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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