Changes in the Structural-Phase State and Dislocation Density of Ti49.8Ni50.2 Alloy Depending on the Isochronal Annealing Temperature after Severe Plastic Deformation by abc Pressing at 573 K

X-ray diffraction studies were conducted to examine changes in the structural-phase state and dislocation density of Ti_49.8Ni_50.2 alloy depending on the isochronal annealing temperature after severe plastic deformation by abc pressing at 573 K. The total true strain achieved in the alloy specimens during abc pressing was e = 9.55. Isochronal annealing was carried out for 1 h at 573, 673, 773, 873 and 973 K. Analysis of all studied specimens at room temperature revealed the coexistence of R and B19′ phases, whose relative fractions varied with annealing temperature. The high-temperature B2 phase was not detected. It was found that the most rapid decrease in the dislocation density, which was measured at 393 K (in the B2 state), occurred after annealing at 673 and 773 K. Specimens annealed at 773 K had the minimum dislocation density, which is more than an order of magnitude lower than the dislocation density immediately after abc pressing. In the same temperature range, there is a significant decrease in the root-mean-square B2 lattice microdistortions <ε²>^1/2 and a slight increase in the average size of coherently diffracting domains (crystallities). After abc pressing and isochronal annealing, the main contribution to the intrinsic X-ray line broadening is made by B2 lattice microdistortions, while the contribution from crystallite size is insignificant. The obtained results show that intense recrystallization in Ti_49.8Ni_50.2 alloy after abc pressing at 573 K begins at T ≥ 773 K.