Alumina Failure and Post-failure Oxidation in the NiCoCrAlY Alloy System at High Temperature

Abstract

This paper examines the oxidation behavior of thin specimens of cast NiCoCrAlY alloys at 1150 • C through successive stages, from Al 2 O 3 growth to complete alloy conversion to oxide. Five alloy compositions were used, with varying fractions and compositions of γ and β. The time evolution of the alloy composition during Al 2 O 3 growth was simulated using the DICTRA module of Thermo-Calc, and calculated analytically in the approximation of flat profiles. Simulated and experimental profiles were found to be in good agreement, indicating that the phase equilibrium and mass balance were correctly reproduced in the simulations. Local variations of alloy composition were observed in thinner specimens and found to be comparable with the variations expected from the uncertainty on the initial specimen thickness. The variations observed in the time-to-Al 2 O 3 failure were greater than expected on this basis, suggesting that additional sources of variability were in effect. Alumina failure was followed by the growth of a Cr 2 O 3 layer at the alloy-scale interface. Similarly, Cr consumption eventually led to Cr 2 O 3 failure, and Ni-and Co-containing spinel oxide formed, converting the Cr 2 O 3 at the alloy-scale interface and the Al 2 O 3 at the scale-gas interface. The remaining NiCo alloy was then converted to (Ni,Co)O. This sequence occurred without abrupt increase of the mass gain, due to the continued presence of the remnant Al 2 O 3 layer, and to the small amount of metal left to oxidize when the (Ni,Co)O eventually broke through the scale. The evolution of the scale composition throughout the oxidation stages is discussed based on an analysis of the thermodynamic conditions at the alloy-scale interface.

Size effects on high temperature oxidation of MCrAlY coatings processed via APS and HVOF depositions

“…The formation of the Al 2 O 3 scale and intrusions, together with the interdiffusion with the substrate, causes aluminum depletion in the coating. In thin products, this may bring the Al concentration down to a critical level where Al 2 O 3 is no longer stable [30,[32][33][34][35][36][37][38][39]. Other oxides may then form at the oxide/gas and metal/oxide interfaces on large regions.…”

“…Other oxides may then form at the oxide/gas and metal/oxide interfaces on large regions. This evolution of the protective Al 2 O 3 scale is called intrinsic chemical failure (InCF [34]) and is not necessarily associated with an increase of the mass gain [30,33,35,38]. Mechanical, thermal cyclic and thermal transient loading promotes mechano-chemical coupling and the onset of early strain-and oxidation-assisted damage and cracking/spallation of the external oxide scale [40][41][42].…”

“…In order to document the intrusive oxidation within APS and HVOF materials, the oxidation behavior and lifetime of both the projected materials were directly compared to the ones of a -defect-free‖ cast material with a similar chemical composition solely developing and external Al 2 O 3 scale (see specimen UT2 in Ref. [38] for additional information on the microstructure, oxidation behavior and lifetime).…”

Modeling Oxidation of AlCoCrFeNi High-Entropy Alloy Using Stochastic Cellular Automata

“…In computational material space, the oxidation modeling of alloys is traditionally performed by using thermodynamic analysis, first-principles energetic calculations, atomistic modeling [ 25 , 26 ], and continuum-level modeling. A large part of the computational efforts on modeling alloy oxidation has been restricted to binary alloys, although recent efforts have focused on ternary systems [ 21 , 27 ], and even HEAs, as well [ 28 , 29 ]. In a nutshell, CALPHAD-based modeling and ab initio modeling on the oxidation of alloys provide insight into the reaction and thermodynamic behaviors of oxides and their interaction with the unoxidized alloy [ 30 ].…”

Simulation of diffusion with non-equilibrium vacancies, Kirkendall shift and porosity in single-phase alloys

“…Diffusion simulations have become widely used to predict composition evolutions in high temperature materials, e.g. in alloy-coating systems or in alloys subject to selective oxidation [1][2][3][4][5][6]. In a substitutional alloy subject to vacancy-mediated diffusion, a composition gradient will generate diffusion, which in turn may have consequences associated with the Kirkendall effect [7][8][9][10].…”