Research On Microstructures And Properties And Aging Kinetics Of New High Strength And High Conductivity Cu-Ag-Cr Alloy

The higher properties for contact wire materials are required to satisfy the rapid development of high-speed electrical railway .The ideal contact wire materials should reach tensile strength of 550MPa, the electrical conductivity 80%IACS, and the decline rate of tensile strength less than 10% at 300℃for 2h. Nowadays, many countries, such as Japan, Germany and France are developing higher properties copper alloy materials for contact wire.It was a successful design of new high strength and high conductivity Cu-Ag-Cr series alloys based on the alloying principle. It can satisfy the requires of the higher properties of high-speed railway contact wire materials. Contrasting with Cu-0.1Ag alloy widely used for contact wire, the strengthening increased greatly and the recrystallization was restrained effectively with the trace addition of Cr; the over-aging was restrained obviously with the trace addition of Ag, which made the alloy having good high-temperature performance. The micro-hardness and electrical conductivity can reach to 162HV and 82%IACS, the tensile strength of 594MPa, the decline rate was only 5% at 300℃for 2h of Cu-Ag-Cr alloy after proper heat processing. The electrical sliding wear performance of Cu-Ag-Cr alloy was 2~3 times than that of Cu-0.1Ag alloy. The results showd that Cu-Ag-Cr alloy having high strength, high conductivity and good wear performance, which is the better substitute materials for contact wire being good general performance.The transformation kinetics Cu-Ag-Cr was studied by dynamic resistance technique. The variation of the relative resistance with time was measured with self-made dynamic resistance testing instrument, when Cu-Ag-Cr alloy was treated in continuous heating, cooling and isothermal at various heating/cooling rates. Model of relative resistance for the various reactions was first presented, transformation volume fraction was calculated and continuous cooling transformation curve of Cu-Ag-Cr is plotted in this work.By means of transmission electron microscope (TEM), high-resolution transmission electron microscope(HREM) the aging precipitates in the Cu-Ag-Cr alloy was studied. It was first to apply VEC(Visual Electron Crystallgraphy)technique to analysis the coherent, semi-coherent and the coherency loss between precipitates and Cu matrix. The HREM observations suggested that the coherent and semi-coherent particles coexist when aged at 440℃～520℃. The coexisting radium of Cr coherent / semi-coherent precipitates was about 15～45nm, the calculated value of critical transition radius for the coherency loss was given as 12.5nm.The coherent strengthening mechanism operated for Cu-Ag-Cr alloy at the primary aging stage. The degree of coherent strengthening of Cu-Ag-Cr alloy aged at 480°C for 2h was consistent with that calculated by the dislocation theory, and the Orowan mechanism operated at the over-aging stage.It was the first time to analysis the growth and coarsening and the regularities of distribution of aging precipitates in Cu-Ag-Cr alloy by means of TEM, image analysis and statistic analysis. The results showd that the growth behavior of spherical Cr precipitates was in good agreement with LSW theoretical model when Cu-Ag-Cr alloy aged at 480℃.The mean radius of Cr precipitates with t1/3 is linear relation, which suggesedt that the growth and coarsening of Cr precipitates were dominated by diffusion. The size distribution of Cr precipitates was proximity normal distribution curve, which was also in good agreement with LSW theoretical.The interaction of aging precipitates and recrystallization on microstructures and properties of deformed Cu-Ag-Cr alloy were intensively explored. A phenomenon of simultaneous in-situ and discontinuous recrystallization has been observed during aging. The in-situ recrystallization cause the alloy to maintain relatively high strength, while the discontinue recrystallization cause the precipitates coarsening and the strength rapidly decreased. The pre-aging process for the Cu-Ag-Cr alloy were responsible for the significant strengthening effect in re-aging process, the re-aging strengthening with pre-aging at 440℃for 8h was the most remarkable. Upon re-aging at 350℃～400℃, the detected strengthening was 30HV～50HV in terms of micro-hardness increment.The wear behavior of Cu-Ag-Cr alloy and Cu-Ag alloy were contrasted using self-made wear apparatus. The electrical sliding wear performance of the high strength and high conductivity Cu-Ag-Cr alloy increased 2~3 times than that of Cu-Ag alloy due to the aging strengthening action of Cr, which can increase of service life by far. Adhesive wear and abrasive wear were the dominant mechanisms under unlubricated conditions. During the electrical sliding processes, the adhesive wear, abrasive wear and electrical erosion were the dominant mechanisms.The ideas of on-line solution technique and device for precipitation strengthen copper alloys was put forward. It was first to propose the way of the combination the copper alloys on-line solution and aging with the continuous casting and rolling process, thus it is possible to manufacture the solution high strength and high conductivity copper alloys rod and wires.