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

Abstract

The analysis presented here shows that in B2-phase of Ti49.1Ni50.9 (at%) alloy, hydrogenation with further aging at room temperature decreases the temperatures of martensite transformations and then causes their suppression, due to hydrogen diffusion from the surface layer of specimens deep into its bulk. When hydrogen is charged, it first suppresses the transformations B2↔B19′ and R↔B19′ in the surface layer, and when its distribution over the volume becomes uniform, such transformations are suppressed throughout the material. The kinetics of hydrogen redistribution is determined by the hydrogen diffusion coefficient DH, which depends on the grain size. In nanocrystalline Ti49.1Ni50.9 (at%) specimens, DH is three times greater than its value in coarse-grained ones, which is likely due to the larger free volume and larger contribution of hydrogen diffusion along grain boundaries in the nanocrystalline material. According to thermal desorption spectroscopy, two states of hydrogen atoms with low and high activation energies of desorption exist in freshly hydrogenated Ti49.1Ni50.9 (at%) alloy irrespective of the grain size. On aging at room temperature, the low-energy states disappear entirely. Estimates by the Kissinger method are presented for the binding energy of hydrogen in the two states, and the nature of these states in binary hydrogenated TiNi-based alloys is discussed.

Countermeasure for corrosion and hydrogen embrittlement of Ni-Ti superelastic alloy in acidic fluoride solution by adding sodium bicarbonate and hydrogen peroxide

“…Hydrogen absorption is enhanced owing to the high hydrogen solubility limit and diffusion rate in the martensite phase [41]. Moreover, hydrogen absorption is further enhanced by dynamic lattice changes due to transformation [44,45,[48][49][50][51]. Nevertheless, it is likely that the present countermeasure for hydrogen absorption is effective because of the inhibition of hydrogen evolution, although corrosion resistance somewhat decreases in the presence of the martensite phase or during martensite transformation.…”

Hydrogen’s Effect on the Shape Memory Effect of TiNi Alloy Single Crystals

“…By now, hydrogen's effect on MT has been studied mainly on TiNi polycrystals [1,[3][4][5][6][7][8][22][23][24][25][26]. These studies have shown that hydrogen's effect on the mechanical and functional properties is determined by the state of hydrogen in the initial B2 phase (as metal hydrides or atoms in a solid solution), its concentration, and the type of initial phase itself (austenite or martensite) upon hydrogenation.…”

“…At low concentrations below 400 wppm (wppm represents weight part per million), hydrogen contributes to an increase in the plasticity of the martensite phase and superelasticity. When hydrogen concentrations are greater than 400 wppm, hydrogen reduces plasticity; suppresses shape memory; and in TiNi alloys with a one-stage B2-B19 MT, induces the R phase through stresses [1,[3][4][5][6][7][8][9][10][22][23][24][25][26]. Nevertheless, despite numerous experimental data, interest in studying the effect of hydrogen on the functional properties of binary TiNi alloys remains due to their wide use in the dental and medical fields [11,18,21].…”

Source for In Situ Positron Annihilation Spectroscopy of Thermal—And Hydrogen-Induced Defects Based on the Cu-64 Isotope

“…However, the in situ investigation of the defect structure evolution during hydrogen exposure is more useful in terms of the mechanisms of hydrogen embrittlement. Experiments on the hydrogen interaction with materials are carried out either using the liquid phase (electrolytic saturation) [14] or gas phase (hydrogen gas sorption at high pressure and various temperatures) [15]. These conditions impose more stringent requirements for the potential positron sources to be used for in situ studies.…”