Study On The Microstructure And Properties Of Tungsten Based On Multiple Direction Compression

Tungsten is widely used in aerospace and nuclear industry, due to its wonderful mechanical properties, chemical stability and strong heat resistance at high temperature. Traditional materials fabricated by powder metallurgy always have some detects, such as coarse grains and porous microstructure. Severe plastic deformation (SPD) which has the advantages of densifying the microstructure and refining the grains can effectively decrease the defects. Multiple direction compression (MDC) is easy to be carried out and at low cost, therefore it is the most prospective method for preparing bulk fine-grain materials among the SPD processes. It’s meaningful to the industrial application of tungsten that the preparation of high-property tungsten by SPD with the microstructure densification and refinement.Based on the mechanism of deformation strengthening, sintered tungsten was processed by MDC at high temperature with different parameters (the reduction after single pass and the number of deformation passes). The mechanism of the grain refinement and the mechanism of the microstructure evolution during the process were deeply discussed via optical microscopy, XRD and EBSD. The strengthening and toughening effects were verified by microhardness tests. The results show that the obvious compacting and grain-refinement can be obtained by one-pass MDC having the reduction of 50% and that the microstructure can be elevated much. Slip deformation at the crystalline plane of (110) are dominant in the crystals of tungsten and twinning deformation also occurs at the crystalline plane of (211). The dislocation density inside the microstructure, the microstrain and the amount of the grain boundaries are enlarged obviously. Meanwhile, there is a trend that low-angle grain boundaries change into high-angle grain boundaries, which is related with grain refinement and the transmission of the grin boundaries during MDC. Dislocation multiplication and the increasing dislocation density emerge inside W grains when initial coarse grains are guided by deformation strain. Since the nature of the grain boundary is dislocation, the emergence of grain slipping leading to the tangling of low-angle grain boundaries (dislocation). New boundaries having large misorientation will form when the accumulation of distortion comes to a certain degree and the transmission of grain boundary misorientation is obtained. The emergence of new boundaries means that the purpose of the microstructure refinement and the strengthening of tungsten are achieved.The thermal expansion and diffusion of the material before and after deformation were tested at different temperature ranges. Based on the knowledge of thermology of materials, the influences of microstructure evolution on the thermal properties of tungsten were studied. Preparatory analyses were carried out on the recrystallization behavior via differential scanning calorimetry. The analyses show that deformed organization has the better performance of thermal diffusion in the environment of high temperature. However the thermal expansion properties of deformed material increased in high temperature. Compared with the initial sample, oxidization degree of the deformation microstructure at high temperature can be largely decreases and the material exhibit the better performance of the high-temperature stability. The recrystallization temperature of pure tungsten is not affected by the change of microstructure. So the processing temperature range of pure tungsten before and after deformation by MDC unchanged. It will be of great significance to the subsequent forming process of pure tungsten was deformed by MDC.