Microstructure Studies Of Metal Tungsten Wires,Rods And Tungsten Coatings

Tungsten and tungsten alloys have been widely used in many fields,such as in the electronics and electrical engineering,aerospace,chemical industry and so on,such as the lamp filament,welding electrodes,tube cathode,high temperature resistance furnace heating element,cutting tools and coating materials,etc.Because tungsten and tungsten alloys have high strength,high density,low thermal expansion coefficient and excellent corrosion resistance,etc.Based on the special properties of tungsten and tungsten alloy,the current study of tungsten and its alloys generally about refinement,purification,strengthening and toughening and composite technology.With the development of nuclear industry,tungsten materials are widely used in plasma facing materials.However,the brittleness of the tungsten restricts its application.Coating technology can overcome the shortcoming.The main coating technologies include chemical plating,molten salt plating,plasma spraying,chemical vapor deposition,physical vapor deposition,etc.The material performance depends on the material microstructure,based on the above researching background,in this paper,microstructure of three kinds of products of tungsten,tungsten rods,tungsten wires and tungsten coatings are characterized and analyzed.In this paper,three kinds of microstructure of the samples were characterized by metallographic microscope,scanning electron microscope,transmission electron microscopy and X-ray diffraction and the results show that:(1)The grain sizes of outside and inside of two tungsten rods are different.The grain sizes outside are large,which are called coarse grained area.The grain sizes inside are small,which are called fine grained area.The microstructure distribution of rotary swaging tungsten rod is not uniformity,which are improved by annealing obviously.The pores are found in the grain boundaries and grain interiors in rotary swaging tungsten rods,while the pores are found only in grain interiors in annealing tungsten rod.The occurrence of pores in grain boundary is due to the weak combination of grains as a result of large deformation,solidification or creep deformation.While the occurrence of pores in grain interior is due to the weak combination between tungsten particles during the suppressing process.Annealing process can only remove the pores in grain boundary but can't eliminate the pores in grain interior.The degree of deformation is large in coarse grained area in rotary swaging tungsten rods,and the grain sizes change small after annealing with recrystallization of consolidation mechanism.While the degree of deformation is small in fine grained area in rotary swaging tungsten rods,and the grain sizes change lagre after annealing with recrystallization of grain boundary bowing out.(2)The mechanism of ductile fracture of tungsten wires belongs to the microporous gathered shaped fracture,but the mechanism of brittle fracture is relatively complex.Cleavage fracture lead the tungsten wires failure eventually,with the occurrence of longitudinal fracture,Which prompt the characteristics of tearing slits and stripping area in the fracture of tungsten wires.In order to avoid the formation of crack,we should increase density by controlling porosity,purity by avoiding precipitation elements and improve the deformation degree by avoiding the formation of isometric grains.(3)The cross section of CVD-W coatings on graphite substrates with a PVD-Si intermediate layer presents a columnar crystal structure.The average grain sizes of the coatings after thermal fatigue test are larger than the as-deposited coatings.The interfaces between graphite substrate and PVD-Si interlayer,PVD-Si interlayer and CVD-W coatings are clearly observed,but the results of EDS reflect the inter diffusion of CVD-W and PVD-Si.Melting pools and steps form in the top of the surface grains during the thermal fatigue test with the appearance of shallow melting.The range of shallow melting area are different because of the inhomogeneous heating.The size of grains in the shallow melting area where the grains are exposed change small and the small pyramid-like grains are grown in the pools during the cooling stage.