Behavior of copper‐containing high‐entropy alloys in harsh metal‐dusting environments

Metal dusting is still an unresolved issue at high temperatures. Currently, two material‐related strategies to mitigate metal dusting are described in the literature. On the one hand, highly alloyed materials are used, which contain large amounts of protective oxide‐forming elements, such as Cr, Al, and Si. The second mitigation strategy is based on inhibiting the catalytic effect of Fe, Ni, and Co. These elements all strongly catalyze the formation of solid carbon from the gas phase. Combining the catalytic protection of Cu alloying for metal dusting with protection by a classical alumina/chromia barrier is a native feature that high‐entropy alloys (HEAs) can offer. In this study, the behavior of different equiatomic HEAs with and without Al and/or Cu are studied when exposed at 620°C in a highly aggressive metal‐dusting environment

Pt accelerated coarsening of A15 precipitates in Cr-Si alloys

The effect of alloying Cr-rich Cr-Si alloys with Pt was investigated by a combination of complementary experimental methods and atomic scale modelling. The investigated Cr-Si and Cr-Si-Pt (Cr ⩾86 at.%) alloys developed a two-phase microstructure consisting of Cr solid solution (Crss) matrix and strengthened by A15 precipitates during annealing at 1200\ub0C. It was found that additions of 2 at.% Pt increase the coarsening rate by almost five times considering annealing times up to 522 h. Pt was found to change the precipitate matrix orientation relationship, despite its low influence on the Crss matrix/A15 precipitate misfit. Through this experimental and modelling approach new insight has been gained into mechanisms of enhanced coarsening by Pt addition. The increased coarsening is principally attributed to a change in interface composition and structure resulting in different thermodynamic stabilities: Pt-containing A15 phase was found to have a broader compositional range if both elements, Pt and Si, are present compared to only Si. Additionally, the Crss phase was found to have a higher solubility of Pt and Si over just Si. Both factors additionally facilitated Ostwald ripening

Development of a metal dusting resistant functional coating by Sn and Al pack cementation

A new coating against metal dusting is developed by the combination of a catalytic inhibition approach with a classical oxide barrier. Coatings were manufactured by co-deposition of tin and aluminum on nickel plated alloy 800H via powder pack cementation with different dwell times. The resulting coatings consist of two phases, a nickel-aluminide phase as oxide forming reservoir and an underlying nickel-tin phase which catalytically inhibits carbon deposition. Coated samples were exposed to H2-22%CO-10%H2O as a laboratory metal dusting environment as well as air at 620 °C. Cross sectional analysis revealed no sign for metal dusting and the formation of a thin alumina layer at the surface

Oxidation behaviour and related microstructural changes of two β₀–phase containing TiAl alloys between 600 °C and 900 °C

A comparative study on the isothermal oxidation behaviour of the two β0–containing titanium aluminide alloys GE 4822 and TNM-B1 in the HIPed condition has been conducted at temperatures between 600 °C and 900 °C for up to 1000 h in air. The two alloys exhibit clear differences in their oxidation behaviour, however, both suffer from undesirable embrittlement due to oxidation after exposure for 100 h even at 600 °C. Phase transformations in the subsurface zone were shown to have a strong impact on the mechanical properties at room temperature

Influence of copper and aluminum substitution on high-temperature oxidation of the FeCoCrNiMn “Cantor” alloy

In this study, the oxidation behavior of FeCoCrNiMn (HEA + Mn) is compared to three modified HEAs manufactured by substituting Mn with Al, Cu, or Al + Cu. Oxidation tests were conducted between 600°C and 800°C for up to 500 h in synthetic air. Substitution of Mn leads to a significant improvement in the oxidation resistance for the three modified HEAs. For FeCoCrNiCu (HEA + Cu), a local attack of a Cu-rich phase was observed, leading to the formation of CuO blisters on the surface. The FeCoCrNiAl (HEA + Al) alloy was characterized by the formation of a thin Al2O3 surface layer for all temperatures. However, for the HEA + Al alloy the formation of AlN was observed after 300 h at 800°C, leading to a partial breakdown of the protective scale. FeCoCrNiCuAl (HEA + Cu + Al) by far showed the best oxidation resistance, characterized by the formation of a highly protective Al2O3 scale that effectively inhibited nitrogen penetration into the metal subsurface and local attack of the Cu-rich phase.This article is published as Bürckner, Mary‐Lee, Lukas Mengis, Emma MH White, and Mathias C. Galetz. "Influence of copper and aluminum substitution on high‐temperature oxidation of the FeCoCrNiMn “Cantor” alloy." Materials and Corrosion 74, no. 1 (2023): 79-90. DOI: 10.1002/maco.202213382. Copyright 2022 DECHEMA. Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0). Posted with permission. DOE Contract Number(s): AC02-07CH11358

High-temperature oxidation behavior of polymer-derived SiHfBCN ceramic nanocomposites

Within this study, the oxidation behavior of SiHfBCN ceramic powders and monoliths was studied at temperatures from 1200 to 1400 degrees C. Both powder and monolithic samples exhibited parabolic oxidation behavior characterized by very low rates (10(-9)-10(-8) mg(2) cm(-4) h(-1) The activation energy of 112.9 kJ mol(-1), which was determined for the SiHfBCN powder, is comparable to that of other silica formers such as silicon or SiC and relates to the diffusion of molecular oxygen through silica scale. Whereas, the values determined for the SiHfBCN ceramic monoliths (174 and 140 kJ mol(-1), depending on the Hf content) indicate the complex nature of their oxidation process, leading at temperatures below 1300 degrees C to a continuous oxide scale consisting of borosilicate, silica, m-and t-HfO2. At higher temperatures, the oxide scale consists of silica, HfSiO4 as well as m-and t-HfO2 and becomes discontinuous, probably due to the evaporation of boria. (C) 2015 Elsevier Ltd. All rights reserved