| CuCrZr alloy has attracted extensive attention in electrical and electronic fields because of its high strength and high conductivity.The high strength of CuCrZr alloy comes from various strengthening measures for its matrix structure,such as grain refinement strengthening,deformation strengthening and precipitation strengthening.As a typical strengthening method of this kind of alloy,the effect of precipitation strengthening is restricted by the initial solidification structure and subsequent heat treatment process.In this paper,Cu-0.67Cr-0.3Zr alloys with different solidification rates were prepared by liquid metal cooling(LMC)directional solidification technology,and the solid solution and aging treatment were carried out.The microstructure of samples in different states was characterized by metallographic microscope,XRD diffractometer,scanning electron microscope(SEM)and transmission electron microscope(TEM).The changes of strength,wear resistance and electrical conductivity of the alloy with solidification rate and aging process were studied.The following conclusions can be drawn.The microstructure characterization of samples under different solidification rates shows that: In the slow solidification rate,below 5 μm/s,the alloy grows with a flat solid-liquid interface;In the solidification rate range of 5-500 μm/s,the alloy growth model is cellular growth;Under the condition of rapid solidification,more than 1000 μm/s,the alloy exhibits dendrite growth.This result shows that in practical production,the alloy often solidifies in the form of cellular growth.In the range of cellular growth rate,eutectic structure exists at the cell crystal interface.The cell size and eutectic structure spacing decrease with the increase of solidification rate.In addition,there are coherent FCC-Cr nanoparticles with different densities in the cells of each solidification rate sample.The mechanical properties test results show that with the increase of the solidification rate,the refinement of the eutectic structure and the increase of the density of FCC-Cr nanoparticles improve the strength and hardness.However,the electrical conductivity is decreased due to the increase of the solute content in matrix,which is resulted from the increase of the solidification rate.After the solution treatment at 950 ℃ for 2 h,the test results of samples with different aging temperatures show that: The properties of the alloy cannot be effectively improved after aging at 350 ℃ for 6 h;After aging treatment at 450 ℃,the coherent FCC Cr precipitates in matrix transformed into BCC Cr precipitates,which keep K-S orientation relationship with the matrix.This transformation improved the strength of the alloy(202.36 MPa-299.72 MPa)and purified the matrix.As a result,the electrical conductivity is also improved(49.88 %IACS-84.1 %IACS);The over aging occurs when the aging treatment exceeds 450 ℃.The BCC Cr precipitates gradually grow up and the density decreases under the action of the Gibbs-Thomson effect,resulting in a decrease in the strengthening effect.Analysis of strengthening mechanism of FCC and BCC Cr precipitates show that:Those FCC precipitates smaller than 35 nm in size follow dislocation cutting mechanism,and the BCC precipitates with a size larger than 1.4 nm follow dislocation bypass(Orowan)mechanism.The test results of the samples aging at 450 ℃ show that: The strength and electrical conductivity are significant improved after 2 hours of aging;With the extension of aging treatment time,the change of strength and electrical conductivity are not obvious.The test results of friction experiments show that: The main wear mechanisms are adhesive wear,abrasive wear and oxidative wear.The variation of wear rate is opposite to the change trend of the strength.The higher the strength is,the lower the wear rate is. |
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