When the
titanium alloy tube is heated, in addition to the recovery and recrystallization process of the cold working arrangement, there are also the solution of the compound and the polymorphic change of a→β. In order to improve the properties 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 a process of eliminating most of the second type of internal stress that occurs 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 in the arrangement after deformation, in the process of nucleation and growth of crystal grains. At this time, the lattice type does not change, but there are changes in mechanical properties. This process is affected by the degree of cold deformation, heating temperature and holding time, and can be recrystallized through three-dimensional recrystallization of cold deformation rate, heating temperature and recrystallized grain size.
The effect of alloying elements on the initial 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 improve the recrystallization temperature of titanium. The determination of recrystallization is mainly based on the combination of metallographic investigation and X-ray diffraction. When recrystallization occurs, fine equiaxed grains appear on the deformed fiber arrangement, while the diffraction rings on the X-ray back-reflection Laue diagram phase begin to become disconnected mottles.
For heat-treatable beta alloys, the recrystallization structure can also be revealed by incomplete aging (500°C/4-8 hours, air cooling), and the unrecrystallized grains after incomplete aging appear dark after corrosion. It has been determined that the initial recrystallization temperature of TA2 pure titanium is about 550 °C, TA7 titanium alloy is about 600 °C, TC4 titanium alloy is about 700 °C, and TB2 alloy is about 750 °C.
It should be pointed out that in titanium alloy tubes and titanium plates, the recrystallization process is often accompanied by some other structural changes. For example, in near-a alloys and a+β alloys with a small content of β stable elements, the dissolution of the a phase and the change of the β composition are accompanied by the recrystallization process in heat-treatable β alloys. Or form a gestational effect on the subsequent aging.
