A Study On Fabrication Of NbC Nanoparticles Reinforced Nanocrystalline Cu Matrix Nanocomposites And Their Microstructure And Properties

Cu and Cu alloys are widely used in industry applications,and the development of industry ask for higher strength of these Cu materials.However,the prevelant strengthening mechanisms such as solid solution and aging,second phase strengthening failed to meet the increasing requirement.Such that futher improving the strength of Cu materials and maintaining its excellent ductility and electrical conductivity become an important research topic.Based on the detailed discussions on the effects of solid solution strengthening,dislocation strengthening,grain boundary strengthening and nanoparticle strengthening on yield strength,ductility and electrical conductivity,a novel approach combined grain boundary strengthening and NbC nanoparticles strengthening is believed to further improve the comprehensive properties of Cu materials.Here,we successfully fabricated novel Cu-NbC nanocrystalline nanocomposites consisting of a grain size about 100 nm and NbC nanoparticles size about 10 nm by high energy mechanical milling,followed by thermal mechanical processing,showing a high strength,high thermal stability,excellent electrical conductivity and ductility.Furthermore,we investigated the evolution of microstructure of the nanocrystalline nanocomposite during processing and its effects on mechanical and electrical properties.During milling,the grain size of Cu-5wt.%Nb-1wt.%SA decrease as the increase of milling time,and dislocation density increase as the increase of milling time.After milling for 36 h,grain size and dislocation density show insignificant change.However,the Fe contamination increase monotonously as the increase of milling time.Thus,a milling time of 36 h was chosen to process powder materials.No formation of NbC was observed during milling and the in-situ formation of NbC nanoparticles happened during heat treatment.The particle size of NbC nanoparticles is about 8 nm after annealing at 700℃ for 1 h.The thermally stable NbC nanoparticles strongly inhibit the grain growth of nanocrystalline Cu matrix,resulting in barely grain growth after annealing at 800℃ for 1 h.Thus,such highly stabled Cu-NbC nanocrystalline nanocomposites yield a high microhardness.Moreover,its hardness increases as the increase of annealing temperature below 700℃,from 2128 MPa of the as-milled powder to 2665 MPa of the powder annealed at 700℃.Bulk Cu-NbC nanocrystalline nanocomposites show a good combination of strength,ductility and electrical conductivity.For example,a composition of Cu-6.4vol.%NbC shows a tensile strength of 868 MPa,elongation to failure of 6% and electrical conductivity of 56 %IACS.A typical size of NbC nanoparticles is about 10 nm,and showing insignificant coarsening during thermal mechanical processing,resulting in a grain size of about 100 nm with insignificant change of grain size and dislocation density before and after hot extrusion.So as to the highly stabilization effects on the Cu matrix of NbC nanoparticles,the change of volume fraction of NbC shows insignificant effects on the grain size,dislocation density and particle size of NbC.Furthermore,the increase of volume fraction of NbC results in dramatic increase of strength when the volume fraction of NbC is low.However,when the volume fraction of NbC is high,no increment was realized with the increase of NbC volume fraction.During deformation,grain rotation and grain boundary immigration result in grain growth and grain shape change,twin boundary immigration and dislocation activities also happened under strain.Besides,the change of NbC particle size and its volume fraction results in significant influence on electrical conductivity.High structural stability of nanocrystalline Cu matrix and NbC nanoparticles was observed under long term high temperature annealing at 1000℃.Such high structural stability is realized by the thermal stability of NbC nanoparticles.The solid solubility of C and Nb in Cu is so low that growth rate of NbC nanoparticle through Ostwald ripening mechanism is low.Besides,the shape of NbC nanoparticles changes from spherical to cubic during its growth and forms a cubic-cubic relationship to the matrix,such relationship dramatically lowers the growth rate of NbC.As a result,the Cu grain boundaries are pinned by these stable NbC nanoparticles showing insignificant grain growth.Meanwhile,the recrystalization of Cu matrix contribute to the stablization of the grain size.Though the slightly growth of Cu grain and NbC nanoparticles results in a decrease of strength and increase of electrical conductivity,no significant increase of ductility was observed.During hot rolling,the deformation and growth of grains could change the shape of grains and the distribution of NbC nanoparticles.For example,when rolled at 800℃,the deformation of grains predominates,thus the grains are elongated and NbC nanoparticles distribute along the grain boundary;when rolled at 1000℃,the growth of grains predominates,thus the grains are equiaxial,and NbC nanoparticle distribute homogeneously.And the increase of rolling temperature results in particle coarsening,for example,the particle size of NbC is 8.7 nm and 13.9 nm that of rolled at 800℃ and 1000℃,respectively.Meanwhile,after hot rolled at 800℃,the grains along the direction perpendicular to the rolling direction show significant growth;however,after hot rolled at 1000℃,both grains along the direction parallel to extrusion direction and perpendicular to the rolling direction show significant growth,indicating the strain distribution and high temperature could lie dramatic effects on the growth of grain.After hot rolling,both yield strength and electrical conductivity of the nanocomposite decrease.