1. High Entropy Alloys Ryan Oliver, University of Wisconsin – Milwaukee Dr. Abraham Munitz, Colorado School of Mines Dr. Michael Kaufman, Colorado School of Mines
Introduction
High entropy alloys are a fairly new area of research that explores that potential of nontraditional alloy systems. Typical alloys are made up of a base metal, which makes up the majority of the composition, combined with small amounts of alloying elements. High entropy alloys on the other hand, are comprised of five or more principle elements with between 5% and 35% of each by atomic concentration. These alloys have a tendency to form simple structures, like fcc and bcc, instead of intermetallic compounds. This is because the high entropy of mixing reduces the free energy of the system and stabilizes these simple structures.
By changing the elemental composition, the number of elements, or the ratio of each element in the alloy, practically limitless alloys can be designed with a wide range of properties. In this study, three similar alloys, each differing by only one element, are compared in physical properties and microstructure. The composition of the three alloys is given as follows.
HEB6 – AlCrGaTiV HEB7 – AlFeGaTiV HEB8 – AlCuGaTiV
The resulting XRD plots are inconsistent with those of an alloy containing a simple structure such as fcc or bcc.
Conclusion
Significant changes can be seen in high entropy alloys by changing a single element in the composition. HEB6 and HEB7 are distinctly harder and more brittle than HEB8, and have much simpler microstructures. None of the three alloys formed simple solid solutions. A more thorough analysis of potential heat treatments to each alloy, especially HEB8, needs to be done in order to fully asses the possibilities of these alloys.
References
J.-W Yeh, Y.-L Chen, S.-J Lin, S.-K Chen: “High Entropy Alloys – A New Era of Exploitation.”
Yong Zhang, Yunjun Zhou: “Solid Solution Criteria for High Entropy Alloys.”
Acknowledgments
This material is based upon work supported by the National Science Foundation and the Air Force Office of Scientific Research under Grant No. DMR
Looking at the DSC data, one can see that all three alloys have high temperature peaks associated with phase transformations. In particular, HEB8 has several peaks including peaks after melting has occurred. Further analysis of these alloys can be done by heat treating samples of each alloy to the temperature associated with each peak in order to identify which transformations are taking place through crystallography.
Test 1
Test 2
Test 3
Average
HEB6-as cast
847
876
850
858
HEB7-as cast
872
830
820
841
HEB8-as cast
559
565
569
564