Regulation Of Microstructure And Properties Of Cu-8.33ni-1.67si Alloy By Severe Plastic Deformation And Heat Treatments

With the rapid development of modern industry and information industry,the demand for high-strength copper alloy has increased significantly,and higher requirements have been put forward for the comprehensive properties of materials.Aging strengthened Cu-Ni-Si alloy is considered to be one of the most potential high-strength conductive materials,which needs further development.The strength of Cu-Ni-Si alloys can be further improved by increasing the content of Ni and Si,since the increase on the amount of precipitated phases.However,excess Ni and Si addition always results in the formation of network structure of intermetallics at the grain boundary of Cu-Ni-Si alloy,and thus decrease the electrical conductivity dramatically.According to the previous work of this thesis's research group,Cu-8.33Ni-1.67Si alloy was the best chemical composition from th e viewpoint of strength and electrical conductivity,but integrated network structure formed at the grain boundary.To get a excellent combination of strength and electrical conductivity,multi-direction hot forging,multi-direction cold pressing and heat treatments are employed to regulate the microstructure of Cu-Ni-Si alloy,especially the network intermetallics at the grain boundary.The main conclusions of the present thesis are as follows:(1)Phase composition of the Ni-Si intermetallics,which is controversial until now,in as-cast Cu-Ni-Si alloy is identified by means of extraction technology combined with SEM,TEM and XRD.The network intermetallic phase at the grain boundary is Ni31Si12,while the precipitate phase in the alloy is Ni2Si.(2)Based on Arrhhenius constitutive model,the constitutive equation of Cu-Ni-Si alloy considering strain effect is established,and then hot working diagrams were established by using the dynamic material model and Prasad instability criterion.The suitable hot deformation parameters are determined as defomation temperature 810?890 ? and strain rate 0.01?0.1s-1.(3)By processing the real stress-strain curve data of Cu-Ni-Si alloy,the critical formation model of dynamic recrystallization is obtained,where the minimum value criterion of-(?)?/(?)?-?curve and the Zener-Hollomon parameter temperature compensation factor of strain rate is used.Meanwhile,a dynamic recrystallization grain growth model is established.(4)Multi-directional hot forging is helpful for the broken of the network grain boundary Ni31Si12 phase in as-cast alloy,resulting in spherical particles with an average diameter of 0.62?m,along with a very small amount of short rod or ellipsoid particles.At the same time,hot forging also leads to the rapid decomposition and solid solution of Ni2Si phase.The broken of network Ni31Si12 is beneficial while the decomposition and solution of Ni2Si is harmful for the electrical conductivity,but both of them is good for the improvement of strength.As a reult,the electrical conductivity increases and then decreases with increase of forging passes,while the hardness of the alloys increases continuously.(5)A large number of intersecting deformation band zones are formed after multi-directional cold pressing.When the cumulative strain reaches 6.4,ultrafine grains with average grain size of 0.35um as well as a large number of deformation defects inside them are obtained.The hardness and electrical conductivity are 261.3HV and 8.72%IACS.Compared with the solid solution alloy,the hardness is increased by 39.87%,and the conductivity is decreased by 37.36%.(6)During the aging process of the alloy with accumulated strain of 9.6 multi-directional hot forging+950?/4H solution treatment+accumulated strain of 6.4 multi-directional pressing,the initial Ni-Si compounds dispersion and precipitation lead to the rapid increase of hardness and electrical conductivity,and then the coarsening of grains and precipitates of copper matrix and the decrease of deformation structure leads to the significant decrease of hardness,while the conductivity increases slowly and then decreases,and the hardness reaches a peak value of 302HV at 450?/0.5h,and the conductivity reaches a peak of 49.6%IACS at 500?/8h.The comprehensive properties of Cu-Ni-Si alloy are the best at 400?/8h:hardness 269.9HV,tensile strength 899.8MPa,percentage elongation 13.1%,electrical conductivity 39.8%IACS.(7)The alloy kinetics transformation equation and electrical conductivity equation are established.The high-density deformation defects formed by multi-directional pressing accelerate the diffusion of Ni and Si atoms and the precipitation process of Ni-Si compounds,and the apparent activation energy for aging of Cu-Ni-Si alloys is greatly reduced.