Assessing Refractory High Entropy Alloys for Potential High Temperature Applications

As the demand for higher turbine inlet temperatures increases, there is an ever increasing need to develop new materials capable of operating in these high temperature environments. Traditionally superalloys, Nickel or Cobalt-based, have been used in the hot sections of gas turbine engine. However, as operational temperatures begin encroaching on the upper temperature limits of superalloys, new classes of materials are being explored as potential replacements. One such class of materials is the High Entropy Alloy, a metal alloy composed of four or more principal alloying elements. This study will present the results from compression testing at room and elevated temperatures of high entropy alloys developed for potential high temperature applications. These alloys were specifically designed using refractory elements with high melting temperatures, such as tungsten and tantalum, as major components of its composition to increase the melting temperature of the resultant alloy, with the objective to elevate its potential operational envelope, while also including relatively soft elements, such as niobium, to ensure the alloys ductility. These results will be compared to yield strengths reported for existing superalloys, as well as other high entropy alloys, from literature. Additionally, this work will include a discussion of the difficulties observed during testing due to the alloy’s unique properties/behavior and their implications on the tested materials’ practicality. Some notable difficulties include the difficulty in manufacturing test coupons, and excessive oxidation resulting in early sample failure for high temperature tests conducted outside of a vacuum.