Preparation And Fundamental Research Of The Cu-15Ni-8Sn Alloy With High Strength And Ductility

Cu-15Ni-8Sn alloy possesses high mechanical properties,excellent stress relaxation resistance,outstanding wear and corrosion resistance,being considered as an ideal substitute for Cu-Be alloys.This alloy shows great potential in commercial applications in electronic industries and is emerging as a material of choice for large mechanical equipment,such as aircraft bearings,heavy haul vehicles and marine engineering equipment,which has garnered particular attention as a high-performance Cu-based alloy.However,it is difficult to reach the level of Cu-Be alloys in terms of strength and toughness.With the development of large equipment in China,the properties of this alloy are difficult to satisfy the urgent demands of the production of key components in large equipment.Therefore,improving the strength-ductility synergy of the alloy has an important promoting effect on meeting the national major demand and expanding its application.Aforementioned,this research focuses on the preparation of the Cu-15Ni-8Sn alloy with a combination of high strength and toughness,which was carried out via composition design,optimization of forming process and heat treatment.The influence of addition of Si and Ti on the microstructure and mechanical properties of the alloy was investigated.Microstructural evolution in the modified alloy was investigated deeply by using optical microscopy(OM),scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD),small angle neutron scattering(SANS)and three-dimensional atom probe(3DAP).Additionally,the preparation of Cu-15Ni-8Sn alloy using selective laser melting(SLM)technology was studied.The main conclusions of this research were drawn as follows:(1)With addition of Ti or Si,needle-like Ni₃Ti or granular Ni₃Si phases are formed in the alloy,while granular Ni₁₆Si₇Ti₆particles are precipitated with the additions of both Si and Ti.Due to the existence of these insoluble second phases,the dendritic structure in the as-cast alloy was refined and the formation of uniform recrystallized grains was promoted in the subsequent hot extrusion process.When the content of trace elements exceeds 0.3%(mass fraction),the second phases are coarsened,resulting in the increase of segmenting the matrix and thereby leading to a decrease in ductility of the alloy.With the additions of 0.3%Si and0.1%Ti,the alloy exhibits the optimal mechanical properties with a tensile strength of 909MPa,a yield strength of 708 MPa and a high elongation of 29.6%.(2)The constitution relationship with strain compensation and hot processing map of Cu-15Ni-8Sn-0.3Si-0.1Ti alloy were established based on hot compression tests.The optimal thermal processing parameters window of the alloy with a strain of 0.9 was determined as follows:the deformation temperature of 825?925°C,the strain rate of 1×10^-3?0.6 s^-1.Ni₁₆Si₇Ti₆particles with a size of?100 nm are formed owing to the additions of Si and Ti,promoting the nucleation process of dynamic recrystallization during hot deformation.Micron Ni₁₆Si₇Ti₆and?particles induce particle stimulated nucleation(PSN)effect that facilitates dynamic recrystallization process.Dispersed Ni₁₆Si₇Ti₆particles with a average diameter of16 nm are precipitated in the alloy during air-cooling process,effectively inhibiting the growth of recrystallized grains in the process of air-cooling.(3)Part of Si addition is dissolved into?phase in the Cu-15Ni-8Sn-0.3Si-0.1Ti alloy,improving the thermal stability of?phase.The existence of?phase significantly hinders the grain growth during the solid solution and subsequent aging treatment.According to the observations through SANS and 3DAP,the additions of Si and Ti suppress the occurrence of spinodal decomposition and ordering transformation,which retards the aging strengthening effect and modifies the ductility of the aged alloy.As a result,the alloy obtains a high strength-ductility synergy after aging for 4 hours at 400°C,whose tensile strength,yield strength and elongation are 1117 MPa,967 MPa and 16.4%,respectively.At the late stage of aging treatment,the lamellar(?'+?)discontinuous precipitate is formed in the Cu-15Ni-8Sn alloy,where?phase shows an orthogonal structure with lattice parameters of a=0.449 nm,b=0.529 nm and c=0.430 nm.The additions of Si and Ti result in the formation of Sn free zones near grain boundaries,which reduces the driving force of the discontinuous precipitation.At the same time,Ni₁₆Si₇Ti₆and?particles formed at grain boundary occupy the nucleation positions of discontinuous precipitation and inhibit the migration of reaction front interfaces of discontinuous precipitation.Discontinuous precipitation reaction is completely suppressed in the alloy with additions of 0.3%Si and 0.1%Ti,resulting in no discontinuous precipitation areas in the alloy after aging for 200 hours.(4)The inverse segregation,which is inevitably formed in the traditional casting alloy,can be eliminated in the dense Cu-15Ni-8Sn alloy manufacturing by SLM technique.SLM manufactured alloy is mainly composed of fine equiaxed grains and columnar grains.Fine dispersed?phase is precipitated in the center of molten pool while high dislocation density is formed in the hatch overlaps.Compared with the traditional casting alloy,the mechanical properties of SLM produced alloy are obviously improved.According to the analysis of strengthening mechanism,the high strength of SLM fabricated alloy is mainly caused by the grain refinement and high dislocation density.