Results of investigations of structural and phase transformations that occur in the titanium-nick-Elide-based alloy Ti _49.5Ni _50.5 with a shape memory effect during severe plastic deformation by torsion under high pressure (HPT) are reported. The studies were performed using transmission and scanning electron microscopy, neutron and X-ray diffraction, and measurements of temperature dependences of electrical resistivity. The martensitic B2 → B19' transformation was found to be induced in the alloy when applying a high pressure. After unloading, the martensitic B19' phase is retained in the alloy. The fine structure of the B19' martensite and its evolution into nanocrystalline and, subsequently, amorphous state during HPT with 1/4, 1/2, 1, 5, and 10 rev have been studied. It was shown that, after HPT, all nanosized crystallites whose sizes are less than 30-50 nm have a B2-type structure and, therefore, the reverse martensitic B19' → B2 trans-formation is realized in the alloy at room temperature after unloading.