Microstructures,Thermal Stabilities And Mechanical Properties Of Nanocrystalline And Ultrafine-grained Cu-Zr(-B) Alloys

With intrinsic physical and chemical properties such as excellent electrical and thermal conductivities as well as superior corrosion resistance,copper-based alloys have been widely used in electronic,electrical and nuclear industries.In the meantime,the emerging of different application conditions requires copper alloys to become stronger and more stable at elevated temperatures.According to the empirical Hall-Petch relationship,grain refinement is an effective way in improving the strength of metallic materials without sacrificing their tensile ductility.However,when the grains in metallic materials are refined down to submicrometer or even nanometer scale,the volume fraction of grain boundary will increase significantly,leading to the substantially increased driving force for grain growth.Consequently,unique mechanical strength associated with fine grains will be gone.In this thesis,Cu-Zr and Cu-Zr-B alloys were used as model materials to prepare nanocrystalline and ultrafine-grained metallic materials using a combination of mechanical alloying and hot isostatic pressing.Subsequently,microstructures,thermal stability and mechanical properties of the prepared samples were systematically studied.Nanocrystalline supersaturated solid solution Cu-Zr and Cu-Zr-B alloys were prepared by mechanical alloying,and then annealed at temperatures in the range of 600 to 900? to allow Cu nanograins to grow and Zr-rich second-phase particles to form in-situ.By using a combination of in-situ X-ray diffraction(XRD),ex-situ XRD,transmission electron microscopy(TEM),scanning transmission electron microscope-energy dispersive spectrometer(STEM-EDS),high angle annular dark field-STEM(HAADF-STEM)and Vickers microhardness tests of as-milled and annealed powders,it was found that 50%of the Cu nanograins in the as-milled powders after annealing at 900?(0.83 Tm,Tm is the melting point of pure copper)for 1 hour were still nanocrystals(less than 100 nm),and their microhardness were almost unchanged.The grain growth results showed that at 700? or lower,Cu nanograins were mainly stabilized by the segregation of Zr solute atoms and Zr particles on Cu grain boundaries.During annealing at 700? or higher,Zr-rich(ZrC and ZrO2)nanoparticles were formed in-situ.Such Zr-rich nanoparticles exerted a strong pinning force on Cu grain boundaries,which would effectively impede the motion of boundaries and thus arrest Cu grain growth.The limited grain growth due to Zener pinning effect warranted slight change in hardness of the prepared samples during annealing.Nanocrystalline Cu-5 at.%Zr alloy powders prepared by mechanical alloying were consolidated with hot isostatic pressing to produce bulk ultrafine-grained sample.The effect of the rolling temperature on its microstructures,mechanical properties and electrical conductivity was investigated.The results showed that the Cu matrix of the alloy consisted of particle-free and particle-rich regions,and in-situ formed second-phase particles in the alloy were identified as ZrC and ZrO2.After hot rolling,obvious Cu grain growth was not observed.The yield strength of the alloy decreased slightly from 497 MPa for the as-hipped sample to 438 MPa for the 1000? hot-rolled sample,while the elongation to fracture of the alloy increased monotonously from 2.7 to 4.5%with increasing the rolling temperature to 1000?.The analysis of strengthening mechanisms demonstrated that grain boundary strengthening and dispersion strengthening made major contributions to the yield strength of the alloy.The electrical conductivity of the bulk ultrafine-grained Cu-5 at.%Zr alloy decreased first and then increased with the increase of the rolling temperature.The factors affecting the electrical conductivity of the alloy include the macroscopic sintering defects,grain boundaries,solute atoms and second-phase particles.