Introduction and Objectives: This study redefines the 1400 °C experiment as an over-temperature degradation test rather than a conventional service-temperature oxidation test, and evaluates the microstructural stability limit of conventional and germanium-modified aluminide coatings on Hastelloy X.
Materials and Methods: Ge-containing precursor layers were produced by CVD using 600 °C/4 h and 900 °C/2 h routes, followed by CVD aluminizing at 1020 °C for 3 h. The coated specimens were exposed at 1400 °C for 55 h and then characterized by mass-change measurement, cross-sectional BSE observations, semi-quantitative EDS, elemental mapping, and XRD.
Results: In the available dataset, ‎Al‎ and Ge900-Al showed net mass changes of -0.09687 and -0.09421 mg/cm², respectively, whereas Ge600-Al retained a positive net mass change of +0.05971 mg/cm². Ge900-Al showed clearer XRD evidence of retained β-NiAl, while Ge600-Al showed the most favorable mass-change response within the present 55 h window.
Conclusion: The results should be interpreted as a short-window over-temperature degradation response. The behavior of the coatings is controlled not only by Ge addition but also by the initial coating architecture, the continuity of Ge/Cr-bearing barrier-like regions, and the relative retention of β-NiAl. Because the test temperature exceeds the nominal melting range of Hastelloy X, the results are not intended to represent normal service oxidation behavior.