Unusual precipitation and its hardness enhancement in Fe-alloyed 60NiTi alloy

•An unusual precipitation phenomenon is observed in Fe-alloyed 60NiTi.•The unknown Ni-rich nano-size phase is formed due to the addition of Fe.•Fe alloying considerably increases the hardness of 60NiTi. In this work, the microstructure and hardness of Fe-alloyed 60NiTi alloys, (Ni55Ti45)100-xFex (x = 0.5, 1, 3, 5 at. %), were studied. The addition of Fe leads to an unusual precipitation phenomenon that significantly influences the microstructure of the studied alloys. In the Fe-alloyed samples, an unknown secondary phase (X phase) precipitates with bcc crystal structure and nano-scale size. The X phase that evolves as thin plates in a densely dispersed manner in the microstructure brings about a hardening effect similar to Ni4Ti3. The hardness of 60NiTi is considerably increased owing to the microstructural evolutions resulting from the addition of Fe, with a maximum value of 717 HV at 5 at.% Fe content in the alloy.