Treasure maps for magnetic high entropy alloys

The new class of high entropy alloys (HEA) holds potential for advanced manufacturing and possesses hardness and resistance to wear and corrosion. Recently magnetic HEAs and their magnetic properties such as the magnetic Curie temperature have attracted attention. However, due to the immense configurational phase space of these alloys (Fig. 1) theoretically guided materials design becomes critically important. Our present work represents an experimentally validated computational guide for the discovery and design of materials with specifically desired properties.

Fig. 1: 3D representation of the quaternary FeCoNiCr phase diagram. Regions explored in conventional material design are indicated by the red areas. The idea behind HEAs is to explore the large, so far unexplored region, e.g., by starting at the indicated equiatomic composition in the middle of the diagram.

For the computational approach we combine ab initio based density-functional theory with a mean field magnetic model to allow efficient Curie temperature predictions. The predictive power of this theory is validated with experimental data from a variety of CoFeNi-based HEAs. Our approach allows us to screen a wide compositional range of HEAs, which essentially provides “treasure maps” of the enormous and unexplored parameter space occupied by four and five component alloys.

To verify the performance of our approach, we computed Curie temperatures for a number of different alloys for which Curie temperature values have been reported in literature (black and red bars in Fig. 2). The trend in the measured Curie temperatures is in very good agreement with our theoretical results indicating the predictive strength of the approach.

Fig. 2: Theoretical (red bars) and experimental (black bars) Curie temperatures for various CoFeNi-based HEAs. The Curie temperatures marked with the star have been derived from an empirical linear interpolation (J. Appl. Phys. 113, 17A923 (2013)).

Our theoretical predictions were further tested by fabricating alloys of CoFeNiCrPdX ranging from x=0 to 0.5. Fig. 3 shows the experimental Curie temperatures (red stars), which are in excellent agreement with the theoretical predictions. In addition to the previously experimentally identified alloying strategies to achieve room temperature ferromagnetism in CoCrFeNi alloys, we suggest three alternative alloying candidates, namely Ag, Au, and Cu, each of which has a variety of compositions that should exhibit room temperature ferromagnetism. The developed trea-sure maps provide an extensive set of compounds that have not yet been synthesized and we have identified hundreds of new alloy combinations that could be useful.This work has been done in a broad collaboration with research teams at the Rochester Institute of Technology, Air Force Research laboratory and Delft University of Technology in the Netherlands

Fig. 3: Left: Comparison between theoretically predicted (solid lines) and experimentally measured (symbols) Curie temperatures. The stars denote experimental results obtained in the present study. The agreement with the new theoretical approach is very good allowing to faithfully scan experimentally unexplored regions, e.g., the orange line showing a modified Cr content or the contour plot in the right panel showing a 2D ‘treasure map’.

1.
F. Körmann, D. Ma, D. D. Belyea, M. S. lucas, C. W. Miller, B. Grabowski, M. H. F. Sluiter
“Treasure maps” for magnetic high-entropy-alloys from theory and experiment
Applied Physics letters 107, 142404 (2015).

Authors: Fritz Körmann, Duancheng Ma & Blazej Grabowski

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