Effects of high O contents on the microstructure, phase-transformation behaviour, and shape-recovery properties of porous NiTi-based shape-memory alloys

•Porous NiTi shape-memory alloys with 0.47–4.30 wt.% of oxygen were fabricated.•The volume of Ni2Ti4Ox phase increased with the oxygen content.•Different aging conditions leaded to different phase transformation behaviours.•Oxygen reduced the shape recovery rate while improved the elastic modulus.•Aging at a temperature above 600 °C was not recommended.

Porous NiTi-based shape-memory alloys (SMAs) were aged at different temperatures and holding times to study the effects of high O contents on the composition, microstructure, phase-transformation behaviour, and shape-recovery properties of SMAs. NiTi2 and Ni2Ti4Ox phases were precipitated during the aging process; the concentration was not significant until an aging temperature of 500 °C was applied for 2 h (O content = 1.87 wt.%). Aging the SMAs at 600 °C for 2 h produced a coarse NiTi2 phase and an O content of 4.3 wt.%. Ti atoms were consumed during the oxidation process, which caused the excess Ni atoms to dissolve into the matrix or generate Ni-rich phases, such as Ni4Ti3 or Ni3Ti. The presence of excess Ni atoms resulted in different phase-transformation behaviours. A high O content increased the elastic modulus of the porous NiTi-based SMAs, but decreased the shape-recovery rate. The samples aged at 600 °C exhibited inferior mechanical properties, and hence, it was concluded that high-temperature heat treatments should be avoided.

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