On the Role of Entropy-'Excited' Surface Layers in the Formation of High Temperature Corrosion Resistant Barrier Oxide Scale on Fe-Cr-Al-La Alloy

摘要

Using scanning electron microscopy, Auger electron spectrometry, fast electron diffraction "on reflection" regime and wavelength dispersive spectrometry, complex investigations were carried out on the hierarchical sequence of entropy-"excited" surface (Beilby layer) transformations into the protective thermal-barrier and tribological oxide scales. The later was developed due to the self-organizing and dissipative processes on the surface of a high temperature resistant Fe-Cr-AI-La system alloy with the high (>40%) chromium content. We established that, the surface and near surface layers of the mechanically brilliant polished specimens of a Fe-44%Cr-4%Al-0.3%La alloy consisted of an amorphous Beilby layer and an adjacent matrix layer distorted due to the plastic deformation as entropy "excited" functional system. At a temperature of 1200°C this system transforms into a surface layer with a micro-wrinkled modulated scale structure having an uniform thickness in the form of a mixture of nanocrystallites. The latest is composed out of oxides of all those elements found in the alloy matrix. Together, with increasing oxidation temperatures there take place the regrowth of nanocrystallites and also the recrystallization processes, accompanied by solid-phase reactions between oxide nano-particles. The mentioned processes lead to scale delamination and the formation of a thin uniform Al_2O_3 layer on the alloy matrix. Developed alumina is characterized by high adherence with the alloy substrate and by its protective features against both high temperature (1200°C) corrosion of matrix and abrasion. By the pretreatment at 1200°C of investigated alloy surface, a low thickness scale which has an ultra fine grain size (～1 μ) with no porosity, and blocked grain boundary short-circuit diffusion paths is formed. The characteristics of this scale allow it to protect the alloy substrate against high temperature corrosion and to easily relax the oxide strains caused by the cycling at high temperatures and mechanical loading.