Effects of one- and two-step aging on superelasticity in Ni-rich TiNi single crystalsTuesday (18.06.2019) 20:22 - 20:23
Present work is aimed at investigation of stress-induced martensitic transformations (MTs), superelasticity (SE) and hystereses in quenched and aged Ni-rich -oriented Ti-51.5at.%Ni single crystals in compression.
Because of the Ni supersaturation the strain glass transition at stress-free cooling is observed in quenched crystals instead of thermoelastic MT. Transition into В19'-martensite is occurred under load. SE was obtained only in high-strength -orientation at low temperatures (<250K) and high stresses (1200MPa).
In order to achieve high-temperature SE (T>373K) and to control its temperature interval the one- and two-step aging was applied.
Aging at 823K, 1h was carried out to precipitate large Ti3Ni4 particles (400nm), which result in the Ni reduction in matrix, the appearance of В2-R-В19' MT at cooling/heating and the decrease of minimal critical stresses in 5-8 times down to 150-250MPa, in comparison with quenched crystals. This aging provides both high-strength properties of B2 phase (>2GPa) and also SE in temperature interval from 223K to 473K, which is the widest one between TiNi alloys. During SE the В2-В19' MTs are realized and the stress hysteresis does not depend on temperature and stresses.
The two-step aging at 823K, 1h + 673K, 1h was carried out to provide bimodal distribution of Ti3Ni4 particles: simultaneously with large particles the nanosized particles (30nm) are precipitated. Nanosized particles lead to the additional strengthening of B2-austenite, increase of MT temperatures and shift of SE interval to the high temperatures. At low stresses (<500MPa) and temperatures 273K÷315K the stress-induced R-B19' MT is observed and thermal and stress hystereses are reduced with temperature and stress. At T>315K and high stresses (>500MPa) the sequence of stress-induced MT changes to В2-В19' and the stress hysteresis becomes constant.
This work was supported by the Russian Science Foundation (No.18-19-00298).