Oxidation of alloys of the system Ni−Ta. I. Oxidation of the intermetallic Ni₃Ta

Abstract

Oxidation kinetics for the intermetallic Ni₃Ta in air at 600–1000°C are studied by a thermogravimetric method. The alloy has an ordered crystal structure (D¹³ _2h-Pmmn) with rhombic lattice parameters a=0.512 nm, b=0.423 nm, and c=0.452 nm. The kinetic isotherms of Ni₃Ta oxidation are described by a parabolic equation. With t≤800°C there is a periodic increase in the rate constant of parabolic oxidation, but with t>800°C there is a periodic decrease of it. In the range 850–875°C the alloy oxidation rate decreases as a result of scale sintering. Oxygen diffusion slows down in the compact scale. X-ray and metallographic analysis of the scale that forms on Ni₃Ta indicates that it contains NiO, NiO·Ta₂O₅, Ta₂O₅ and also Ni and the solid solution Ni(Ta). These phase components are distributed in layers of the scale: NiO (first), NiO+NiO·Ta₂O₅ (second), NiO·Ta₂O₅+Ni+Ta₂O₅ (third), Ta₂O₅+Ni(Ta) (fourth). With a low temperature and short periods of heating there is no Nio·Ta₂O₅ or Ni(Ta) in the scale. Oxidation of Ni₃Ta is controlled by oxygen diffusion in the scale over the direction towards the alloy. With t>850°C this mechanism changes. By analogy with oxidation of tantalum it is assumed that structural changes in the Ta₂O₅ lattice may be responsible for this.