Oxidation resistance of aluminized refractory HfNbTaTiZr high entropy alloy

In the present study, an equimolar HfNbTaTiZr refractory high entropy alloy was subjected to powder-pack aluminizing at 950 degrees C for 4 h. High-temperature isothermal oxidation tests were carried out in open air on both as-cast and aluminized samples at 1000 degrees C for the durations of 5, 25, and 125 h. Microstructural analysis and oxidation kinetics revealed the formation of a complex oxide layer consisting of (Hf,Zr)O2 and Al2O3 with AlN inclusions in the aluminized samples following oxidation. Significant cracking of the complex oxide layer occurred after the prolonged exposure of 125 h. The as-cast samples followed similar oxidation kinetics under all the conditions. However, the magnitude of mass gain for the aluminized samples was 1.18, 1.55, and 3.60 times less than the as-cast samples after 5, 25, and 125 h of oxidation, respectively. The lower mass gain in the aluminized samples indicated the formation of an adherent oxide layer on the surface of the alloy and that the molar volume of the oxide equaled or exceeded the molar volume of the alloy. As a result, the aluminized alloy was shielded from direct contact with gaseous oxygen and oxidation rate was governed by diffusion through the oxide layer.