Origin of age softening in the refractory high-entropy alloys

A graphical representation of the TKD analysis of boundary precipitates

Refractory high-entropy alloys (RHEAs) are emerging materials with potential for user under extreme conditions.  As a newly developed material system, a comprehensive understanding of their long-term stability under potential service temperatures remains to be established.

In the paper 'Origin of age softening in the refractory high-entropy alloys' published in Science Advances, Dr Junliang Liu headed a team made up of this department, the University of Twente and the University of Wisconsin-Madison, to examine a titanium-vanadium-niobium-tantalum alloy.  This is a promising RHEA known for its superior high-termperature strength and room-temperature ductility.  

Using a combination of advanced analytical microscopies, Calculation of Phase Diagrams (CALPHAD) software and nanoindentation, the authors investigated the evolution of its microstructure and mechanical properties upon aging at 700oC.  Trace interstitials such as oxygen and nitrogen initially contributed to solid solution strengthening, and promoted phase segregation during thermal aging.  As a result of the depletion of solute interstitials within the metal matrix, a progressive softening was observed in the alloy as a function of aging time. 

The paper underscores the need for better control of impurities in future development and application of RHEAs.