Results are presented for a study of the structural and phase transformations that occur in the titanium-nickelide shape-memory alloy Ti _49.5Ni_50.5 subjected to heat treatment after deformation-induced amorphization by megaplastic high-pressure torsion (HPT) using five or ten revolutions of Bridgman anvils. The investigations were performed using transmission and scanning electron microscopy, X-ray diffraction, and measurements of the temperature dependences of electrical resistivity and magnetic susceptibility. It has been established that the crystallization of the alloy already occurs upon low-temperature treatment, beginning with ∼500 K. The evolution of the structure and the stage character of the development of crystallization and recrystallization processes depending on temperature have been determined. It has been shown that the annealing of the amorphized alloy makes it possible to obtain highly homogeneous nanostructured, submicrocrystalline, or bimodal states in the B2 austenite. A complete diagram of thermoelastic martensitic transformations of the B2 austenite has been constructed in the region from a nanostructured to a conventional polycrystalline state (with a grain size of 20-50 μm). The effect of size on the stabilization of austenite has been revealed and its specific features have been studied for the B2 → R and B2(R) → B19′ martensitic transformations depending on the structural state of the alloy.