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 Ti49.8Ni50.2 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 Ti49.8Ni50.2 alloy after abc pressing at 573 K begins at T ≥ 773 K.