Structure and Phase State of Ti_49.4Ni_50.6 (at%) Hydrogenated in Normal Saline

The paper analyzes the surface structure and phase state of Ti_49.4Ni_50.6 (at%) hydrogenated at 295 K in normal saline (0.9% NaCl aqueous solution with pH = 5.7) at 20 A/m² for 0.5-6 h. The analysis shows that the average hydrogen concentration in the alloy increases with the hydrogenation time t_H as follows: slowly to 50 ppm at t_H = 0.5-1.5 h, steeply to 150 ppm at t_H = 1.2-2 h, and linearly to 300 ppm at t_H = 2-6 h. According to Bragg-Brentano X-ray diffraction data (θ-2 θ, 2 θ ≤ 50°, CoKα radiation), the alloy in its scanned surface layer of thickness ~5.6 µm reveals a TiNiH_x phase with x = 0.64 and x = 0.54 after hydrogenation for 4 and 6 h, respectively. The structure of this phase is identifiable as an orthorhombic hydride similar to β₁-TiFeH_0.94 (space group Pmcm), rather than as a tetragonal TiNiH_x hydride with x = 0.30-1.0 (space group I4/mmm). Time curves are presented to trace the lattice parameters and volume change during the formation of such an orthorhombic phase from the initial cubic B2 phase in Ti_49.4Ni_50.6 (at%).