In addition to the recovery and recrystallization process of the cold-worked structure when heating the titanium plate, there is also a solution of the compound and a polymorphism transformation of a → β. In order to improve the performance of titanium alloys and titanium plates, in addition to the necessary alloying, appropriate heat treatment is generally used. The recovery process of titanium alloys and titanium plates is also the process of removing most of the second type of internal stress generated during deformation through the movement of vacancies and dislocations at a certain temperature. The temperature at which the recovery process occurs is lower than the recrystallization temperature, generally 500 to 650 ° C.
Like other metals, the recrystallization process of titanium alloy plates and titanium plates is also the process of nucleation and growth of crystal grains on the deformed structure. At this time, the lattice type does not change, but there is a change in mechanical properties. This process is affected by the degree of cold deformation, heating temperature and holding time, and can be recrystallized by three-dimensional recrystallization from the cold deformation rate, heating temperature and recrystallized grain size.
The influence of alloying elements on the starting recrystallization temperature of pure titanium has been described in the previous section. In addition to niobium and cobalt, commonly used alloying elements and impurity elements can increase the recrystallization temperature of titanium.
Determination of recrystallization mainly uses a combination of metallographic observation and X-ray diffraction. When recrystallization occurs, fine equiaxed grains appear on the deformed fibrous structure, and at the same time, the diffraction rings on the Laue diagram phase of the x-ray back reflection begin to become unconnected spots. For heat-treated β alloys, incomplete aging (500 ℃ / 4 ~ 8 hours, air cooling) can also be used to show the recrystallized structure. The unrecrystallized grains after incomplete aging are dark after corrosion. It has been determined that the initial recrystallization temperature of TA2 pure titanium is about 550 ℃, TA7 titanium alloy is about 600 ℃, TC4 titanium alloy is about 700 ℃, TB2 alloy is 750 ℃.
It should be pointed out that in titanium alloy plates and titanium plates, the recrystallization process is often accompanied by some other structural changes. For example, in the near-a alloy and a + β alloy with a small amount of β-stabilizing elements, the dissolution of the a phase and the change of the β component are accompanied; in the heat-treated β alloy, the recrystallization process is often performed simultaneously with the recrystallization process Or it may have a contagious effect on the subsequent aging. In addition, different types of alloys have different microstructures at room temperature, different alloy phases involved in deformation, and different recrystallization processes and characteristics.