Protective Alumina Scale Growth at 900 °C for a Ni- and Cr-Free Co-Base Model Alloy with γ/γ’-Microstructure: Synergistic Effects by Combining Shot-Peening and Halogenation

The oxidation resistance of novel γ/γ’-strengthened Co-base superalloys is clearly outmatched by their Ni-base counterparts within the high-temperature regime. Therefore, surface modification strategies to foster protective alumina growth seem auspicious. This study elucidates the impact of fluorination and shot-peening on protective alumina formation at 900 °C for a quaternary Co-base model alloy (Co-Al-W-Ta system) which is well known for an exceptionally low inherent oxidation resistance. Time-resolved isothermal gravimetric analysis (TGA) in synthetic air, detailed electron microscopic analysis, and X-ray diffraction (XRD) were used. For polished samples, no pronounced enhancement of oxidation resistance could be obtained by halogenation. However, in case of shot-peened samples (halogen-free), an increased tendency for alumina formation is found compared to polished surfaces. The very early stages of oxidation were identified to be especially crucial with respect to sustainable protective scale growth. Most noteworthy is the observation of a strong synergistic effect derived by a combination of halogenation and shot-peening, leading to significantly increased oxidation resistance.

STEM investigations of the influence of copper on alumina scale detachment during in-situ wetting experiments of Al-7Si-0.3Mg alloy with 95Sn-5Cu filler metal

Aluminum alloys have a strong tendency to form alumina layers on their surfaces when exposed to atmospheric air, even at room temperature. This is a severe challenge for brazing aluminum alloys as the alumina layer acts as a diffusion barrier and hinders the interactions between the filler metal and the base material. In order to achieve a good metallurgical bond between the filler metal and the aluminum alloy, it is of crucial importance to remove the alumina layer as well as to simultaneously prevent further oxidation of the aluminum alloy. The current investigation focuses on the detailed micro-structural changes that occur during in-situ brazing of liquid filler metal, 95Sn-5Cu (wt.%) on an aluminum alloy, Al-7Si-0.3Mg. These in-situ studies were performed in a large chamber scanning electron microscope in order to monitor the interactions of the filler metal and the base material, particularly the role of Cu on alumina detachment. After the in-situ experiments, the local surface and cross-sectional regions were analyzed by scanning electron microscopy in conjunction with energy dispersive X-ray spectroscopy to understand the variation in chemistry across the wetted region, which includes the interfacial region between filler metal and the base material. As the alumina scale present on the aluminum alloy is very thin (<50 nm), nanoscale characterization techniques such as transmission electron microscopy in scanning mode, including selected area electron diffraction for crystal structure determination, were performed. From this investigation, it was found that the Cu in liquid filler metal diffuses into the base material via the oxide layer, resulting in the formation of Al2Cu intermetallic precipitates.

Effects of Roughness on Stresses in an Oxide Scale Formed on a Superalloy Substrate

The effects of surface roughness on the stresses in an alumina scale formed on a Fecralloy substrate are investigated. Spherical indenters were used to create indents with different radii and depths to represent surface roughness and then the roughness effect was studied comprehensively. It was found that the residual stresses in the alumina scale formed around the rough surface are almost constant and they are dominated by the curvature rather than the depth of the roughness. Oxidation changes the surface roughness. The edge of the indent was sharpened after oxidation and the residual stress there was released presumably due to cracking. The residual stresses in the alumina scale decrease with increase in oxidation time, while the substrate thickness has little effect, given that the substrate is thicker than the alumina scale. Furthermore, the effect of roughness on the oxide growth stress is analysed. This work indicates that the surface roughness should be considered for evaluation of stresses in coatings.

The growth mechanism of oxide scale with Pt on NiCoCrAlY coating in water vapor at 1050∘C

In this study, we focus on the oxidation behavior of the oxide scale with Platinum (Pt) on free standing NiCoCrAlY coating in air with water vapor at high temperature. The Pt layer is deposited on the NiCoCrAlY coating surface by electroplating, and then heat treatment at 1000[Formula: see text]C with Argon (Ar) protection. Cycle oxidation of the oxide scale with Pt is conducted in air and air with water vapor at 1050[Formula: see text]C. The results indicated that the spinels (NiCr2O[Formula: see text] are formed on the surface of the alumina scale in air and air with water vapor. However, this kind of spinel is inhibited in the air with water vapor and the scale on the coatings with Pt presents a good adherence during oxidation process. The thermal stress and crack initiation and propagation will be discussed.