The concentrated distribution of some voids and impurity elements C and N at the crystal boundary results in weak connections between the tungsten crystal boundaries, and micro-cracks will be formed under the action of external forces. The expansion of these micro-cracks will lead to the brittle fracture of tungsten products, For example, tungsten wire becomes very brittle after recrystallization after high-temperature use, it is very easy to break under impact or vibration. Therefore, in order to solve the problem of filament fracture, 3% - 5% rhenium is often added to the tungsten filament, which can reduce the plastic brittle transition temperature of tungsten to or below room temperature.
In addition, the addition of rhenium to tungsten also improves plasticity at high and low temperatures. Rhenium can not strengthen the alloy at high temperatures. The solution strengthening effect comes from the interaction between solution atoms and dislocations. In a high-temperature environment, only the following two mechanisms can work. One is from the interaction of elastic stress field, and the other is from the change of matrix elastic modulus by solution atoms. The above mechanisms assume that the strengthening effect is directly proportional to the solution concentration, However, high rhenium content weakens the strength of tungsten thorium alloy at high temperatures above 2000K because of the shear modulus of tungsten decreases. W-Re alloy is easy to form twins during deformation, which reduces the stacking fault energy and the grain boundary impedance of dislocation movement, resulting in the increase of dislocation mobility and the softening of tungsten solid solution.
Adding rhenium to the tungsten product will also reduce the interaction energy between alloy atoms, and then affect the melting point of the alloy. For example, when the rhenium content is 26%, the melting point of the w-26re alloy is nearly 400k lower than that of pure tungsten. The change of melting point has no significant effect on the mechanical properties at room temperature, but it has a great impact on the mechanical properties at high temperatures. For example, the temperature at 2600k is equivalent to 0.79 times the melting point of w-26re-1tho2 alloy, but only 0.70 times the melting point of the w-tho2 alloy.
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