Abstract
The study focuses on the superelastic effect in single-crystalline boron-doped Fe-based shape memory alloys. The homogenized and quenched single crystals were subjected to a heat treatment at 973 K for variable aging times. As a result, small and coherent nanometer-sized γ′ (Ni3Al-type) precipitates were formed. It was established that Fe-28Ni-17Co-11.5Al-2.5Ta-0.05B single crystals oriented along [001] direction exhibit the fully reversible superelastic behavior up to 14.3% compression strain at 77 K reaching the maximum theoretical value. The boron addition suppressed completely the formation of the brittle β phase and reduced the average precipitate size of the γ′ precipitates. Using high-energy synchrotron radiation and high-resolution transmission electron microscopy analysis the volume fraction and precipitate size of γ′ were determined indicating that both factors are critical in obtaining the largest superelastic reversibility. Boron addition counters the initial effect of mechanical stabilization which was detected in single crystals without boron. Unlike the thermally induced martensitic transformation, applied stresses produce a different austenite/martensite interface composed of interchanging austenite and martensite variants. It is also demonstrated that upon loading/unloading cycles the moving transformation front divides the material into three district regions i.e. single variant of austenite, austenite intermixed with martensite and single variant of martensite.
Highlights
Is a proper precipitation hardening which promotes forward and reverse stress-induced martensitic transformation (SIMT) [27,28,29,30,31,32]
No β phase was observed even after 24 h of aging. This indicates a large change as compared to NCAT single crystals where a significant amount of β (NiAl-type, B2 structure) was detected [21]
Similar to NCAT single crystals, the intensity of γ′ reflections is found to increase with increasing annealing time
Summary
Is a proper precipitation hardening which promotes forward and reverse SIMT [27,28,29,30,31,32]. The effect of boron on the superelastic strain and precipitation hardening in single crystalline materials is not fully understood. Another issue is a large mechanical stability [51,52,53]. The stabilization effect allows to reveal the interface between austenite and martensite upon loading and unloading cycles This provided more insight into the transformation mechanism and interface compatibility between both phases. The superelastic effect has been widely studied by many researches the exact transformation interface is still a subject of interest leaving some space for controversy This becomes important when the stress-strain asymmetry for compression and tension as well as the abnormal large reversible strains (higher than the theoretical transformation ones) are considered. A transition region composed of alternating austenite and martensite variants between single variant of austenite and single variant of martensite was unveiled
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