Preparation Of Cu Cr Nb Alloys By Mechanical Alloying And Spark Plasma Sintering

The strength and conductivity of metals usually restrict each other.In the research of high strength and high conductivity materials,precipitation strengthened copper alloys have attracted much attention due to their high strengthening effect and limited conductive loss.In this research,the preparation of Cu Cr Nb alloys was studied.The elements Cr and Nb were used because they have a lower solid solubility in copper.Alloy materials with nano-grains and precipitated phases were prepared by ball milling and spark plasma sintering.The balance of strength,conductivity and elongation was obtained.The preparation methods of broader high-strength and high-conductivity copper alloy had been widened.In order to improve the ball-milling method,the effects of ball-milling speed and ball-milling duration on the performance of the alloy were investigated.The results show that the lower ball-milling speed can only make the micron-size phase distribute widely among the copper particles.This affects the interface bonding between particles and has little effect on strengthening.Therefore,pre-high speed ball milling with the added phase can not only refine the particles,but also achieve the solid solution of the elements.During high-speed ball milling,copper powder was grown up by impact of cold welding and enclosed with the added phase,which acted as ball-milling medium;the added phase was further refined and dispersed.In addition,high-speed ball milling could make the matrix form supersaturated solid solution,accumulate a large amount of deformation and lattice distortion,and realize precipitation strengthening and fine crystal strengthening in subsequent sintering.Extending the milling time can give full play to the strengthening effect of the added elements,but it would affect the binding of the particles during sintering.To find a suitable sintering process,the effects of sintering pressure,sintering time,sintering temperature and heating rate(mould)on the performance of the alloy were investigated.The bonding process of large copper particles was restored by comparing the microstructural differences of samples with different sintering parameters.At the beginning of sintering,the sample had a high resistance.Spark discharge mainly occured in the interface contact area between two copper particles and produces a large amount of Joule heat to form the sintering neck.Therefore,the facets are relatively tightly bound.However,due to the limited liquidity and deformation capacity,the multiparticle linear junction area had poor contact.Therefore,it produced less heat and mainly depends on the hot-pressing effect in the subsequent stage of sintering.The linear contact area is liable to become the weak link of bonding due to the insufficient sintering pressure and time.It was found that the poor bonding between macro-particles has little effect on the conductivity,but it will affect the strength and elongation of the material.Increasing sintering pressure and prolonging sintering time can improve the bonding,while eliminating holes and defects in the material and further improving the material conductivity.Metallographic observation,fracture analysis,and EBSD analysis were carried out to explore the microstructural evolution during sintering.During sintering,recrystallization of copper occurred due to the accumulation of a large amount of lattice distortion.The grain size of the matrix was refined to nanometer scale.Enhanced elements with lower solid solubility would precipitate under the action of current and temperature activation preferring regions with large lattice distortion.It results in the widespread distribution of nano-precipitates at the nano-crystal boundary,which prevents the grain growth.There are three main types of precipitates distributed on the grain boundary: Cr,Nb and Cr2 Nb.The sintering process test of the sample shew that the precipitated phase coarsens due to the high sintering temperature and the enhancement effect was reduced.Shorter sintering holding time(5 minutes)allows the solid soluble elements to precipitate sufficiently.If the sintering time was further extended,the dynamic process of dissolution,diffusion and precipitation of the precipitated phase would occur and the performance fluctuations can occur.In addition,there are some relatively large Cr,Nb phases and Cr2 Nb phases produced by solid-state phase transformation reaction in the materials.They contribute partially to the strength but have little effect on the conductivity of the material.With the purpose of comparing the effect of different elements and find a reasonable proportion of elements,the alloy samples with different compositions were prepared.It is found that the conductivity loss of pure copper sintered sample is about10% IACS.This is mainly due to the widely distributed nanoscale grain boundaries and lattice mismatch left by incomplete recrystallization.The fine dispersed precipitates will further produce lattice distortion and cause electron scattering.This further reduces the conductivity.Among them,cu-1cr-0.5nb,cu-1.5cr-0.75 nb and cu-2cr-1nb show more comprehensive and balanced performance compared with the existing literature.It shows that the process innovation of this study has a good application prospect.