The influence of Hf content on oxide scale microstructure and HfO2 formation mechanisms in two-phase (γ' + β) Ni-Al alloys

Abstract

Hafnium is often doped into alumina-forming alloys to enhance their oxidation resistance, though the effect varies with the doping amount. In this paper, two-phase (γ' + β) Ni-30Al alloys with different Hf contents were prepared and the cyclic oxidation behaviour at 1100 °C was investigated. The oxidation rate increased remarkably with the increase of Hf content, because excessive Hf leads to severe internal oxidation. And it is interesting to note that oxide scales that formed on the alloys exhibited very different microstructures. On the Ni-30Al-0.1Hf alloy the scale mainly consisted of single α-Al2O3. But on the Hf over-doped alloys (Hf > 0.1 at.%), the scales microstructure varied with locations. The scale in-situ formed on β phase consisted of an outer spinel layer and an inner α-Al2O3 layer; while, the one formed on γ' phase consisted of an outmost NiO layer, a middle spinel layer dispersed with fine HfO2 particles and an inner α-Al2O3 layer. Moreover, Hf-rich oxide pegs were prone to be formed at scale/alloy interface on original γ' phase. However, when Hf content reached to 2 at.%, the pegs were also found at scale/alloy interface on original β phase. The different oxidation behaviour and peg formation mechanisms of the alloys are discussed.