| Refractory metal tungsten has wide applications in many fields such as aerospace,nuclear industry,due to its excellent mechanical properties,high temperature strength and corrosion resistance.Commercial pure tungsten prepared by powder metallurgy method always has some problems of porosity,coarse structure and weak grain boundary caused by the high sintering temperature and impurity pollution,which result in poor plastic properties,higher ductile brittle transition temperature and easily oxidized of tungsten,and that limits its application and development.High-pressure torsion(HPT)process is based on the principle that high hydrostatic pressure and severe shear deformation can effectively close the pores,improve the microstructure and properties of the material,can fabricate ultrafine grain with non-equilibrium high angle boundaries at lower temperature,promote the deformation of grain boundary sliding and dislocation evolution,and improve the low temperature deformation ability of material with low plastic and high strength.Using HPT process to prepare pure tungsten material with excellent comprehensive properties has positive effect on promoting its engineering application.Based on the theory of deformation strengthening,sintered pure tungsten was undergone shear plastic deformation under different turning numbers at lower temperature using semi-constrained HPT process.The deformation characteristics of pure tungsten,the evolution of microstructure and deformation substructure during HPT process were deeply analyzed by microstructure observation,X-ray diffractometer and electron back-scattered diffraction.The results indicate that the microstructure of the sintered pure tungsten is highly compact and refined after HPT,and the fiber structure with obvious orientation is formed.The deformation of pure tungsten is mainly caused by the slip deformation of the {110} crystal plane,and with the increase of the number of the torsion ring,the slip deformation along the {112} and {200} planes is enhanced to make up for the deficiency of the single-slip.The microstrain of deformed microstructure increases gradually,and the subgrain size and dislocation density increase first and then decrease with the increase of strain.The microstructure is mainly refined by shearing fragmentation and deformation induced dislocation evolution during HPT deformation,accompanied by the evolution of low angle boundaries transform to high angle boundaries and the formation of specific grain orientation.When the deformation reaches a certain degree,grain growth of dynamic recrystallization occurs in the deformed microstructure,with the recrystallization cube texture formed.Due to the deformation mechanism,stress state and dynamic recrystallization behavior of the material during HPT process,the microstructure of pure tungsten exist the "texture fluctuation" effect.The microhardness,indentation morphology and recrystallization behavior of pure tungsten under different turning numbers were tested,and strengthening and toughening mechanism of HPT deformed pure tungsten and the thermal stability of deformation organization were analyzed combined with microstructure evolution.The results show that the microhardness and the toughness of tungsten are improved obviously at room temperature after HPT deformation,and the toughening effect and uniformity increased significantly with the increase of turning numbers.The strength and tough of HPT deformed tungsten is mainly dominated by grain refinement and dislocation proliferation.The recrystallization temperature of pure tungsten does not decrease obviously with the better dimensional stability of recrystallized microstructure under different turning numbers.Although the strain-stored energy increases and the recrystallization activation energy decrease after HPT deformation,but due to the release of stored energy resulted from dislocation rearrangement and the transformation of non-equilibrium boundaries,thermal stability of pure tungsten is relatively improved. |
PDF Full Text Request