and other fields.Literatures show that the addition of the fourth component will improve the overall performance of the Cu-Cr-Zr alloy,but the existence of the fourth component in the alloy and the mechanism of their effect are not well understood yet.Based on the above background Cu-Cr-Zr-Ti alloy has been taken as research object.By exploring the effects of micro-alloying elements of microstructure,the segregations of Cr,Zr and Ti elements during precipitation was studied,and the precipitation mechanism of Cr phase in the simultaneous presence of Zr and Ti elements was analyzed.In addition,the effects of different preparation processes on the properties of Cu-Cr-Zr-Ti alloy rods prepared by two processes were investigated,and the effect of the morphology and distribution of the precipitated phase particles during the microstructure evolution process on the properties of the alloy was investigated.The relationship between the preparation process,microstructure and mechanical properties of Cu-Cr-Zr-Ti alloy was established.The Cu-Cr-Zr-Ti alloy ingot is prepared by the process of "melting and casting",The results show that the as-cast grains are coarse and there are a large number of dendrites,and the primary phase is mainly the Cr-rich phase with developed dendrite and the Zr-rich phase with rod shape.The different contents of Cr,Zr,Ti elements have a great influence on the morphology and quantity of the primary phase of the alloy.The increase of Cr content will lead to more developed dendrite of Cr-rich phase in as-cast structure,but less effect on Zr-rich phase.The increase of Zr content will increase the number and size of Zr-rich phase in the matrix,and has no obvious effect on the morphology of Cr-rich phase.The increase of Ti content can hinder the growth of Cr-rich phase,which leads to the underdevelopment of Cr-rich phase dendrite,but has no effect on the morphology of Zr-rich phase.The precipitation strengthening mechanism of Cu-0.31Cr-0.29Zr-0.15 Ti alloy during aging was studied.The results show that the precipitated phase does not grow up significantly and the size of the precipitated phase is maintained at 3~5 nm,only the metastable fcc-Cr phase coherent with the matrix was found.The Cr,Ti atoms segregate at the center of the Cr-rich phase during aging,while the Zr atoms segregate at the interface between the Cr-rich phase and the Cu matrix.Many fcc-Cr precipitates containing Ti atoms can form and grow to a certain scale.This causes Cr-and-Ti-lean zones between Cr-rich precipitates,which will not provide any supersaturation of solutes for further growth of fcc Cr-rich precipitates,inhibiting the formation of bcc-Cr phase.Preparation of ?=3 mm alloy rods are carried out by two casting processes.The tensile strengths of the rods prepared using the upward continuous casting process and hot-extrusion process are 621 and 571 MPa,and the softening temperatures are 590 oC and 563 oC,respectively.Conductivities are about 65% IACS and the elongations are about 5% for both alloys rods.After the alloy is extruded and deformed,the as-cast crystal grains are crushed into fine grains with a small grain size of about 20~40 ?m,and the secondary phases are dispersed in a granular form in the copper matrix.After solid solution treatment,the alloy grains are abnormally coarse,and the grain size is above 200 ?m,and a large number of annealed twins are formed.The as-cast microstructure of upward continuous casting process alloy is coarse and dendrite developed,and the primary Cr phase is mainly distributed at the grain boundary.After solid solution treatment,the alloy is completely recrystallized,and the grain size is about 20 ?m,and the primary Cr phase is distributed at the crystal boundary.The primary Cr phase particles at the grain boundary have an effect on grain boundary migration.By calculating of driving force and resistance of recrystallization grain growth,the driving force of grain growth is greater than that of resistance during the solid solution treated(950 oC/10?20?40 min)of the hot-extrusion process alloy,and the grain grows up continuously.The driving force of the grain growth of the upward continuous casting process alloy is less than the resistance during the solution treated,and the pinning action of the second relative grain boundary makes it difficult to recrystallize and obtain fine grains.