Features of Multistage Thermoelastic B2–R–B19′ Martensitic Transformations in Heterophase Single Crystals of Ti–Ni Alloys

In heterophase Ti_50.0–xNi_50.0+x (х = 0.8–1.0 at.%) single crystals after aging at 823 K for 1.5 h at the heating/cooling rate (15 ± 1) K/min and stepwise aging at 823 K for 1.5 h plus aging at 573–623 K for 1 h, multi-stage martensitic transformations are observed: there are three peaks of heat release in the DSC (differential scanning calorimetry) curves. The first peak is associated with a B2–R-transformation and increased electrical resistance ρ and the other two correspond to the development of an R–B19′-martensitic transformation occurring in two stages, and are followed by a two-stage drop of electrical resistance in the ρ(T) plot. The high-temperature stage of the R–B19′-martensitic transition is associated with transformations under the action of local stress fields in the vicinity of disperse Ti₃Ni₄ particles measuring ~400 nm, while the low-temperature stage is associated with transformations in the spaces between large particles containing fine disperse β′-phase particles isostructural with respect to the matrix, measuring 3–5 nm, which are formed during slow cooling from 823 K or in the course of low-temperature aging at 573–623 K.