Niobium

Niobium has the properties of high melting point, low vapor pressure, good cold working performance, high chemical stability, strong resistance to liquid metal and acid and alkali corrosion, and large dielectric constant of the surface oxide film. It is an important functional material.

Metal niobium has good toughness, elongation and low temperature processing performance, but it has strong chemical activity and high melting point. Its performance is very sensitive to the content of impurities. Nearly several hundred parts per million of oxygen, hydrogen or carbon can make niobium become Hard and crispy. The powder metallurgy method that usually uses vacuum high temperature sintering, electron beam furnace (EB), vacuum consumable electric arc furnace (VAR) vacuum smelting method are common methods for preparing high-purity niobium.

During melting and casting, due to the rapid cooling rate, the gas in the ingot cannot be removed in time, and the alloy contains unstable phases and segregation and other defects in the crystal. These defects have a great influence on the uniformity and stability of the structure or the process performance, and can be effectively removed by the homogenization annealing treatment.

The homogenization annealing temperature of niobium ingot is 1800~2000℃. The degree of vacuum is mmHg. The holding time depends on the size of the ingot, generally 5 to 10 hours. The changes in gas content and hardness of niobium ingots before and after homogenization annealing are as follows:

The deformed metal has higher energy and is in a thermodynamically unstable state. At lower temperatures, a series of changes in tissue and properties occur, called reversion. During the recovery period, only a small part of the energy is released, and the properties of the metal after deformation are only partially recovered. It can be seen that the use of stress relief annealing in the cold working process can continue to process the metal

The cold working recovery activation energy of niobium and niobium alloys is less than the natural diffusion activation energy, so recovery occurs very quickly at relatively low temperatures. Niobium and most niobium alloys relieve stress and reduce strain energy when heated at a given temperature for 1 hour. Almost all strain centers that create new grains are eliminated. If reheated to the recrystallization temperature, only equiaxed grains are formed in the remaining strain centers, and in other places, the grains continue to grow, but the cold working direction is maintained.
Niobium and niobium alloys are often subjected to stress relief annealing during cold working.

After the deformation, when the metal is heated to a high enough temperature, nucleation and growth are carried out again in the original crystal to form a new crystal without distortion in equilibrium. This process is called recrystallization. After recrystallization, the processing structure of the metal is eliminated, and the mechanical and physical properties are restored to the state before cold deformation. The new crystal has the same crystal structure as the original, but a different phase. Unleashed all potential. The recrystallization temperature of niobium and niobium alloys is mainly related to the degree of deformation and the addition of alloying elements. The pure niobium smelted by electron beam has a cold deformation of 80~85%, and starts to recrystallize at 1070~1100 °C: the deformation amount is 60%, and it starts to recrystallize at 1200 °C. The recrystallization diagram of pure niobium is as follows:

Recrystallization diagram of Niobium

The addition of titanium has little effect on the recrystallization of niobium. The addition of molybdenum and tungsten can greatly increase the recrystallization temperature.
Niobium and niobium alloys undergo recrystallization annealing during rework. Through the interaction of deformation and annealing, the segregation of the ingot can be eliminated; the grains are broken and the structure is uniform; Thereby improving product performance and yield.

The effect of alloying elements on the recrystallization temperature of Niobium

Non-uniformly distributed second phases are often seen in the as-cast structure of some alloys. After high temperature annealing treatment, the second phase can be completely or partially dissolved. Most of the second phases in niobium alloys are carbides and oxides of active metals (titanium, zirconium, hafnium, etc.). In addition to dissolving these compounds at high temperatures, precipitation reactions sometimes occur with changes in temperature. Therefore, aging treatment can be performed. Because the precipitates are preferentially precipitated on the dislocation lines generated by cold deformation. Therefore, the dispersed strengthening phase can be obtained through “solid solution→cold deformation→aging”, so as to obtain excellent comprehensive properties. This combined treatment is called thermomechanical treatment.

The temperature and time of solution and aging and the amount of intermediate cold deformation have a great influence on product performance. When the solution temperature is low, the supersaturation of the solid solution is not enough; if the solution temperature is too high, the grains are coarse. Excessive aging temperature will result in overaging or recrystallization. If the amount of intermediate cold deformation is insufficient, the precipitated phase will be discontinuously distributed along the grain boundary, resulting in the cupping brittleness of the product. It can be seen that reasonable selection and strict control of these parameters are very necessary. Also note the interrelationships between them. Solution or aging is sometimes carried out at the same time as recrystallization, so that many niobium alloys have different degrees of “saddle shape” in the relationship between mechanical properties and annealing temperature. The curve decreases, indicating that the softening effect caused by recrystallization is the main aspect of the contradiction. As the temperature changes, the strength increases and precipitation strengthening dominates.

Many unique properties of niobium determine that it is a multi-purpose functional material. In the electronics, steel, metallurgy, chemistry, cemented carbide, atomic energy and aerospace industries, modern strategic weapons, superconducting technology, scientific research, medical equipment, and even arts and crafts and decoration industries, all involve niobium metal, alloys and compounds The wide range of applications. Niobium is mostly used in industries and superconducting technologies such as steel, ceramics, and nuclear energy.
Niobium has good thermionic emission properties. Can make getters and low pressure rectifiers for vacuum tubes. Niobium is used in cryotubes, which can replace electron tubes and transistors, greatly reduce the size of the equipment, and have a great effect on improving computers and radars.