Strengthening mechanisms and high-temperature oxidation properties of laser-clad TaNbZrTi refractory high entropy alloy coatings

In the quest to enhance the performance of conventional high-temperature alloys, the limitations of existing materials have become apparent. Refractory high entropy alloys (RHEAs) have emerged as promising candidates, exhibiting exceptional high-temperature properties and offering a novel perspective on advanced materials for extreme environments. This study systematically determined optimal process parameters via single-channel laser cladding (LC), resulting in the development of multi-channel TaNbZrTi RHEA coatings. The investigation encompassed an in-depth analysis of microstructure and oxidation behavior across a temperature range spanning from 800 to 1200 °C. The findings elucidated the complex composition of the coating, characterized by the interplay of BCC1, BCC2, and Fe2Nb phases. The theoretical yield strength of the coating was calculated at 1816.1 MPa, with contributions from solution strengthening, precipitation strengthening, fine crystal strengthening, and alloy foundation strength, standing at 41.1%, 41.5%, 5.5%, and 11.9%, respectively. At 800, 1000 and 1200 °C, the oxide thickness of the coating is 1.4, 2.9, and 34.5 μm, respectively. The transition of BCC2 phase at elevated temperatures and the diffusion of Fe from the substrate emerged as pivotal factors contributing to catastrophic oxidation observed at 1200 °C. Importantly, the diffusion of substrate elements facilitated the formation of Fe2Nb phase, which exhibited robust precipitation strengthening effects, further enhancing the coating overall strength. However, the protective role of Fe oxides proved inadequate, leading to the vulnerability of the oxidation layer and its propensity to fracture, thereby compromising the high-temperature oxidation resistance of the coating.