Isothermal oxidation behavior of Ni3Al-0.1B base alloys containing Ti,Zr, or Hf additions

The isothermal oxidation behavior of Ni₃Al-0.1B and of this alloy containing additions of approximately 2% Ti, Zr, or Hf was studied in purified oxygen at atmospheric pressure over the temperature range 1300 to 1500 K and for periods up to 400 ks. Ni₃Al was also studied similarly for comparison. The oxidation of Ni₃Al and Ni₃Al-0.1B resulted in an extensive formation of geometric voids on the substrate surface leading to poorly adherent scales. The addition of B to Ni₃Al did not change the oxidation behavior much, however, both the activation energy of oxidation and the relative thickness of the Al₂O₃ layer in the scale were increased. The addition of Ti led to the formation of adherent scales at 1300 K, but the oxidation was accelerated after an initial period at 1300 and 1400 K. At higher temperatures the scale was protective, and the parabolic rate constant, k_p, decreased, however, the scale adherence was impaired by the formation of interfacial voids. The addition of Zr resulted in very adherent scales at all temperatures, but at the same time it increased k_p considerably. The addition of Hf also resulted in very adherent scales at all temperatures and decreased k_p except at 1300 K. The improved scale adherence can be accounted for by the keying effect, since the additives resulted in roughened scale/alloy interfaces. A complex of oxide particle and an associated void was found on the exposed surface of the Hf-containing alloy. This supports the vacancy-sink mechanism.