Theoretical critical metastability temperature to interpret phase formation in a lamellar-like-structured high entropy alloy

Control over a lamellar-like-structured high entropy alloy (HEA) system is found to be possible by replacement of aluminium with boron into the FeCoNi(Bx Al1-x)0.1Si0.1 composition in the range of x = 0 to 1.0. The BCC/B2 microstructure of FeCoNi(Al0.1Si0.1) HEA is changed into a multiple phase system comprising of BCC/B2, FCC and Fe2B-type intermetallic phases. The microstructures of as-cast alloys were seen to be a lamellar-like structure comprised of nanostructured lamellae with alternating FCC and Fe2B phases. The concept of critical metastability temperature from nucleation theory is employed phenomenologically, and found to correlate with the presence of Fe2B in this alloy at different boron additions. With the substitution of boron, the stability of the disordered BCC solid solution is reduced, promoting the formation of secondary phases. We show a link between the interfacial energy of the phases present, the interlamellar spacing, and alloy metastability as a function of boron addition, the key relationship being that Fe2B phase formation correlates with a drastic reduction in the interfacial energy. These correlations bear further investigation and may be useful in the design of lamellar-like-structured multi-component systems.