Effects Of Rare Earth And Aging Treatment On Microstructure And Properties Of Cu-Ni-Si Alloys

With the improvement of generator unit capacity,people have put forward higher requirements for the safety and reliability of generator units.The rotor slot wedge is one of the most important parts of a generator unit,and Cu-Ni-Si alloy is currently the most widely used slot wedge material.Due to its precipitation strengthening mechanism,the cold deformation,aging temperature and aging time are the key factors influencing the aging performances.Besides,the rare earth and rare earth oxides can act as effective additives which can improve the microstructure and properties of copper alloys.In this paper,the changes of microstructure,mechanical and electrical properties of Cu-Ni-Si alloys after aging treatment have been investigated by means of X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM,HREM),tensile test and conductivity measurement.Moreover,the behaviors of rare earth and rare earth oxides in Cu-Ni-Si alloys and their influences on microstructure and properties are also discussed.The main conclusions are shown as follows:1.Since the as-cast Cu-Ni-Si alloy has typical dendrite organization,there is an obvious coring and dendrite segregation.Hot extrusion could eliminate some as-cast defects,at the same time the dynamic recovery and recrystallization occur because of the high extrusion temperature.The alloy still remains supersaturated solid solution after hot extrusion with no obvious second phase precipitation.Cu-Ni-Si alloys get the optimal overall performance after 60%cold drawing deformation and aging at 450 ℃for 3 hours with the microhardness of 265.6 HV,tensile strength of 760 MPa and conductivity of 40.1%IACS,respectively.Some ordering transformation has been detected during aging at 430 ℃ and the main precipitated phase is Ni2Si after aging at 550℃,but these precipitations have grown up obviously.The behavior of phase transformation kinetics of the Cu-Ni-Si alloy during aging is investigated through measuring the variation of electrical conductivity with aging time increasing.The Avrami phase transformation kinetics can be explained as:400 ℃:φ=1-exp(-0.7561t0.72678);430℃:φ = 1-exp(-0.9094t0.70005);460℃:φ = 1-exp(-1.1868t0.53943),respectively.2.The lanthanum addition can refine as-cast grains of Cu-Ni-Si alloys.With the mass fraction of lanthanum increase from 0.05 wt.%to 0.5 wt.%,the grain sizes decrease from 2.5 mm to 1.1 mm.Besides the α-Cu phase,other phases such as LaSi2,LaNi3 and LaCu2 are also found by XRD.With the increase of lanthanum contents,the morphology of second phases change from sphere to long strip or block,and the size increase from 3～5μm to 10～12 μm.With the addition of 0.1 wt.%La,the tensile strength and elongation ratio of as-cast Cu-Ni-Si alloys reache to their peaks of 430 MPa and 17.15%,respectively.Furthermore,the precipitation sequences and precipitated phases have obviously changed due to the lanthanum addition.As for the Cu-Ni-Si-0.1La alloy,the main precipitated phase is Ni3Si after aging at 430℃ for 0.5 hours,which keeps the coherent relationship with the Cu matrix.After aging for 2 hours,the Ni3Si and some other phases such as LaNi3,LaSi2,and Ni2Si are formed,and the Ni3Si as well as Ni2Si still keep the coherent relationship with the Cu matrix.3.The addition of rare earth oxide La0.67Sr0.33MnO3(LSMO)can facilitate the formation of equiaxed grains,and a large number of equiaxed grains have been found in the center of Cu-Ni-Si ingot with the LSMO content of 0.1 wt.%and 0.2wt.%.There is a close correlation between the distribution and the content of LSMO.When the content is 0.1 wt.%,the LSMO particles will gather in dendrite junction,while the LSMO particles distribute in both grains and grain boundaries of the matrix with the content of 0.5wt.%.At the LSMO addition of 0.1wt.%,the as-cast Cu-Ni-Si alloys have the optimum plastic deformation capacity with the elongation of 22.8%,increased by 71.4%,and the tensile strength of 382 MPa,which falls slightly compared with the alloy without LSMO addition.The cold rolling process of Cu-Ni-Si alloys with 0.1 wt.%addition is carried out,and the results shows that under the 78%deformation,the samples are smooth and crack-free,which indicates the good cold deformation ability.