Although
tantalum has been used in many fields, the basic research on it is still relatively weak, especially the research on deformation microstructure and annealing behavior is not perfect.
In the early 1970s, in order to have a better understanding of the recrystallization mechanism of the body centered cubic metal, Fujii et al and Vandermeer et al studied the deformation structure and annealing behavior of single crystal tantalum with different orientations in detail. The results show that the four single crystals with different orientations (001)[1-10], (112)[1-10],(111)[1-10],(111)[-1-12] show different deformed and annealed structures. Some of the orientations show good stability during the rolling process, and the orientations after deformation are basically consistent with those before deformation, while some orientations will rotate after deformation and gradually deviate from the initial orientations. Among them, (001)[1-10] and (112)[1-10] oriented crystals show good stability in the rolling process. When the shape reaches 80%, the grains basically do not rotate or split, and the orientation after deformation is still (001)[1-10] and (112)[1-10]. The analysis of the transmission images shows that the dislocations of the rolled grains of the two orientations are highly entangled and have a considerable number of small dislocation rings, but no obvious dislocation cells are formed. (001)[1-10] was annealed at high temperature for a long time (1400℃, 36000s), only recovery but no recrystallization occurred. The recovery process will be activated in a short time at low temperature (800℃, 600s), when the dislocation density decreases rapidly and the dislocation ring will disappear. With the increase of annealing temperature and annealing time, the dislocation density will decrease further, forming a stable dislocation network and large subgrains. However, the grain boundary orientation of these large subgrains is less than 1°. (112)[1-10] oriented crystals exhibit different behaviors during annealing. After annealing at high temperature for a long time (1200℃, 36000s), some recrystallized crystals occurred, but only a few recrystallized grains were nucleated at the highest temperature. Essentially, the response of (112)[1-10] oriented crystals is similar to that of (001)[1-10], but differs to some extent. During short time low temperature annealing, a large number of entangled dislocations were rapidly rearranged to form an obvious dislocation cell structure. After further annealing, the subgrains grew, but recrystallization did not occur.
However, the orientation of (111)[1-10] oriented crystals changed after rolling, rotating 10° in the direction of (112)[110]. On the other hand, obvious cellular structures were formed in the deformed crystals, and these cells elongated along the transverse direction, resulting in a large angular misorientation of 18°. After annealing at 1200℃ for 7200s, the recrystallization of (111)[1-10] oriented crystals is complete and the grain size is about 230μm. The recrystallization texture is deflected by about 23° relative to the deformed matrix around the [111] axis. The ideal orientation of the new recrystallization texture is close to {111}<253> orientation. After annealing for a long time at 600℃, 36000s), the crystal has a little recovery, and the dislocation distribution basically does not change. There is no evidence that there is an incubation stage of recrystallization. After increasing the annealing temperature (700℃), the dislocation substructure changes significantly, and the average cell size increases, but the accompanying mechanical properties change little. Some very large subgrains (about 2μm) will become recrystallized grains. Well developed sharp subgrain boundaries with a maximum misorientation of 6° to 8° appeared. (111)[-1-12] single crystal remains stable after cold rolling at 60%. After annealing at high temperature (2000℃), two texture types (110)[001] and (115)[-5-52] will be produced, of which (110) is slightly stronger. However, the average grain size of (110) oriented grains is much larger than that of (115) oriented grains. In addition, from the orientation relationship, (110)[001] orientation rotates 35° clockwise around the axis of [-110], while (115)[-5-52] orientation rotates 39° counterclockwise around the axis of [-110], but these two orientations have the same ∑9 matrix relationship with the matrix.
Deformation and annealing behavior of thin coarse tantalum. The results show that the deformation behavior of coarsed tantalum is inconsistent, and the grain orientation has a significant effect on the deformation structure. Coarsed grains with (001)[110] orientation will not split significantly in the rolling process, and their internal adjacent orientations will be small (less than 8°), while grains with the other orientation will split into two orientations [110] and [114], resulting in a large internal orientations (more than 15°)[12]. The recrystallization does not occur even when annealed at high temperature because of the low storage energy in the deformed grains with small misorientation. These deformed grains always exist in the microstructure of tantalum in the form of long strip, which adversely affects the performance of tantalum.
On the rolling and recrystallization texture of
tantalum plate, Raabe et al. made a preliminary study. The texture of 70% of cold-rolled tantalum has incomplete α fiber texture between {001}<110> and {111}<110>, and weak γ texture. After annealing at 1000℃ and 1h, the orientation of {001}<110> and {111}<110> were enhanced, and the strength of {001}<110> was twice that of the rolling texture, while the component of {112}<110> was weakened. When the annealing temperature rises to 1200℃ or 1300℃, the annealing texture changes, and the strength of α fiber texture decreases, while the strength of γ fiber texture increases, with the maximum value at {111}<112> component. The texture of 80% cold-rolled tantalum is basically similar to that of 70% cold-rolled tantalum, except that the α fiber texture becomes stronger and γ fiber texture becomes weaker. After annealing at 1000℃ and 1h, the strength of {001}<110> and {111}<110> components in α fiber texture increased, while the strength of {112}<110> components decreased. When the annealing temperature is increased to 1100℃, the strength of {001}<110> component decreases and the texture of γ fiber becomes stronger. The α fiber texture of 90% cold rolled tantalum continues to be stronger. After high temperature annealing, the α fiber texture will disappear, and the γ fiber texture will become stronger.
The recrystallization texture of tantalum is closely related to its processing history. The effects of four cogging methods (side forging (force perpendicular to spindle axis), top forging (force parallel to spindle axis), top forging combined with edge forging and extrusion) on the texture of annealed rolled sheet were studied. After three kinds of forging, all the annealed billets have the texture of {001}<110>, and the edge forging slab has the strongest texture of {100}. There are many kinds of texture after extrusion, such as {001}<100>, {113}<110>, {011}<322>. However, the edge forging cobbleplate has mixed texture after rolling, including {100}<011>, {111}<011>, {011}<211> texture components, etc., while the other three kinds of cobbleplate have typical BCC metal rolling texture after rolling, that is, the texture is between {001}<011>+ and {111}<011>. But the top forged cobbleplate has the strongest {100}<011> texture. After annealing, the top forged
tantalum sheet has strong {111}<110> and {111}<112> texture components and fine grains, which is suitable for further stamping processing. The edge forged tantalum plate contains a texture component of {001}<100>, which cannot be eliminated even after rolling annealing. The annealed rolled sheet with top forging and coaling has mixed texture, including {111}< UVW > and {100}< UVW > texture components. After extrusion and rolling annealing, the rolled annealed tantalum sheet has a strong {111}< UVW > texture similar to that of the rolled annealed tantalum sheet.
In China, the research on tantalum is relatively late, mainly focusing on the control technology of grain structure and texture of tantalum sheet, and the basic research on deformation microstructure evolution and texture formation mechanism is relatively weak. Among them, the microstructure evolution of tantalum sheet in the process of processing is fully understood. The results show that the preparation method, intermediate coiling method, final rolling condition and annealing system all have important effects on the annealing and recrystallization behavior of tantalum sheet. The change of rolling mode affects the microstructure and mechanical properties of tantalum plate. After annealing at 1100℃, the grain size of tantalum sheet is fine and has strong {111} texture. It is very beneficial to eliminate the ear-making process of tantalum sheet deep drawing.
