Control Mechanism Of Trace Co And P On Microstructure And Properties Of Cu-15Ni-8Sn Alloy

Cu-15Ni-8Sn alloy is widely used in the manufacture of wear-resistant bearings,bushings,bearings,high-power electronic components,and precision connectors and related fields due to its high strength,high elasticity,high wear resistance and good stress relaxation resistance.It is one of the most potential substitute materials for Cu-Be alloy.However,the properties of Cu-15Ni-8Sn alloy will be seriously deteriorated by the formation of discontinuous precipitation at the later stage of aging.As one of the most effective methods to improve alloy segregation and inhibit discontinuous precipitation,microalloying had attracted much attention in recent years.Based on this,the effect of Co/P addition on the microstructure and properties of Cu-15Ni-8Sn alloy was studied.Combined with deformation and heat treatment,the comprehensive properties of Cu-15Ni-8Sn-x（Co/P）alloy were further improved,and the strengthening mechanism of the alloys was revealed.In addition,combined with the application of the alloy in different fields,the friction and wear resistance and stress relaxation resistance of the alloy were analyzed.The purpose of the study was to explore the relationship between"alloy composition,microstructure,preparation and processing technology,mechanical and functional properties".The main results are as follows:The results showed that the addition of Co in Cu-15Ni-8Sn alloy has the potential to improve the composition segregation.Further,it exhibited negligible effect on the phase transformation sequence of Cu-15Ni-8Sn alloy,and the Co element is mainly enriched in the ordered phase andγ-DO₃phase during the aging process,which was influenced the coarsening of modulated structure and the formation of discontinuous precipitation.The coarsening activation energies of Cu-15Ni-8Sn and Cu-15Ni-8Sn-0.5Co alloys were 148 k J/mol and 177k J/mol,respectively.The increase of coarsening activation energies in Cu-15Ni-8Sn-0.5Co alloy was related to the decrease of diffusion coefficient D,interfacial energyσ,equilibrium solid solubility X_e and transformation free energy.The peak hardness and over aging resistance of Cu-15Ni-8Sn alloy was improved by adding 0.5 wt%-0.8 wt.%Co.Meanwhile,the nucleation and growth of discontinuous precipitation were significantly inhibited.Based on the JMAK equation,the activation energy of discontinuous precipitation growth for Cu-15Ni-8Sn and Cu-15Ni-8Sn-0.5Co alloys were 105.45 k J/mol and 178.44 k J/mol,respectively.The inhibition effect of Co addition on discontinuous precipitation is related to the decrease of matrix supersaturation,the flattening of grain boundary morphology and the increase of interlayer spacing.In addition,the relationship between the interlayer spacing of discontinuous precipitation and the hardness of Cu-15Ni-8Sn and Cu-15Ni-8Sn-0.5Co alloys conformed to the Hall-Petch type relationship.The interlayer spacing of Cu-15Ni-8Sn-0.5Co alloy was larger than that of Cu-15Ni-8Sn,which was related to the increase of interface energy betweenα-Cu matrix andγ-DO₃ phase.The addition of appropriate amount of P to Cu-15Ni-8Sn alloy can refine the grain size and improve the composition segregation.During the solution treatment,the addition of P promoted the occurrence of spinodal decomposition and ordering of Cu-15Ni-8Sn alloy,which is related to the increase of atomic size factor.Further,hexagonal Ni₁₀Sn P₃ phases with different shapes was observed after solution treated and aging of Cu-15Ni-8Sn-0.2P alloy.Ni₁₀Sn P₃phase was incoherent with copper matrix,and its orientation relationship with matrix was（02?0）_Cu∥（01?12）_{Ni10Sn P3},[001]_Cu∥[011?1]_Ni10_{Sn P}3.Furthermore,the addition of P in Cu-15Ni-8Sn alloy can also delay the coarsening of spinodal modulated,and the coarsening activation energies of the Cu-15Ni-8Sn-0.2P alloy was 198.61 k J/mol,which was higher than that of Cu-15Ni-8Sn and Cu-15Ni-8Sn-0.5Co alloys.The reason for this phenomenon was related to the decrease of equilibrium solid solubility X_e and transformation free energy.In addition,the nucleation and growth of discontinuous precipitation was effectively inhibited by addition of0.1 wt.