Study On Microstructure And High Temperature Properties Of Cu-Cr Alloys

CuCr alloy is a typical aging strengthening alloy and widely applied in manufacturing electrical equipment.The ultimate tensile strength of CuCr is relatively low,thus the addition of third element is needed to further enhance the strength of CuCr alloy.Multiple studies have shown that the addition of Zr can effectively accelerate the precipitation of Cr phase and optimize the phase distribution,what's more,the formation of CuZr composites could also result in the mechanical property improvement of CuCr by precipitation-strengthened mechanism.However,due to the severe loss of Zr in smelting process,the preparation of CuCrZr alloy is generally achieved by vacuum smelting,which greatly increases the production cost and limits the sizes of cast ingots.Vacuum melting process can not promote large-scale preparation and application of CuCrZr alloy at present.Comparing with Mg and Ti,there is little effect on the electrical conductivity of the copper alloy with minor addition of Ag.What's more,the physicochemical property of Ag is stable which means less loss in smelting process,and CuCrAg alloy could be prepared by non-vacuum smelting.Concerning that it is difficult to realize mass and continuous production of CuCrZr alloy,there is potential for CuCrAg to be an alternative material of CuCrZr under some conditions(such as high temperature).Besides,it is conducive to optimize the melting process of CuCrZr by replacing Zr with Ag element.There are many reports about CuCrAg and CuCrZrAg,but few reports about their high temperature properties exist.In this study,CuCrZr,CuCrAg,and CuCrZrAg alloy with the same atomic ratio of Zr,Ag and(Zr+Ag)were designed on the basis of CuCr binary alloys.The differences of room temperature,high temperature performance and creep resistance of three alloys were compared through mechanical tensile test,creep test,hardness test and conductivity test.Further,microstructure of alloys were investigated by optical microscope(OM),transmission electron microscope(TEM)and scanning electron microscope(SEM),furthermore,inner connections between properties and microstructure were established.Lastly,it was clear and verified that whether CuCrAg and CuCrZrAg alloy could take the place of CuCrZr alloy under some conditions.The mechanical properties test showed that the CuCrZr,CuCrAg and CuCrZrAg alloys homogenized at 900? for 12h are suitable for cold processing combined with aging treatment.After being aged at 450? for 3h,CuCrAg alloy exhibits more excellent comprehensive property than CuCrZr and CuCrZrAg alloy with tensile and conductivity reaching 318MPa and 96.2%IACS respectively.After 27%cold deformation and an aging treatment of 450?×4h,the tensile strength of Cu-0.24Cr-0.04Zr(at%)alloy reaches 428MPa,the electrical conductivity is 93.3%IACS;after an aging treatment of 400?×8h,the tensile strength of Cu-0.22Cr-0.06Ag(at%)reaches 451 MPa and the conductivity is 96.1%IACS;after an aging treatment of 400 ?×4h,the tensile strength of Cu-0.24Cr-0.02Zr-0.03Ag(at%)alloy reaches 479 MPa,the conductivity is 93.2%IACS.With the temperature increasing from 300? to 600?,the tensile strength of CuCrZr decreases from 176MPa to 121 MPa,the tensile strength of CuCrAg decreases from 208MPa to 92MPa,and the tensile strength of CuCrZrAg decreases from 184MPa to 116MPa.In the high temperature softening test,the softening point of CuCrZr and CuCrZrAg alloy is between 600-650?,and the softening behavior of CuCrAg occurs between 500? and 550?.TEM analysis showed that the size of the precipitated phase in CuCrZr and CuCrZrAg alloys changes little with the temperature increasing,and the precipitated phase at high temperature can still maintain the strong dislocation movement resistance;the precipitated phase in CuCrAg is easily coarsened at high temperature,therefore the performance of CuCrAg severely deteriorates in high temperature.It was obtained that the strain of CuCrZr and CuCrZrAg decreases with the experiment goes on while the same phenomenon was not observed for CuCrAg alloy at 400? with the applied stress of 30MPa.With temperature and stress increasing,negative creep disappeared and creep curves transformed into a typical type.The creep life of three alloys are largely affected by temperature and stress,the effect of temperature and stress on creep life of CuCrZr alloy is minimal and maximal for CuCrAg.When the stress is 30 MPa,the creep life of CuCrAg is 7.7h and creep life of CuCrAg and CuCrZrAg is more than 50h at 600 ?.The stress exponents of CuCrZr,CuCrAg and CuCrZrAg are 5.2,3.7 and 5.3 respectively at 600? and the creep behavior of CuCrZr and CuCrZrAg is controlled by dislocation climbing mechanism while the creep mechanism of CuCrAg is sticky motion of dislocation.The creep rupture mechanism of CuCrAg is brittle intergranular fracture while the creep rupture mechanism of CuCrAg and CuCrZrAg is ductile fracture.In conclusion,CuCrAg and CuCrZrAg alloy could be an alternative material of CuCrZr alloy under lower temperature condition(?300?).Taking poor performance of CuCrAg under high temperature,it is not recommended to be applied at high temperature.However,with Zr element being replaced by Ag for CuCrZr alloy,the high temperature property of CuCrZrAg is similar with CuCrZr and could meet the needs of relative applications,thus there is essential for CuCrZrAg to partly take the place of CuCrZr alloy.