The effective low-temperature stress-induced martensite ageing (SIM-ageing) at T=420-500 K under applied compressive stress of 500-700 MPa is carried out to improve the functional properties of B2-oriented single crystals of NiFeGaCo ferromagnetic shape memory alloys.
The SIM-ageing leads to an increase of martensitic transformation temperatures by 50-70 K, and 2-fold decrease in the values of thermal and stress hysteresis as compared to as-grown crystals. As the result, the SIM-aged crystals with a narrow stress hysteresis and high cyclic stability of superelasticity at elevated temperature up to 500 K are attractive for efficient high-temperature actuator.
For the first time it is shown that SIM-aged -oriented NiFeGaCo crystals demonstrate the two-way shape memory effect and rubber-like behavior in martensite. The change in the loading axis at SIM-aging from B2 to B2 directions give the possibility to control the magnitude and direction of internal stress fields and the reversible strain. The SIM-aging of NiFeGaCo single crystals under compression stress along B2 direction induces, firstly, the two-way shape memory effect with a reversible tensile strain up to + 8.5% along the B2 direction at stress-free cooling/heating cycles. Secondly, the rubber-like behavior with reversible compression strains up to -7.0% along the B2 direction at loading/unloading cycles in martensite. Such excellent functional properties can actually be exploited to design the thermal and magneto-controlling actuators based on NiFeGaCo single crystals.
This work was supported by the Russian Science Foundation (grant No. 16-19-10250).