| Refractory tungsten has been widely used in the fields of electronics,aerospace and nuclear energy due to its high strength and hardness,remarkable thermal conductivity and excellent high temperature stability.Compared with tungsten prepared by other methods,tungsten prepared by chemical vapor deposition?CVD?exhibited high density,high-purity,and excellent thermal conductivity.However,tungsten prepared by CVD is prone to show brittle fracture at room temperature,and it has a higher ductile-brittle transition temperature?DBTT?.This limitation makes it difficult to apply for room temperature machining and assembly.In order to improve the deformation capability of high-purity tungsten prepared by CVD,HR-W,CR1.1-W and CR1.9-W with different equivalent strains were obtained by rolling deformation.Through the test of hardness and three-point bending,and the characteriziation of scanning electron microscope?SEM?,X-ray diffraction?XRD?and electron backscatter diffraction?EBSD?,the evolution of mechanical properties and microstructures of high-purity tungsten during deformation was investigated,and the main factors influencing the plasticity of high-purity tungsten were analyzed.Based on the optical microscope and hardness test,the effects of equivalent strain and annealing parameter on the recrystallization behavior of high-purity tungsten were investigated through isochronous and isothermal annealing of rolled tungsten.Finally,in order to explore the effect of the preparation process on the recrystallization behavior of rolled tungsten,the differences in metallurgical structure and hardness during isothermal annealing of cold-rolled tungsten?CVD?and cold-rolled tungsten?PM?with the same equivalent strain were compared.The main conclusions have been reached in the study as follows:?1?As the amount of rolling deformation increases,the hardness,fracture strength,and plasticity room-temperature gradually increase,and DBTT can be reduced below the temperature of 100?.Compared with the CVD state,the deformation strain and stress of the deformed high-purity tungsten increased to 0.99%and 2349 MPa,which are 5.5 times and 4.0 times than that of pre-deformation,respectively.As the amount of deformation increases,the grains gradually refine,and at the same time the number of layers of heterostructures formed by the gradient strain inside the material increases.The combined effect of the above factors contributes to the improvement of the plasticity and DBTT for high-purity tungsten.?2?When the annealing time is constant,the amount of deformation increases and the recrystallization temperature becomes lower;the initial recrystallization temperatures of HR-W,CR1.1-W and CR1.9-W are 1400?,1200?and 1100?respectively.After fitting,the recovery kinetic coefficients R/V of the three specimens are 0.95×10-3,1.16×10-3,and 1.44×10-3 Kg·f·K-1·mm-2,respectively.The rerecovery process becomes more easy to get activated.During the isothermal annealing process,with the increase of the annealing time,the hardness decreases monotonously and the recrystallization volume fraction increases monotonically.?3?Compared with cold-rolled tungsten prepared by PM at the same equivalent strain,cold-rolled tungsten prepared by CVD shows the microstructure characteristic of finer grains and exhibits a higher hardness.Although the initial recrystallization temperature is the same,the complete recrystallization temperature of cold-rolled tungsten prepared by CVD is as high as1500?,which is higher than that of cold-rolled tungsten prepared by PM.The grain size after recrystallization is nearly smaller than that of PM cold-rolled tungsten,so cold-rolled tungsten prepared by CVD displays better thermal stability. |
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