The urge to improve the properties of various materials to suit different applications have increased over the past years. Recently, development of high entropy alloys with single or multi-phase alloys have attracted significant attention of researchers. Among the phases formed by high entropy alloys, body centered cubic and face centered cubic are the most common ones. Generally, high entropy alloys exhibit unique properties that take into account the main effects including distorted lattice, entropy effects, cocktail effect and sluggish diffusion. Consequently, they have provided the opportunity for discovering and developing more high-performance alloys.
Just like the conventional super alloys, high entropy alloys have potential for numerous applications in turbine blades, disks and thermal protective sheets and this is attributed to their microstructural stability at very high temperatures. Despite the great efforts undertaken in improving the mechanical properties of high entropy alloys, little has been done about their oxidation resistance. Unfortunately, the oxidation resistance may prevent the application of the alloys at elevated temperatures if not properly investigated. Even though some high entropy alloys form protective complex oxides, other types form unprotective oxide layers due to linear oxidation kinetics. In a recently published literature, NbTiZrV alloy exhibited linear oxidation behaviors. It underwent oxidation due to lack of oxidation resistance.
Researchers at Institute for Complex Materials IFW Dresden in Germany: Dr. J. Jayaraj, Mr. Pramote Thirathipviwat, Dr. Junhee Han and Dr. Annett Gebert developed a cast AlNbTiZr high entropy alloy comprising of solid solution base centered cubic dendrite and inter-dendrite Zr2Al intermetallic phase. Their main objective was to investigate the microstructure, phase formation, oxidation behavior and mechanical compression properties of the AlNbTiZr refractory high entropy alloy. Their work is published in the research journal, Intermetallics.
The mechanical compression properties including the specific yield strength and fracture strength were impressive as compared to solid solution of HfNbTiZr. This was attributed to the advantage of modifying the microstructure by replacing the Hf with Al. Moreover, the AlNbTiZr is similar towards the light-weight property of the AlNbTiV high entropy alloy. The authors observed that the developed AlNbTiZr alloy exhibited high thermal stability at elevated temperatures. Interestingly, unlike the linear oxidation behavior of the HfNbTiZr high entropy alloy, the AlNbTiZr exhibited parabolic mass gain behavior due to the formation of protective oxide layers at different oxidation temperatures. Furthermore, the formation of thick complex oxides resulted in sluggish oxidation kinetics of AlNbTiZr high entropy alloy at an elevated temperature of 1273K.
The successful development of the new AlNbTiZr high entropy alloy exhibited improved mechanical and oxidation resistance properties as compared to the initial HfNbTiZr alloy. Therefore, it can be utilized in various structural and oxidation resistance applications. This will improve the efficiency and performance of such structures. Furthermore, the study by IFW Dresden scientists will pioneer further investigations and development of more efficient high entropy alloys.
Jayaraj, J., Thirathipviwat, P., Han, J., & Gebert, A. (2018). Microstructure, mechanical and thermal oxidation behavior of AlNbTiZr high entropy alloy. Intermetallics, 100, 9-19.Go To Intermetallics