Investigation On Preparation,Microstructure And Properties Of Cu-Cr-Nb Alloys

Owing to the combination of high strength,good electrical and thermal conductivities,Cu-Cr alloys have been applied in electric resistance welding electrodes,the integrated circuit lead frames,the liner tube of continuous casting crystallizer and the railway contact wires.In order to improve the strength and electrical conductivity of Cu-Cr alloy,Cu-Cr-Nb alloy were prepared by vacuum induction melting and in situ reaction method combined with different cooling methods in the present work.The effects of Nb addition and aging treatment on the microstructure and properties of Cu-Cr alloy were investigated,and the effects of different cooling method on the properties and microstructures of Cu-Cr-Nb alloys were discussed.The effect of trace La element addition on the mechanical properties of Cu-Cr alloy was further investigated.The main results are as follows:(1)The as-cast microstructures of Cu-Cr-Nb alloys have dendrite structure which consists of primary?phase dendritic and eutectic phase.With the addition of element Nb,the microstructures were refined and the secondary dendritic arm spacing of the alloys were decreased obviously.The Nb added to the Cu-Cr alloy is partially dissolved in the matrix,and others formed Cr₂Nb particles during the solidification process,which distributed in the interdendritic and hindered the growth of the dendrites.The value of SDAS of Cu-1.2Cr-0.3Nb alloy reaches the minimum 47.73?m when the Nb content is 0.3%.The value of Cu-1.2Cr-0.3Nb decreased 36.6%than the Cu-1.2Cr alloy.(2)For the Cu-Cr-Nb alloy cooled by furnace,after solid solution and aging treatment,two kinds of reinforcing phases of Cr and Cr₂Nb are precipitated from the copper matrix.The Cu-Cr-Nb alloys have the best comprehensive properties when the Nb content is 0.3%,the electrical conductivity is 83.9%IACS,and the hardness reaches 134.3HB.Without degrading its electrical conductivity,the hardness increased by 9.2%compared with that of Cu-1.2Cr alloy.(3)The cooling rates of graphite mold cooling and copper mold cooling are calculated by ANSYS,and the cooling rates are about 300K/s and 600K/s,respectively.With increase of cooling rates,the microstructure of the alloy is refined obviously,and the solid solubility of the alloy elements increased.As a result,the hardness increases greatly,but the electrical conductivity decreased slightly.Compared with the Cu-Cr-Nb alloys prepared by furnace cooling,the SDAS of alloys cooled by graphite mold have decresed about 60%.The microstructure of Cu-1.2Cr alloy cooled by copper mold has a characteristic of cellular growth.With the addition of Nb,the primary?phase exhibits columnar growth characteristics due to the composition undercooling.(4)The precipitates of Cu-Cr-Nb alloys prepared by copper mold are more dispersive and homogeneous,and the nano Cr₂Nb phases have a coherent relationship with the copper matrix,which further improves the properties of alloy.For quasi rapidly solidified Cu-Cr-Nb alloys,the direct aging treatment method can achive the higher performance.After aging treatment,the hardness and electrical conductivity of Cu-1.2Cr-0.4Nb alloy cooled by cooper mold can reach149.0HB and 83.2%IACS,respectively.(5)The addition of La to the Cu-Cr-Nb alloys refines the microstructure and breaks the continuous network structure of the eutectic phase.The tensile strength of Cu-1.2Cr alloy cooled by copper mold is 380.8MPa.With the addition of 0.4%Nb,the tensile strength and yield strength of the alloy are improved obviously,but the elongation is reduced slightly.While the addition of 0.1%La can not only increase the yield strength of the alloy,but also make the tensile strength and elongation of the alloy reach 427.8MPa and 13%,respectively.(6)The Cu-Cr-Nb alloy exhibited ductile fracture with dimple and tear ridges.With the addition of 0.4%Nb,the size of dimple is decreased and the number of dimples is increased.It can be seen that dimple size is uniform,and the amount of dimples is greatly increased after adding 0.1% La.