Additively Manufactured NiTi and NiTiHf Alloys: Estimating Service Life in High-Temperature Oxidation

Area of Science:

• Materials Science
• Metallurgy
• Surface Engineering

Background:

• Nickel-Titanium (NiTi) alloys are known for their shape memory properties.
• High-temperature oxidation is a critical factor limiting the application of NiTi alloys in demanding environments.
• The role of alloying elements like Hafnium (Hf) in modifying oxidation behavior requires detailed investigation.

Purpose of the Study:

• To investigate the effect of Hafnium (Hf) addition on the high-temperature oxidation behavior of additively manufactured and cast NiTi alloys.
• To analyze the oxidation kinetics and oxide scale formation mechanisms.
• To determine the activation energy for oxidation of NiTi and NiTiHf alloys.

Main Methods:

• Thermogravimetric analysis (TGA) at 500, 800, and 900 °C in dry air for up to 75 hours.
• Post-oxidation analysis using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS).
• Kinetic analysis using logarithmic and parabolic rate laws.

Main Results:

• NiTiHf alloys exhibited a two-stage oxidation process: an initial rapid stage followed by a slower stage after approximately 10 hours.
• Oxidation kinetics followed a logarithmic rate law initially, transitioning to a parabolic rate law.
• Multi-layered oxide scales composed of Ti oxides, Hf oxides, and NiTiO3 were observed on NiTiHf alloys.
• Binary NiTi showed a significant increase in oxidation rate with temperature, fitting a parabolic rate law.
• Lower activation energy (60.634 kJ/mol) for additively manufactured NiTiHf compared to additively manufactured NiTi (175.25 kJ/mol) indicates enhanced oxidation resistance.

Conclusions:

• Hafnium addition significantly enhances the oxidation resistance of NiTi alloys at high temperatures.
• The formation of protective multi-layered oxide scales contributes to the improved oxidation behavior of NiTiHf alloys.
• The findings suggest NiTiHf alloys are more suitable for high-temperature applications compared to binary NiTi.