Investigation On The Preparation,Microstructure And Property Of Cu-15Ni-8Sn Alloy With High Strength And High Elasticity

Cu-Ni-Sn alloy has been widely used in precision instruments,electronic appliances,aerospace,oil and gas industry,and mechanical system due to its high strength,hardness and elasticity,excellent electrical conductivity,thermal conductivity,corrosion resistance,wear resistance,stress relaxation resistance,high temperature oxidation resistance,which can be regarded as the candidate to replace the toxic Cu-Be alloy.However,the micro-segregation and inverse macro-segregation of Sn easily existed in the Cu-Ni-Sn alloy ingot prepared by casting method due to its large liquid-solid mushy zone,which leads to the crack during subsequent deformation process.In addition,as a typical aging-strengthened copper alloy,discontinuous precipitation（DP）reaction would happen at the later stage of the aging process.The formed lamellar structure at grain boundary would cause grain boundary brittleness,and ultimately degrade the properties of the alloy.In this study,the Cu-15Ni-8Sn alloy was selected as the research object.Firstly,the non-equilibrium solidification process of Cu-15Ni-8Sn alloy was studied by high temperature quenching experiment.In addition,the inverse macro-segregation behavior of Sn element was investigated by unidirectional solidification experiment under different cooling rates.Secondly,the vertical semi-continuous casting equipped with mechanical vibration（MV）and electromagnetic field（EMF）was applied to successfully prepare Cu-15Ni-8Sn alloy ingot with uniform microstructure and chemical composition.Moreover,the heat treatment and deformation process of Cu-15Ni-8Sn alloy was investigated.Finally,the high-melting stable second phase was introduced into grain boundary and grain interior to inhibit the nucleation and growth of DP through micro-alloying,the effect of V addition on microstructures and properties of Cu-15Ni-8Sn alloy was investigated.The main results are obtained as follows:（1）The solidification structure evolution process of Cu-15Ni-8Sn alloy was investigated by high temperature quenching experiment.The nucleation and growth of primary Sn-depletedα₁ phase（L→α₁）took place when the temperature decreased to the liquidus temperature（1114 ^oC）,and the growth ofα₁ phase lead to the higher contents of Ni and Sn in residual liquid;the separated eutectic reaction of the residual liquid took place when the Sn content increased to 26 wt.%（L₂→α₂+γ）.During subsequent cooling process（700～600 ^oC）,the supersaturated solid solution（α₂）decomposed into two phases containing lamellar Sn-depletedα₁ matrix and Sn-richγphase（α₂→α₁+γ）.（2）The vertical semi-continuous casting equipped with MV and EMF simultaneously was applied to successfully prepare Cu-15Ni-8Sn alloy square ingot with a length of 600 mm and a section of 100 mm×100 mm.The process parameters were as follows:casting temperature was 1350 ^oC,casting speed was 120 mm/min,primary cooling water was 1000L/h,secondary cooling water was 2500 L/h.Applying MV and EMF at the same time can refine the grain size and improve the macrosegregation of Sn.When the amplitude of MV was2 mm,the frequency was 57 time/min,the magnetic field power was 10 k W,the size of equiaxed grain on the whole cross section of ingot decreased from 2.24 mm to 0.94 mm,the segregation rate of surface inverse segregation and center positive segregation improved from11.5%and-12.6%to 2.1%and-1.4%.（3）The properties of Cu-15Ni-8Sn alloy were further optimized by applying rolling deformation between solution and aging treatment.After solution treatment at 850 ^oC for 2 h and 90%rolling deformation,the grain size decreased from 36.1μm to 1.4μm and the dislocation density increased to 2.34×10¹⁵ m^-2,leading to the hardness and yield strength increase from 140 HV and 326 MPa to 316 HV and 898 MPa.The hardness,yield strength,and electrical conductivity can be significantly improved with increasing rolling deformation,while the peak aging time shortens greatly compared to that of the un-deformed alloy.The optimal properties with a hardness of 409 HV,a yield strength of 1256 MPa,an elasticity modulus of 137 GPa,and an electrical conductivity of 7.5%IACS can be obtained in the sample with 90%rolling deformation after aging at 400°C for 0.5 h.The contributions from precipitation strengthening,dislocation strengthening,solid solution strengthening and grain boundary strengthening were 36%,30%,18%and 12%,respectively.（4）The trace V element was added into Cu-15Ni-8Sn alloy to suppress the DP and improve microstructures and properties.The grain size can be significantly reduced by adding trace V content in Cu-15Ni-8Sn alloy,which improves the segregation of Sn.When the V content increased from 0 wt.%to 1.0 wt.%,the grain size decreased from 762μm to 30μm.The supersaturated single-phaseα-Cu（Ni,Sn）solid solution of Cu-15Ni-8Sn alloy can be obtained after solution treatment at 850°C for 4 h.However,a great number of Ni₃V particles formed intoα-Cu（Ni,Sn）solid solution for V-containing alloys.Ni₃V particles could effectively inhibit the formation of DP in the aging process,and not affect the spinodal decomposition and ordered phases.The optimal combination property with a hardness of 368HV,an ultimate tensile strength of 990 MPa,an elasticity modulus of 131 GPa,an electrical conductivity of 8.0%IACS can be obtained for Cu-15Ni-8Sn-0.4V alloy after aging at 400°C for 4 h.（5）The growth kinetics of DP in Cu-15Ni-8Sn-x V alloy was evaluated by using the Johnson–Mehl–Avrami–Kolmogorov（JMAK）equation and Arrhenius equation.The activation energy（Q）of DP was determined to be about 75 k J/mol in Cu-15Ni-8Sn alloy.It is much lower than the element diffusion activation energy of Ni and Sn in the Cu matrix,thus the formation of DP in Cu-15Ni-8Sn alloy was controlled by grain boundary diffusion.While in Cu-15Ni-8Sn-0.2V alloy,the Q of DP was ascertained to be about 350 k J/mol,which is much larger than the element diffusion activation energy of Ni and Sn in the Cu matrix.The results indicated that the formation of DP in Cu-15Ni-8Sn-0.2V alloy was completed by the bulk diffusion of Ni and Sn elements in the Cu matrix.In Cu-15Ni-8Sn-0.4V alloy,the Ni₃V particles segregated at grain boundaries could reduce the grain boundary energies,decreasing the nucleation rate of DP at grain boundaries.The pinning effect of Ni₃V particles on grain boundaries is greater than that of in grain interior.