Design of Refractory High Entropy Alloys for Extreme Environment by using CALPHAD

High-temperature structural materials for a combustion region of gas turbine engine, such as turbine blades and liners have been researched continuously to increase their heat resistant temperature during operation. Ni-based superalloys are a typical conventional material, and a heat resistance temperature of 1100 degrees Celsius has been achieved in a Ni-based single crystal superalloy (TMS-238). Although Ni-based super alloys are sophisticated materials and a still candidate for advanced heat resistant components, the melting point of Ni is 1455 degrees Celsius, and the improvement of the applicable temperature of Ni-based superalloys is approaching its limit. Therefore, a new concept of material is needed to further improve the operating temperature.<br/>We are focusing on High-Entropy Alloys (HEAs) composed of refractory metals (hereafter denoted as RHEAs). High-Entropy Alloys are composed of 5 or more elements and their configuration entropy exceeds 1.5R (R: Gas constant, 8.314J/mol K). Since lattice distortion and unexpected interactions occur, it is expected that HEAs has unique properties compared to conventional alloys.<br/>We have designed and fabricated TiZrHfNbX (X = Ta, Cr) alloys (hereafter denoted as based-RHEAs) and evaluated their oxidation behavior. It was found that the fabrication of the alloys by arc melting resulted in a single phase of Nb remaining in the alloys, and the remining of Nb caused rapid oxidation of alloys. Based on these evaluation results, we performed a thermodynamic equilibrium simulation (by FactSage 8.1) and succeeded in increasing the homogeneity of the alloy by preparing a Ti1.25ZrHfNbCr2 alloy with an increased Ti and Cr content.<br/>The Ti1.25ZrHfNbCr2 alloy was fabricated using the arc melting method by the same condition for based-RHEAs. Oxidation behaviors were evaluated and compared to the result of based-RHEAs. Thermogravimetric analysis (TGA) and isothermal oxidation tests at ~1200 degrees Celsius in air showed that the weight gain and the thickness of oxide scale for Ti1.25ZrHfNbCr2 were smaller than those of based-RHEAs which is equimolar composition with the highest entropy in the same element system.<br/>Observation of the oxide scale revealed that it was divided into two layers. In the outermost layer, multiple complex oxides (e.g., CrNbO4-like structure, Zr6Nb2O17-like structure (from the result of X-ray diffraction)) were densely intermingled. Similarly, complex oxides (e.g., Hf6Nb2O7-like structure) were formed in the inner layer. These results suggest that the formation of these complex oxides and the structure of the scale probably have acted as a barrier to oxygen diffusion, leading to the suppression of oxidation.