What Are The Main Factors Affecting Refractory Metal DBTT?

May 11, 2020

(1) The higher the purity, the lower the DBTT. Refractory metal brittleness of interstitial impurity (0, N, C, S, P, etc.) is very sensitive, such as target set over industrial production will make the DBTT significantly increased industrial pure V at room temperature with Ⅵ race differences in metal DBTT XiaWen gap is related to the solid solubility of the impurity. In secondary cooling speed cooling to room temperature, preserved in the V and Ⅵ clan metal solid solution of interstitial impurity composition as shown in table. It can be seen from the table that the solid solubility of tungsten and molybdenum to interspace impurities is extremely low at room temperature, while the tantalum and niobium v group metals are several orders of magnitude higher. Generally, the interspace impurity content of the tungsten molybdenum powder metallurgy blanks produced by industry is controlled in the range of 0.003% ~ o.006% (i.e., 30×10-6 ~ 60×10-6), which is far beyond its solid solubility at room temperature. Therefore, the industrial pure W and Mo are still polyphase alloys saturated with impurities at room temperature, and the processing brittleness is serious at room temperature. With the increase of temperature, the solubility of interstice impurity increases and the plasticity increases, so the plastic brittle transition temperature of tungsten and molybdenum is higher than room temperature. The tantalum and niobium billet prepared under vacuum, with the gap impurity content still within the solid solubility of room temperature, is single-phase, the plastic brittle transition temperature is lower than room temperature, and has good room temperature plasticity. At high temperature (in the air), it is very easy to absorb gas impurities and become brittle, and the billet is all processed into wood at room temperature. The final component of Si, Al and K doped electric light source tungsten wire is equivalent to pure tungsten, but due to the presence of potassium bubbles (see potassium bubble enhancement of doped tungsten wire), its DBTT is about 200℃ higher than that of pure tungsten wire. The addition of rhenium in moderation will reduce DBTT.

(2) Ⅵ DBTT of refractory metal and material is closely related to the microstructure. Firstly, DBTT has a linear relationship with the logarithm of the grain size of the metal material, and DBTT decreases with the grain size refinement. The processing of equiaxed coarse crystal blank must be carried out at high temperature, otherwise it will be brittle. In addition, the metal in a certain temperature range, with the increase of deformation degree, the change of microstructure, the DBTT is gradually reduced, so there is a deformation toughening effect, that is not to change the material composition, only increase the degree of plastic deformation processing, cooperate with stress annealing, make the material deformation along the main direction of the distribution of fiber streamline processing organization, and the more fiber, less DBTT. For example, after the tungsten billet is processed to the filament at about 1300℃, DBTT is reduced to about 400℃. After 80% deformation, DBTT was reduced from about 1000℃ to near room temperature. With annealing stress, cold deformation at room temperature could be realized. So Ⅵ family of refractory metal brittleness is sensitive to the structure. DBTT of high purity tantalum and niobium metals is not very sensitive to the structure, but the tantalum wire bending experiment shows that it is still good in fiber toughness.

(3) The stress state has an important influence on DBTT. A refractory metal with uneven deformation and notched surface is liable to brittle fracture due to the tensile stress, especially the three-way tensile stress load. Tungsten is a material that is very sensitive to notches. Different experimental methods, due to different stress states and deformation rates, test DBTT is different. The DBTT measured by the stamping experiment or bending experiment is in good agreement with the actual engineering situation. However, the degree of brittleness of tungsten and molybdenum in recrystallization at room temperature is different. When isometric crystal is formed in the early stage of recrystallization, severe brittle fracture will occur at room temperature. When recrystallized isometric grains are formed by high temperature recovery annealing, the isometric grains will have high toughness at room temperature. With the completion of recrystallization, the isometric grains will grow, resulting in room temperature embrittlement. This is due to the formation of recrystallized grains and the accumulation of interstitial impurities at grain boundaries. Therefore, the formation and growth of recrystallized grains should be prevented in the processing and use of tungsten and molybdenum. In 1990, a study by zhou meiling et al. in China showed that trace rare earth La2O3 was added to molybdenum. The brittleness of recrystallization of molybdenum at room temperature can be improved obviously by adding 3% La2O3. After annealing at 1900℃, the elongation of the fine molybdenum wire and the pure molybdenum wire was as high as 20%, while the latter was completely brittle. In 1992, Chinese zhang jiuxing and zhou meiling joined 0.2% ~ 2% La2O3. The impact experiment of high temperature recrystallization annealing of molybdenum plate and pure molybdenum plate proved the toughening effect of La2O3 molybdenum plate again, its impact toughness was 4.5 times that of pure molybdenum plate. They also measured their DBTT, 2% La2O3, by bending. The DBTT of molybdenum plate has been reduced to one 83℃, while the pure molybdenum plate has shown complete brittle fracture (recrystallization brittle fracture) at room temperature.

Zirconium tube with hydrogen embrittlement has excellent nuclear properties and is widely used as cladding material in nuclear reactors. Hydrogen is easily absorbed into zirconium, and it is often unavoidable to inhale hydrogen into zirconium tubes in the manufacturing process and application environment. Hydrogen occupies the tetrahedral space in zirconium, and when the hydrogen content exceeds 0.001%, it is precipitated as hydride. The existence and distribution of hydride fragments will lead to serious damage of zirconium alloy tubes. Hydrogen embrittlement can be prevented by improving the distribution of hydride. In use, the plasticity of zirconium decreases when the tensile stress is perpendicular to the thin and one-sided hydride. When the tensile stress is parallel to the thin one - sided, it has little effect on the plasticity of the material. Therefore, when the hydride is distributed in a circumferential direction (or tangential direction), it is beneficial to the material. When the hydride is distributed in the radial direction, it is easy to produce brittle crack. F48 means that the Angle between the hydride and the tangent of the tube is 48. ~ 90. The percentage of the number of slices in the range. The larger the f48, the more radial distribution and the more severe brittleness. The distribution of hydride is related to the tube processing and heat treatment process. F48 is one of the important indexes for the quality test of zirconium tubes. F48 ≤ 0.3 is the qualified standard.