Study On Aging Behavior Of High-strength Conductive Cu-Ti-Fe-Cr Alloy Foils

Age-strengthened Cu-Ti alloys,which have well mechanical and electrical properties and the highest level of stress relaxation resistance among copper alloys,are widely used in components of various electronic devices,such as switches,connectors,conductive spring.With recent rapid advances of densely-integrated and the reductions in size and thickness of electronic components,Cu-Ti alloys' low electrical conductivity is gradually emerged.Therefore,it is the pre-requisites and key to enlarge application field of Cu-Ti alloys by improving electrical conductivity without sacrificing the mechanical properties.In this paper,the effects of Cr element and combined aging process on the microstructure and properties of Cu-3Ti-0.2Fe alloy were studied by optical microscopy,scanning microscopy,EDS spectroscopy,Vickers hardness,conductivity and tensile tests.By means of the optimized alloy components and combined aging process,the novel high strength conductive Cu-Ti-Fe-Cr alloy foils were obtained.The main research contents and results are as follows:(1)The microstructures of the as-cast Cu-3Ti-0.2Fe-xCr(x=0,0.1,0.2,0.3)alloys are typical dendrite structure,whereas the average dendrite arm spacing of as-cast alloys decreases with the increment of Cr content.According to the variation of grain size,second phase content and electrical conductivity,the optimized solution treatment of Cu-Ti-Fe-Cr alloys is 900 ?×240 min.With the increment of Cr content,the rod-shaped phases are transformed to be granular phase in the grain boundary,and the second phases are(FeCr)2Ti intermetallic phases analyzed by EDS test.For the four alloy foils with the thickness of 0.1 mm aged at 450 ?,the hardness increases to peak values rapidly but then reduces moderately with an increase in the aging time,whereas the electrical conductivity increases and increment tendency reduces gradually.The addition of Cr element has not only increased the peak-aged hardness but also increased the aging time reaching the peak-aged condition.The best comprehensive properties of Cu-Ti-Fe-Cr alloy foil are obtained by addition of 0.2 wt.%Cr,and the hardness,tensile strength,elongation and electrical conductivity of Cu-3Ti-0.2Fe-0.2Cr alloy foil are 355.1 HV,1053 MPa,2.2%and 12.7%IACS.(2)With the increment of aging temperature,the peak-aged hardness of Cu-3Ti-0.2Fe and Cu-3Ti-0.2Fe-0.2Cr alloy foils is gradually decreased,the time reaching the peak-aged condition is reduced gradually,and the decrement velocity of hardness is accelerated in the late aging process.With the increment of aging temperature,electrical conductivity is increasing in the initial period of the aging process,but the increasing rate reduces gradually in the late period of the aging process.Besides,the electrical conductivities of specimens aged at 450 ? are still higher than those of specimens aged at lower temperatures.(3)Phase transformation equations and Avrami conductivity equations of Cu-3Ti-0.2Fe and Cu-3Ti-0.2Fe-0.2Cr alloy foils are obtained by analyzing the evolution of electrical conductivity in different temperatures.According to coefficient of correlation and F-check,the theoterical results coincide well with the measured ones.Precipitation formation activation energy of Cu-3Ti-0.2Fe alloy foil increases by 12.31 kJ/mol by addition of 0.2 wt.%Cr.(4)The slip band and deformation twin of pre-rolled Cu-3Ti-0.2Fe-0.2Cr alloys reduces during the pre-aging process at 450 ?.The hardness of Cu-3Ti-0.2Fe-0.2Cr alloy foils re-aged at 450 ? increases rapidly but then decreases quickly,whereas the electrical conductivity increases and variation tendency reduces gradually.With the increment of pre-aging time,the time reaching to the peak hardness of re-aged alloy foils is reduced.After the optimized combined aging treatment of solution treatment at 900 °C for 240 min,pre-rolling with reduction of 50%,pre-aging at 450 ? for 240 min,re-rolling with reduction of 95.6%and re-aging at 450 ? for 90min,the hardness,tensile strength,elongation and electrical conductivity of Cu-3Ti-0.2Fe-0.2Cr alloy foil are 354.6 HV,1062 MPa,1.9%and 17.1%IACS,respectively.The fractograph of the studied alloy foil treated with optimized process is composed of flat facets resembling cleavage,river pattern and dimples.