Abstract
The paper analyzes the surface structure and phase state of Ti49.4Ni50.6 (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 tH as follows: slowly to 50 ppm at tH = 0.5–1.5 h, steeply to 150 ppm at tH = 1.2–2 h, and linearly to 300 ppm at tH = 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 TiNiHx 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–TiFeH0.94 (space group Pmcm), rather than as a tetragonal TiNiHx 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 Ti49.4Ni50.6 (at%).
Highlights
Materials 2021, 14, 7046. https://TiNi alloys, showing superelasticity and shape memory, good plasticity, high corrosion resistance, and biocompatibility, are efficient materials for manufacturing various engineering devices [1,2] and medical implants [3,4]
At near-room temperatures (290–310 K), the rate of diffusion process in TiNi is low such that a large amount of hydrogen first goes into its surface layers and diffuses deep into the material [9,10]
Ti49.4Ni50.6 after electrolytic hydrogenation in normal saline. (This figure is in color only in th Qualitative of signal distribution due to hydrogen vs. sputtering time in electronic version.)profiles
Summary
TiNi alloys, showing superelasticity and shape memory, good plasticity, high corrosion resistance, and biocompatibility, are efficient materials for manufacturing various engineering devices [1,2] and medical implants [3,4]. At near-room temperatures (290–310 K), the rate of diffusion process in TiNi is low such that a large amount of hydrogen first goes into its surface layers and diffuses deep into the material [9,10]. TiNi alloys with hydrogen decreases the temperatures of thermoelastic martensite transformations from a cubic B2 phase to a rhombohedral R and a monoclinic B190 phase, and this impairs their superelasticity and shape memory effect [8,10,13,14] and causes their cracking and fracture [15,16]. Note that the data currently available on the state diagram of Ti-Ni-H system and the structures of hydride phases are rather scanty and contradictory
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