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 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.

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:

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.

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.