Pseudo-binary diffusion couple approach as a tool for evaluation of microstructural changes as a function of composition (x) in AlCoCrxFeNi, AlxCoCrFeNi and AlCoCrFeNix multicomponent alloys

This study employs a pseudo-binary diffusion couple approach to systematically investigate the microstructural evolution in AlCoCr x FeNi, Al x CoCrFeNi and AlCoCrFeNi x multicomponent alloys. In the Al x CoCrFeNi system, the study identified a single-phase face-centered cubic (FCC) structure at low Al concentrations, a dual-phase B2 (ordered body-centered cubic (BCC)) and BCC at intermediate Al levels, and a B2 matrix with fine BCC particles at higher Al concentrations. The solubility limit of Al in the FCC phase was determined to be approximately 8 at.% Al. Contrary to previous assertions that Al stabilizes the BCC phase, this study concludes that Al acts as a B2 stabilizer. For the AlCoCr x FeNi system, the study concluded a single-phase B2 structure at low Cr concentrations, fine BCC particles within the B2 matrix at intermediate Cr levels, and a phase inversion at higher Cr concentrations where the BCC phase became the matrix with B2 particles embedded within it. The threshold Cr content for this phase inversion was identified as approximately 33 at.% Cr. It was concluded that Cr acts as a stabilizer for the BCC phase, while the B2 phase exhibited limited Cr solubility. AlCoCrFeNi x system demonstrated unique behavior. A high diffusion coefficient of Ni was observed in the AlCoCrFe B2/BCC basket-weave microstructure. This area could accommodate up to 25 at.% Ni without significant microstructural changes. At higher Ni contents, the microstructure transformed dramatically into a combination of L1₂ (ordered FCC) and B2 phases. As Ni content increased, the structure evolved to a eutectic microstructure, accompanied by a significant reduction in the rate of Ni diffusion. The similarity of these results to those reported for conventionally processed and heat-treated alloys suggests that the pseudo-binary diffusion couple method is an effective tool for evaluating microstructural changes in multicomponent alloys. Notably, the eutectic microstructure observed in the AlCoCrFeNi₂.₁ diffusion couple aligns with findings in cast alloys. Given the current research focus on eutectic high-entropy alloys (HEAs), this method offers a rapid and efficient means of identifying compositions exhibiting such microstructures. Overall, this research enhances the understanding of multicomponent alloy systems and provides a foundation for future studies aimed at optimizing alloy composition and processing to achieve desired microstructures and mechanical properties. • Al/CoCrFeNi, Cr/AlCoFeNi, Ni/AlCoCrFe diffusion couples were studied. • Changes of the microstructure as function of Cr, Al, Ni content were reported. • Solubility limits of Cr and Al in B2 and FCC, respectively, were determined. • Conclusions were in line with those reported for cast and heat treated alloys. • Pseudo-binary diffusion couple method is an effective tool for studying HEAs.