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The development of inelastic and plastic strains is studied for Ti49.3Ni50.7 (at.%) alloy samples with the structure of high-temperature B2 phase in the state as received from the manufacturer under torsion. It has been established that when the strain of the samples, specified by torsion, is equal to 10.6%, the value of the accumulated plastic strain is 0.35%. Therefore, the actual yield stress of the Ti49.3Ni50.7 (at.%) alloy samples is localized at the end of the pseudo-yield plateau on the stress-strain dependence rather than at the end of the strain hardening stage, as previously thought. The results presented in the work should be taken into account in the practical use of the alloy with this composition as products for technical and medical purposes that experience tensile, bending, and torsional deformations. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Using the backscattered electron diffraction method, the features of grain-subgrain structure of the Ti49.8Ni50.2 (at %) alloy after megaplastic deformation by multiaxial forging (abc pressing) at 573 K and subsequent annealing were studied. It has been shown that the best detection of patterns of grain-subgrain structure of the Ti49.8Ni50.2 alloy after a given true deformation of e = 9.55 and annealing at 773 K for 2 h is observed in zones without martensitic relief on the surface of a samples. The clearest backscattered electron diffraction patterns are observed in the central part of grains, and the quality of the Kikuchi lines deteriorates with distance from the grain center. Calculation based on the equivalent grain diameter showed that the average grain size is 0.203 ± 0.053 μm. Grains are mainly elliptical in shape with an axial ratio of 0.48. Elongated grains along their length have a discrete-continuous misorientation of the crystal structure, which can reach 5 degrees per 1 μm. [ABSTRACT FROM AUTHOR]

Analysis of elastic properties of Ti49Ni51 (at %) and TiNi–TiFe single crystals, where the B2 phase can be both stable and unstable with respect to martensitic transformations, has been performed using the Every's and Blackman's diagrams, which are well known as two characterization methods of elastic properties of cubic crystals. Pair correlation relationships between (s2, s3), (s3, s1), and (F44, F12) have been calculated. The evolution of Cauchy pressure pC has been analyzed as a function of content of iron atoms in the crystalline lattice of the B2 phase of the alloys. It has been established that, irrespective of composition of the alloys, the Cauchy pressure is higher than zero and nearly monotonically decreases from 100.5 to 35.0 GPa with increase in content of Fe atoms in the crystalline lattice of the TiNi–TiFe B2 phase and increase in its stability with respect to martensitic transformations (up to complete stabilization). Such evolution of pC corresponds to the data, according to which significant constituent of bond forces in TiNi is a metallic bond, and with increase in concentration of Fe atoms in the alloys instead of Ni atoms and approaching the composition of TiFe, the fraction of the covalent constituent of bond forces increases. [ABSTRACT FROM AUTHOR]