Investigation of the nitrogen-nickel-titanium system: The isothermal section at 900 °C

Phase relations in the ternary system N-Ni-Ti have been established experimentally for the 900 °C isothermal section assisted by thermodynamic modeling. Phase equilibria are characterized by an appreciable amount of up to ≅11 at. % N dissolved in the octahedral interstices of the crystal structure of Ti₂Ni. Two-phase equilibria are formed at 900 °C among the pairs Ti₂N + Ti₂NiN_x, TiN_1-x + Ti₂NiN_x, TiN_1-x + TiNi_1-x, and TiN_1-x + Ni(Ti)_x. The investigation is based on X-ray powder diffraction, metallography, scanning electron microscopy, and electron probe microanalysis techniques on about 50 alloys, which were prepared by arc melting or high-frequency levitation melting of appropriate blends of Ti,Ni-powders with TiN used to introduce nitrogen. Key experiments related to the N solubility in the Ti₂Ni phase have been triggered by an interactively performed thermodynamic modeling. The experimental results are in fine agreement with the thermodynamic calculation.