Effect of ageing in austenite and martensite on superelasticity of the [001]-oriented FeMnAlNi single crystals

Effect of ageing conditions (ageing in the high-temperature phase (austenite) in free state and under stress and ageing in martensite under stress, ageing temperature and time) on the development of stress-induced BCC-FCC martensitic transformation (MT) and superelasticity (SE) was studied in the FeMnAlNi single crystals along the [001] direction under compression. It is shown that the precipitation of nanosized particles of the β-phase (ageing in austenite for 3 and 6 h at 473 K in a free state and under stress and ageing in martensite under stress for 3 h at 473 K) leads to an increase in the stress level for the onset of stress-induced BCC-FCC MT σ _cr and to the creation of crystals with high-temperature SE at temperatures above 373 K. SE is observed in a wide temperature range: from 273 to 473 K in quenched crystals; from 203 to 573 K after ageing in austenite at 473 K for 3 and 6 h in a free state and under stress and after ageing in martensite under stress at 473 K for 3 h. The maximum SE is 6.3–8.3% and 6.2% after ageing in austenite and in martensite under stress, respectively. It was shown for the first time that after ageing in martensite under stress (when transformation strain ε _tr=3%) for 3 h at 473 K, the SE curves are characterized by two stages, which are not observed after ageing in austenite under stress. The influence of particles on the σ _cr and the mechanical hysteresis Δσ is discussed.