Application of High-Entropy Theory to Structural Steels Utilizing Refractory High Entropy Alloy Design and Characterization

In order to find a superior alternative to aerospace steels and super alloys, a comprehensive computational and experimental study was carried out to investigate application of high entropy theory to structural steels for jet engine components. Candidate refractory high entropy alloys for jet engine components were determined as TiTaHfNbZr, TiTaHfMoZr, TiTaHfNb and Ti_1.5Ta_0.5Hf_0.5Nb_0.5Zr and their phase diagrams were constructed using Thermo-Calc Software. In the critical operating temperature range of 800-900 °C, only TiTaHfNbZr, TiTaHfMoZr and TiTaHfNb HEAs showed stable single phase. The single phase predicted by the Thermo-Calc software was also observed by SEM and XRD analyses. Microstructural analyses demonstrated that the initial microstructures of the three HEAs show clear resemblance to high-manganese steels. Charpy impact energies of TiTaHfNbZr, TiTaHfMoZr and TiTaHfNb were 0.25 J, 0.10 J and 14.8 J respectfully. These impact energies are in line with uniaxial compression tests which demonstrate that TiTaHfNbZr exhibits remarkable strength, TiTaHfNb shows better formability and the TiTaHfMoZr has similar yet relatively inferior properties to those of the TiTaHfNbZr. Average hardness values of the TiTaHfNbZr, TiTaHfMoZr and TiTaHfNb were 6.5 GPa, 6.5 GPa and 3.5 GPa respectfully. Post-mortem micrustructural analyses revealed that TiTaHfNbZr gets its high strength from co-existence of brittle and ductile phases and plastic deformation was facilitated by slip only as evident by the dense dislocation activities. High temperature oxidation tests showed that TiTaHfMoZr was pested but the remaining HEAs were more resistant to corrosion up to 800°C. However, since TiTaHfNb formed a protective surface layer, it showed better oxidation resistance. High temperature compression tests presented that strength levels of the TiTaHfNb and TiTaHfNbZr HEAs remain similar despite the change of temperature from room temperature to 600 ⁰C. Superior alternatives to steels can be utilized by the formation of a stable phase similar to HEAs in a variety of steel grades by applying the HEA theory. So, a HEA-like steel composition which will ensure high strength at elevated temperatures and low temperature ductility and formability can be achieved.