%-0.2 wt.%P in Cu-15Ni-8Sn alloy.Based on the JMAK equation,the activation energy Q values of discontinuous precipitation growth for Cu-15Ni-8Sn-0.2P alloy was 221.90k J/mol,which was higher than that of Cu-15Ni-8Sn alloy.The inhibition effect of P addition on discontinuous precipitation was related to the distribution of Ni₁₀Sn P₃ phase,the formation of free-zone precipitation,the decrease of grain boundary diffusion coefficient and the the increase of discontinuous precipitation layer spacing.Compared with the Cu-15Ni-8Sn alloy,the increase of discontinuous precipitation layer spacing in Cu-15Ni-8Sn-0.2P alloy is related to the increase of free energy caused by the formation of Ni₁₀Sn P₃ phase.The coarsening of the spinodal modulation structure and the growth rate of discontinuous precipitation were improved with the cold deformation.While,the transformation of DO₂₂ to L1₂ ordered was inhibited.Furether,due to the occurrence of discontinuous precipitation and recrystallization reactions during the aging,the properties of deformed Cu-15Ni-8Sn-x（Co/P）alloy was decreased,and the deterioration of properties caused by discontinuous precipitation was greater than recrystallization.In the later aging stage of deformed Cu-15Ni-8Sn and Cu-15Ni-8Sn-0.5Co alloys,discontinuous precipitation reaction plays a dominant role,which can inhibit recrystallization.However,for Cu-15Ni-8Sn-0.2P alloy with smaller original grain size,the recrystallization reaction plays a dominant role at appropriate aging temperature and large deformation,and the recrystallization reaction significantly inhibits the discontinuous precipitation reaction.The peak value of tensile strength（yield strength）and elastic modulus of Cu-15Ni-8Sn,Cu-15Ni-8Sn-0.5Co and Cu-15Ni-8Sn-0.2P alloys aged at 400℃were 1245MPa（1165 MPa）and 144.9 GPa;1250 MPa（1197 MPa）and 149.8 GPa;1303 MPa（1280 MPa）and 145.0 GPa,respectively.Precipitation strengthening,dislocation strengthening and solution strengthening are considered to be the main strengthening mechanisms to improve the strength of Cu-15Ni-8Sn-x（Co/P）alloys.In addition,the increase in yield strength of Cu-15Ni-8Sn-0.5Co and Cu-15Ni-8Sn-0.2P alloys was attributed to the solid solution strengthening and precipitation strengthening.According to the application environment of Cu-15Ni-8Sn alloy,the friction and wear resistance and thermal stress relaxation resistance of Cu-15Ni-8Sn-x（Co/P）alloy were studied.The results show that,the Cu-15Ni-8Sn-0.2P alloy has better friction and wear resistance than Cu-15Ni-8Sn and Cu-15Ni-8Sn-0.5Co alloys.The Ni₁₀Sn P₃ phase with high hardness was formed in Cu-15Ni-8Sn-0.2P alloy.Its distribution along grain boundary and matrix makes Cu-15Ni-8Sn-0.2P alloy have better friction and wear resistance.Meanwhile,the Cu-15Ni-8Sn-0.5Co alloy has better stress relaxation resistance.The binding energy of Co with vacancy is greater,which can accelerate the depletion of vacancies in the matrix,thus greatly reduce the number of vacancies,and eventually inhibit the slip of movable dislocations.Besides,Co was dissolved in theα-Cu matrix,which can pin dislocations and slow down recrystallization.The joint action makes Cu-15Ni-8Sn-0.5Co alloy has better stress relaxation resistance.