Hf-containing Alloys Research Articles

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, kp, 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 kp considerably. The addition of Hf also resulted in very adherent scales at all temperatures and decreased kp 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.

The influence of alloy microstructure on the oxide peg morphologies in a Co-10% Cr-11%Al alloy with and without reactive element additions

The addition of reactive elements such as yttrium or hafnium to heat-resisting alloys has marked beneficial effects on their high-temperature oxidation behavior, and in particular greatly improves the adhesion of the oxide scale to the substrate. In the case of alloys forming Al2O3 protective oxide scales, the improvement in adhesion appears to be related to the formation of intrusions of oxide into the metal—“pegs”—which hold the scale on. The morphology of these pegs is a function of alloy structure and of the reactive element. In particular, it appears that while the pegs of Hf-containing alloys grow as a result of a diffusion process, perhaps along the oxide-metal phase boundary, the pegs on Y-containing alloys seem to form too fast for this, and it is suggested that stresses established by the oxidation of the Co3Y inclusions lead to the formation of porous short-circuit paths.