The γ' solvus temperature, the microhardness and the martensitic transformation temperatures were examined for the Fe-Ni-Co-Al-based shape memory alloys. It was found that Fe-28Ni-17Co-11.5Al-2X (X=Ti, Nb and Ta) alloys have a relatively high γ' solvus temperature and that they show high hardness when aged at 600 °C, especially in the case of Ta alloys. Alloy compositions to obtain a high hardness, a small transformation hysteresis and complete reversible transformation were evaluated as being Ni>28%, Co>13%, Al>10% and Ta>2.5%, and the most suitable aging temperature was 600 °C. Grain boundary precipitation of the NiAl-B2 phase was found to be suppressed by the addition of over 0.03 at.% boron.
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