There are many ways to strengthen metals, such as solid solution strengthening, precipitation strengthening, dispersion strengthening, work hardening, fine crystal strengthening and particle strengthening, fiber strengthening, whisker strengthening and layer strengthening. In addition to fine grain strengthening, the mechanism of other strengthening methods is mainly to use solute atoms and the second phase isolated in the matrix (precipitation phase, dispersion phase, external particles, fibers, whiskers, etc.) to prevent the movement of dislocation to improve the deformation resistance of the metal. Fine grain strengthening takes advantage of the fact that the grain boundary strength in the polycrystal is higher than the inner grain strength at room temperature, and strengthens by increasing the number of grain boundaries. Work hardening uses the increase of dislocation density in metal to cause the distortion of crystal lattice and the interentanglement of dislocation to prevent the movement of dislocation and strengthen the metal.
Tantalum and tantalum alloy structural materials are mostly prepared by vacuum melting method. Solid solution tantalum alloys mainly rely on solid solution strengthening of solute elements, such as TaW alloy, the strength increases with the increase of tungsten content. When the tungsten content is 12%, the strength is about 650MPa, and when the tungsten content is 15%, the maximum can reach 750MPa. However, the increase in tungsten content makes the processing of the alloy more difficult, so the current tungsten content in the alloy is limited to less than 12%. On the basis of tantalum tungsten alloy, hafnium, carbon and other elements are added to the multi-element tantalum alloy, although the high temperature creep resistance has been improved, its strength can only reach about 800MPa, and the processing difficulty is further increased.
When tantalum and tantalum alloy are prepared by powder metallurgy sintering method, more oxygen will be adsorbed on the powder surface, and oxygen diffusion will dissolve in high temperature sintering, resulting in an increase in the overall oxygen content of the material, although the strength is increased, the plasticity is sharply decreased. For example, Ta-7.5W alloy prepared by powder metallurgy sintering method, after pressure processing into wire and sheet, although the strength can reach a higher level of 1170MPa, its elongation is greatly damaged, only about one-sixth of the melted alloy. The strength of the dense material prepared by hot isostatic pressing using the pre-alloyed Ta-10W alloy powder with low oxygen content is comparable to that of the melted alloy. Therefore, there is no more suitable method to further improve the strength of tantalum and tantalum alloys. The low strength makes people have to increase the amount of material, which causes the application range of tantalum alloy to be greatly limited.
