Research On The Deformation Aging Bahaviors And Properties Of CuNiSi-(Cr,Zr) Alloys

As a typical age strengthening alloy,CuNiSi alloys with both high strength and high electrical conductivity were promising materials for lead frames.Recent researches of the CuNiSi alloys mainly focused on alloy design and combinations of thermomechanical treatments.In this article,Cr and Zr elements with low solution limits in copper were selected as the alloying elements to the CuNiSi alloys with 2 wt.%Ni and 0.5 wt.%Si.Besides the traditional room temperature rolling,cryorolling was also applied to CuNiSi alloys.The influence of Cr and Zr elements on the microstructures,properties and aging behaviors of CuNiSi alloys subjected to room temperature rolling and cryorolling were studied.And two step aging were applied to further promote the comprehensive properties.In addition,in situ synchrotron X-ray diffraction was also employed to analyze the precipitation kinetics.The research conclusions are listed below:The microstructures of cast CuNiSi alloys were composed of coarse columnar grains.Cr3Si and Ni2SiZr particles were respectively introduced by Cr and Zr alloying and refined the cast microstructures of the CuNiSi alloy.After homogenization and solution treatment,the dendrite structures of CuNiSi alloys were replaced by equiaxed grains with annealing twins.The Cr3Si and Ni2SiZr particles were stable during the cheat treatment and were capable of pinning the migration of grain boundaries by the Zener force,hence refining grain size after homogenization and solid solution treatment.Both Cr and Zr elements were capable of refining the microstructures of room temperature rolled CuNiSi alloys in the rolled and aged states.Moreover,Zr element could reduce the stacking fault energy of CuNiSi alloys and promote the formation of deformation bands.During aging,the electrical conductivity of CuNiSi alloys were increased by Cr and Zr elements.Cr promoted the mechanical strength of CuNiSi alloys.Zr increased the elongation of CuNiSi alloys at the expense of mechanical strength.During the fracture process,Cr3Si and Ni2SiZr particles acted as the nucleation sites for dimples,which resulted in the transition of fracture mode for CuNiSi alloys from quasi-cleavage fracture to ductile fracture.In the cryorolled states,the addition of Cr and Zr elements reduced deformation twins and promoted the formation of deformation bands and sub-grains in the CuNiSi alloys.Zr element reduced the spacing of deformation twins and refined the microstructures of cryorolled CuNiSi alloys.Cr element inhibited the recrystallization of cryorolled CuNiSi alloys during aging and refined the precipitates.During aging,Cr element increased the mechanical properties and decreased the electrical conductivity of CuNiSi alloys,while the effects of Zr element was opposite.After two step aging,the ultimate tensile strength,elongation and electrical conductivity are 672 MPa,13.7%and 57.6%IACS for CuNiSiCrZr alloy.Compared with room temperature rolling,cryorolling increased the mechanical strength of CuNiSi alloys(505-610 MPa).According to in situ synchrotron X-ray diffraction analysis,cryorolling increased the dislocation density of the as-rolled CuNiSi alloy,hence promoting the mechanical strength in the rolled state.Upon aging,cryorolling accelerated the precipitation process and promoted the electrical conductivity of CuNiSi alloy.However,recrystallization was also promoted by cryorolling,which promoted the annihilation of dislocations and transformed the precipitation mode from continuous precipitation to discontinuous precipitation,deteriorating the mechanical properties of CuNiSi alloy.Cr element increased the dislocation density of CuNiSi alloys and inhibited the recrystallization and discontinuous precipitation during aging,retarding the annihilation of dislocations.Zr element decreased the dislocation density of CuNiSi alloys and suppressed the discontinuous precipitation during aging.In the early stage of aging for the cryorolled CuNiSi alloys.?-Ni2Si precipitates nucleated by heterogeneous nucleation,the nucleation sites of which were determined by the dislocation density.Cr element increased the dislocation density of CuNiSi alloys,increasing number of nucleation sites and promoted the nucleation of ?-Ni2Si precipitates.While Zr element reduced the dislocation density of CuNiSi alloys,decreasing number of nucleation sites and suppressed the nucleation of ?-Ni2Si precipitates.In the late stage of aging,the coarsening of precipitates dominated the precipitation process.Cr element refined the precipitate radius of CuNiSi alloys,hence suppressing the precipitation of CuNiSi alloys.Zr element promoted the coarsening of precipitates and accelerated the precipitation of CuNiSi alloys.In summary,cryorolling were successfully applied to the fabrication of CuNiSi alloys and the microstructures and precipitation behaviors of CuNiSi alloys were modified by the alloying elements Cr and Zr.A new strategy(cryorolling and two-step aging)were proposed to simultaneously increase the mechanical strength and electrical conductivity of CuNiSi alloys.The CuNiSiCrZr alloys fabricated by this method exhibited an ultimate strength of 672 MPa,an elongation of 13.6%and an electrical conductivity of 57.6%IACS.Moreover,the influence of cryorolling on the aging behaviors of CuNiSi alloys were classified by in situ synchrotron X-ray diffraction analysis.And the dislocation densities of CuNiSi alloys were calculated by the Modified William-Hall equation.These efforts introduced synchrotron X-ray diffraction technique to the study of phase transformation of copper alloys,which would promote the application of synchrotron radiation in the field of copper based alloys.