Effect of Y Additions on the Oxidation Behaviour of Novel Refractory High-Entropy Alloy NbMoCrTiAl at 1000 °C in Air

In this work, the influence of 0.5 at.% and 1 at.% Y additions on the high-temperature oxidation resistance of the equiatomic alloy NbMoCrTiAl at 1000 degrees C in air was investigated. Continuous isothermal and cyclic thermogravimetric experiments were conducted to characterize the oxidation kinetics of the alloys. Various analytical methods such as X-ray diffraction and electron microscopy with energy-dispersive X-ray spectroscopy were used to study the growth, morphology and composition of the oxide scales. It was found that the Y additions led to the formation of Al2Y phase along grain boundaries. The isothermal oxidation experiments of both Y-containing alloys showed lower oxidation rates after a short period of transient oxidation probably due to a reduction of fast-growing and voluminous Nb(2)O(5)oxides within the oxide layer. During the steady-state period, however, oxidation of Al2Y caused local stresses resulting in crack formation and subsequent breakaway oxidation, i.e. pronounce increase in the oxidation rates. The cyclic oxidation experiments revealed lower oxidation rates and thinner, more adherent oxide scales formed on the alloy NbMoCrTiAl1Y compared to NbMoCrTiAl. The increased scale adherence was attributed to the formation of pegs due to the oxidation of Al2Y phase at the oxide/metal interface.