Study On Microstructures Of Precipitations And Properties After Aging For High Strength And Conductivity Cu-Cr-Zr Alloy

Cu-Cr-Zr alloys are used in lots of fields for their combination of high strength and high electrical conductivity,as well as high thermal conductivity.The key issue for the study of this alloy system focuses on using various alloying techniques, heat-treatment and cold deformation processes to increase its strength, and meanwhile,maintain its high conductivity.Cu-Cr-Zr alloys are typical materials whose strength can be greatly increased via a thermal process, and therefore called heat-treatable alloys.From the microstructure point of view, the strength increase of the alloy upon heat-treatment is due to the formation of small particles, called precipitates in the Cu matrix.The precipitation behavior of Cu-Cr-Zr alloys during the thermal processes has thus far not yet been well understood.Hence,detailed investigations on the precipitation microstructure of this alloy are needed for understanding the relations between its properties and microstrutures.In this thesis,the decomposition processes in Cu-0.38Cr-0.23Zr(wt.%)alloys, which are closely associated with the mechanical properties,were investigated by means of SEM(Scanning Electron Microscopy) and TEM(Transmission Electron Microscopy), as well as EDS (Energy Disperse Spectrometer).The hardness and electrical conductivity during ageing process were measured by HXD1000T hardmeter and D60k conductometer. On the basis of analysis above,the precipitate evolutions were given out,which could be used to guide the heat treatment.It has been found that the solute elements are distributed homogeneously in the matrix under the condition of solution treatment at 950℃for 1.5h.After solution treatment,the hardness of these alloys increases rapidly with the increasing of ageing duration.After a peak value has reached,the hardness begins to decrease.On the other hand,the electrical conductivity increases rapidly at the earlier stage of ageing, after a critical value reached,it keeps a stable increasing speed.The results of SEM indicate that there are three kinds of lager phases, one is coarsen phase continuously located at grain boundaries,the second is striping phase embed in the matrix,and the last is large particle.EDS analysis reveals that the components of the first two phases are similar, viz.CrCu2(Zr, Mg),and the last is Cr particle.The results of HRTEM images show that there are large amount of coherent Cr-rich precipitates with lobe-lobe contrast,which have bright lines in the middle keeping the same orientation.These coherent particles play a significant role in strengthening the alloys at the earlier stage of ageing.Under the condition of peak ageing,HRTEM and Reconstruction of Exit Wave results show that there are a large number of round-shape and rod-shape particles,except for coherent Cr-rich particles. In fact,the former two morphologies are the projection of the same kind of disc precipitates in different direction identified by electron microscopy technique.The rod shape is the side projection of the disc.These precipitates were measured as CuCrO2,which exhibit an crystallography orientation relationship with the matrix as [011]Cu//[010]CuCrO2,(111)Cu//(003)CuCrO2,with{111}Cu as it's habit plane.Furthermore,cold deformation,introduced into the heat treatment process before the artificial ageing,leads to formation of huge amount of dislocations.It accelerates the process of nucleation and growth of precipitates,which is helpful to refine the precipitates.The experiment results show that this technique contributes to get higher strength and better conductivity.