Abstract
The ATI 718Plus® is a creep-resistant nickel-based superalloy exhibiting high strength and excellent oxidation resistance in high temperatures. The present study is focused on multiscale 2D and 3D characterization (morphological and chemical) of the scale and the layer beneath formed on the ATI 718Plus superalloy during oxidation at 850 °C up to 4000 h in dry and wet air. The oxidized samples were characterized using various microscopic methods (SEM, TEM and STEM), energy-dispersive X-ray spectroscopy and electron diffraction. The 3D visualization of the microstructural features was achieved by means of FIB-SEM tomography. When oxidized in dry air, the ATI 718Plus develops a protective, dense Cr2O3 scale with a dual-layered structure. The outer Cr2O3 layer is composed of coarser grains with a columnar shape, while the inner one features fine, equiaxed grains. The Cr2O3 scale formed in wet air is single-layered and features very fine grains. The article discusses the difference between the structure, chemistry and three-dimensional phase distribution of the oxide scales and near-surface areas developed in the two environments. Electron microscopy/spectroscopy findings combined with the three-dimensional reconstruction of the microstructure provide original insight into the role of the oxidation environment on the structure of the ATI 718Plus at the nanoscale.
Highlights
Water vapor is not found solely in the Earth’s atmosphere; it is a constituent of many gaseous environments closely associated with human activity
It is occasionally observed that the adhesion of the Cr2 O3 scale, which is formed on corrosion-resistant steels and nickel-based superalloys, to the alloy substrate improves in the presence of water vapor despite its increased growth rate [1,2,3,4,5,6]
It was found that up to approximately 1500 h, the mass gain recorded for the 718Plus sample oxidized in wet air was higher than for the sample oxidized in dry air by approximately 20%
Summary
Water vapor is not found solely in the Earth’s atmosphere; it is a constituent of many gaseous environments closely associated with human activity. It is occasionally observed that the adhesion of the Cr2 O3 scale, which is formed on corrosion-resistant steels and nickel-based superalloys, to the alloy substrate improves in the presence of water vapor despite its increased growth rate [1,2,3,4,5,6]. This is caused by a change in the mechanism that underlies the transport of reagents in this scale—when the inward diffusion of oxygen is predominant, a fine-grained Cr2 O3 scale with excellent adhesion to the bulk metallic material forms. Corrosion in pure steam, which occurs on the inner surface of power boiler waterwalls and in gas turbines, among others, is a notable case
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