Study On Preparation,microstructure And Properties Of The New Cu-Ni-Si Alloys With High Strength And High Elasticity

As the representative of high strength and high elastic copper alloy,Cu-Ni-Si alloy has good electrical and thermal conductivity and is a crucial raw material for the manufacture of large-scale integrated circuit lead frame terminals.Compared with Cu-Ni-Si alloys,Cu-Ni-Co-Si alloys enjoy higher strength,electrical conductivity and stress relaxation resistance,which can meet the development trend of miniaturization and complex molding of VLSI lead frame materials.However,relevant technical patents and products have been monopolized by foreign countries and the new Cu-Ni-Co-Si alloys are still in a blank stage in domestic industrial production.Therefore,the influence of different alloying elements on the microstructure and properties of Cu-Ni-Co-Si alloys is studied systematically.The type of precipitation strengthening phase is determined and the phase transformation characteristics of the alloy during solution-aging process are explored.The intrinsic relationship of alloying elements,precipitation strengthen phase,deformation process and mechanical physical properties is investigated,which sheds lights on the local production and application of high-strength and high-elasticity Cu-Ni-Co-Si alloy sheet material.In order to solve the scientific problems and key technologies in the optimization design,preparation and processing of high-strength and high-conductivity copper alloys,the study focuses on the following aspects:By designing different Ni/Si mass ratios,the influence mechanism of Ni/Si ratio on the microstructure and properties of Cu-Ni-Si alloy is systematically investigated.and the suitable Ni/Si ratio is determined.The alloy exhibits excellent comprehensive properties when the mass ratio of Ni/Si is about 4.2-5.1.The physical properties of Cu-Ni-Si alloy are improved after the addition of trace elements of Co,Cr,Ti and RE respectively.When a certain amount of Co is added,the precipitation of the precipitated phase can be developed,and the matrix can be purified,therefore significantly promoting the strength and electrical conductivity of the alloy.The hardness and conductivity of the alloy can be improved by adding Cr element,but the improvement effect is not as good as that of Co element.The single Cr phase is separated out with body-centered cubic structure and has a K-S orientation relationship with the matrix.The addition of Ti element can reduce the distance between precipitated phases and improve the hardness of the alloy,but decreases the conductivity of the alloy.The addition of RE element can can refine the precipitated phase and cooperate to improve the hardness and conductivity of the alloy.The improvement effect is not as good as that of Co element.Therefore,Co is selected as the key alloying element.Taking the Cu-Ni-Si alloy with the best Ni/Si ratio as the research object and replacing some Ni elements with Co,the influence of Ni/Co ratio on the properties of Cu-Ni-Co-Si alloy is systematically studied by designing different Ni/Co mass ratios.When Co is substituted for Ni element,the hardness of the alloy increases rapidly with the increment of Ni/Co ratio,and then decreases slowly.The hardness peaks at a Ni/Co ratio of 1.12～1.95.The conductivity rate decreases with the growth of Ni/Co ratio and the rate is optimal when the Ni/Co ratio is 0.05～0.50.After the addition of Co element in the alloy,the grain and precipitated phase of the alloy gradually increase with the growth of Ni/Co ratio,and the volume fraction and the number density of the precipitated phase increased first and then decreased.The studies of precipitation behavior and precipitation phase characteristics of alloys are conducted from the perspectives of crystal structure,microstructure evolution and so on.The precipitation phase types of Cu-Ni-Co-Si alloys with different Ni/Co ratios are the same—(Ni,Co)2Si composite phase,which is uniformly distributed in the interior of the precipitated phase and shares the same morphology with those of Ni2Si:orthogonal structures in the shape of discs and rods.The diffusion mechanism of solute atoms in the alloy and the microstructure evolution behavior of the strengthening phase are elaborated,manifesting the phase transformation law of Cu-Ni-Co-Si alloy in the early aging period.The formation of the Co2Si phase by Co with Si accelerates the precipitation of the(Ni,Co)2Si phase,and suppresses the process of amplitude modulation decomposition and formation of atomic clusters,which directly precipitates the(Ni,Co)2Si phase.What’s more,the addition of the Co element inhibits the recovery recrystallization transition of the alloy,resulting in improving the high temperature properties of the alloy.By investigating the effects of combined treatment process on the properties of the alloy,the best combined heat treatment process of Cu-Ni-Co-Si alloy is tested:1025℃ solution treatment 1h+60%cold rolling treatment+425℃ aging 1～2h+35%secondary cold rolling treatment+300～350℃ secondary aging 1～2h.Besides,the properties of the alloy are significantly enhanced:the strength properties of NC-5 alloy(Ni/Co=1.95)are the best,the yield and tensile strength are 900MPa and 1000MPa respectively;the electrical conductivity rate of NC-2 alloy(Ni/Co=0.32)is higher,the tensile strength is maintained at 800MPa,and the electrical conductive rate is above 45%IACS;NC-4 alloy(Ni/Co=1.12)combines the comprehensive properties of the two,yield and tensile strength are 800～850MPa and 900～1000MPa respectively,electrical conductivity rate is 40～45%IACS.The relationship between the stress relaxation properties and the microstructure of the alloy was investigated.Although the introduction of the deformation process did not change the type of precipitated phase,the discontinuous precipitated cell structure appeared at the grain boundary,leading to the brittleness of grain boundary and the degrading of stress relaxation resistance.The stress relaxation resistance of NC-5 alloy is better than that of NC-2 alloy.The coarse(Ni,Co)2Si phase appears in the NC-2 alloy,and recrystallization occurs by introducing an additional nucleation site.The precipitate phase grows and coarsens at high temperature,and the stress relaxation resistance further decreases.The number of small angles grain boundaries of NC-5 alloy is more,which hinders the dislocation movement and increases the number of immovable dislocation.At high temperature,the precipitated phase particles in the alloy are fine and dispersed,which has a more significant effect on the pinning of movable dislocation,thus improving the relaxation stability.Through the precise control of alloy composition and processing,a new high-strength and high-elastic Cu-Ni-Co-Si alloy with uniform composition and excellent performance was produced.As a quaternary Cu-Ni-Co-Si alloy,NC-2,NC-4 and NC-5 alloys enjoy great advantages in terms of electrical conductivity and strength.Combined with the alloy design idea of Co-substituted Ni,this study lays a theoretical foundation for the design and development of multi-element Cu-Ni-Co-Si-X alloy.