Effect of hydrogen redistribution during aging on the structure and phase state of nanocrystalline and coarse-grained TiNi alloys

The Effect of Hydrogen on Martensite Transformations and the State of Hydrogen Atoms in Binary TiNi-Based Alloy with Different Grain Sizes

“…Immediately after hydrogenation, the nanocrystalline and coarse-grained specimens preserve the sequence of their respective phase transformations B2↔R↔B19′ and B2↔B19′, where B2 is high-temperature austenite, R and B19′ are the rhombohedral and monoclinic martensite phases, respectively. Pretransition phenomena caused the small increase in the resistivity on cooling, before the onset of B2→B19′ transformation—both in the initial state [7] and after hydrogenation with aging at room temperature for 1 h (Figure 1b). It is possible that the growth of electrical resistivity at cooling of hydrogenated and aged specimens can also be caused by the formation of “strain glass state”, which was discussed for Ti 50−X Ni 50+X (at%), Ti 50 Ni 44.5 Fe 5.5 (at%) and Ti 49 Ni 50−x Pd X (at%) alloys [36,37,38,39].…”

“…High-angle boundaries feature an increased free volume favorable for hydrogen diffusion along with them. According to transmission electron microscopy [7], our nanocrystalline Ti 49.1 Ni 50.9 specimens are dominated by equiaxed grains with high-angle boundaries, and this is likely the cause for fast hydrogen diffusion.…”

“…Its coarse-grained structure with near-equiaxed grains of average size 10 µm was obtained by annealing at 923 K for 30 min with further water quenching. The microstructure of both types of specimens was analyzed elsewhere [7].…”

“…However, when applied to medical products (e.g., implants and dental devices, which are in long-term contact with hydrogen-containing saline and human tissue,), alloys such as these reveal hydrogen embrittlement and influence of absorbed hydrogen on their martensite transformations and functional properties—in particular, SME and SE [2,3,4]. It is this problem that stimulates the research in the interaction of hydrogen with near-equiatomic TiNi [5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35].…”

“…By now, it is known that hydrogenation, whether gas phase or electrolytic, suppresses the B2↔B19′ or R↔B19′ transformations in B2-phase binary TiNi-based alloys [5,6,7,8]. In addition, according to resistivity measurements [7], it extends the temperature range of premartensite effects. During electrolytic hydrogenation, hydrogen atoms in specimens are first concentrated in its near-surface layer and are then redistributed by diffusion into its bulk.…”