Superior oxidation behaviors of stable nano-grain-sized coating layers produced via sequential two-step pack cementation coatings by B and Si of TiTaNbMoZr high-entropy alloys

In this study, Si-pack cementation coating and sequential B-Si-pack cementation coating were applied to TiTaNbMoZr alloy to investigate the resulting microstructural changes and oxidation resistance. The as-cast alloy underwent severe oxidation at 1300 °C, and the Si-pack cementation-coated alloy also exhibited crack formation due to the oxidation of Zr-rich XSi 2 to ZrO 2 within the XSi 2 layer, resulting in compromised coating integrity. In contrast, the B-Si-pack cementation-coated alloy formed a surface layer composed of XB 2 , XSi 2 , and X 5 SiB 2 , which remained structurally stable during oxidation and consequently exhibited enhanced oxidation resistance. The as-cast alloy and the Si-pack cementation-coated alloy exhibited substantial mass gains of 62.8 mg cm -2 and 55.6 mg cm -2 , respectively, after oxidation at 1300 °C for 10 h. Conversely, the B-Si-pack cementation-coated alloy demonstrated markedly lower mass gain, recording only 4.7 mg cm -2 under identical conditions. Even after 20 h of oxidation, the mass gain remained low at 9.85 mg cm -2 . Furthermore, once the initial protective oxide layer formed, the parabolic rate constant was determined to be as low as 1.47 mg cm -2 h -1/2 . These results confirm the potential of high-entropy alloys (HEAs) for use in high-temperature environments and emphasize the critical role of selecting suitable coating strategies tailored to the alloy composition.