UNDERSTANDING THE ROLE OF THE CONSTITUTING ELEMENTS OF THE AlCoCrFeNi HIGH ENTROPY ALLOY THROUGH THE INVESTIGATION OF QUATERNARY ALLOYS

High Entropy Alloys (HEA) consist of at least five elements in 5¸35at% composition of each element. In these systems, a solid solution with simple crystallographic structure is promoted instead of the formation of complex intermetallic compounds. Since the definition of HEA [1], the interest in these alloys continuously grows due to their exceptional high hardness, good wear and oxidation resistance. The AlCoCrFeNi is one of the most studied HEAs. This alloy solidifies in a dendritic regime, where the dendrite area is enriched in Al-Ni and depleted from Cr-Fe, and vise versa in the interdendrite area [2]. Although the regions differ in their morphologies, both areas contain BCC and ordered BCC (B2) phases, as particles and matrix, respectively. In both areas, the BCC particles are richer with Fe/Cr and the B2 matrix is richer with Al/Ni. The mechanism yielding the unique microstructure of this alloy is not yet understood. In particular, the role of each element is not clear. In an attempt to shed light on this question, quaternary alloys of the elements constituing the AlCoCrFeNi alloy were studied. The alloys were cast from the raw elements, 99.9% pure, in a vacuum arc melting furnace under a purified Argon atmosphere. The cast was re-melted five times to improve chemical homogeneity. The phase content was determined by X-ray diffraction (XRD) and the microstructure was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The microstructure of the as-cast quaternary alloys will be presented and the role of each element discussed.

References

[1] W. Yeh et al., Adv. Eng. Mater. 6 (2004) 299.

[2] P. Wang et al, Mater. Sci. Eng. A. 491 (2008) 154.