Al-Co-Cr-Fe-Ni is one of the most studied systems in the high entropy alloys (HEA) field. Although a complete phase diagram of this system was not constructed, several researchers published partial phase diagrams which often contradicted each other, and some were experimentally disputed [1-3]. One of the approaches to study complex systems is to isolate and understand the effect of each element. Previous investigations of this type although shed light on the subject, but were performed nonlinearly, sampling specific compositions. In current study, two systems were studied applying pseudo-binary diffusion couple approach [4-5]: Cr joined with single phase B2 (ordered BCC) AlCoFeNi and Al joined with single phase FCC CoCrFeNi.

Microstructure of the Cr/AlCoFeNi diffusion couple where Cr content was below 20 at% (x=1.0) and above 33 at% Cr (x=2.0) was found to be similar to the dendrite area of the quinary AlCoCrFeNi. When Cr content was between these values, the microstructure was more like the inter dendritic region of quinary alloys. Below c.a. 15 at% Cr (x=0.7) initial B2 single phase was preserved. Fine spherical BCC particles appeared in the matrix as the amount of Cr increased. Intertwined BCC-B2 morphology was observed at 20-24 at% Cr (x=1.0-1.3). At 33 at% Cr (x=1.97) and above, phase inversion occurred, revealing cuboidal B2 particles dispersed in the BCC matrix.

It is interesting to note that Al seems to diffuse out concomitantly with the diffusion of Cr. Same tendency was observed in the Al/CoCrFeNi diffusion couple: while Al diffused in, Cr diffused out. As a function of Al addition, this diffusion couple exhibited similar changes to reported in [6, for example]. The microstructure has changed from single phase FCC to BCC-B2 dual phase.

References

[1] J. Cieslak, et al; Journal of Alloys and Compounds, 2019, 801, 511-519.

[2] Z. Chuan, et al; JOM. 2012, 64, 839-845.

[3] Z. Chuan, et al; Materials & Design. 2016, 109, 425-433.

[4] P. Aloke; Philosophical Magazine. 2013, 93.18, 2297-2315.

[5] D. Anuj, et al; Acta Materialia. 2020, 193, 163-171.

[6] W.R. Wang, et al, Intermetallics. 2021, 26, 44-51.