Abstract
In recent years, the interest in shape memory alloys with high transformation temperatures (HTSMA) has significantly increased not only in the academic sphere, but also due to a market-push, especially from aerospace, automotive, and robotics. However, a real break-through has not been achieved yet because of several problems: Lower chemistry control than in NiTi alloys, instability of martensite, decomposition of the matrix phase, brittleness, oxidation, etc. Despite those challenges, steady progress has been made on specific systems and distinct thermo-mechanical treatments have been elaborated to reach promising HT-SMA behavior. Among several systems, Ni-Ti-Hf-Nb emerged as particular promising due to superior mechanical properties of its alloys compared to those of the ternary Ni-Ti-Hf system. In particular, the ductility of Ni-Ti-Hf increases significantly by adding Nb, allowing for formation of a soft eutectic network composed of shape memory matrix phase and a not transforming β-Nb phase at the grain boundaries. Recently, it was demonstrated that by this alloying approach, it is possible to obtain fine wires using conventional drawing techniques, increasing the opportunities for a successful industrial commercialization. In this work, Laser technology was explored for straight annealing of HT-SMA wires and promoting shape memory behavior. The functional performances of laser treated wires were assessed through differential scanning calorimetry (DSC), microstructure analysis, tensile tests and thermal cycling under constant load. Laser parameters were optimized for tailoring transformation properties in order to get a stable transformation at high temperatures, the results were compared to those ones of furnace annealed wires.
