Alloying behavior in multi-component AlCoCrCuFe and NiCoCrCuFe high entropy alloys

Multi-component high entropy alloys (HEAs) are observed to form simple solid solutions in contrary to general perception that complex compounds may form in such multi-component equi-atomic alloys. In the present study, alloying behavior was investigated using XRD in AlCoCrCuFe and NiCoCrCuFe equi-atomic high entropy alloys synthesized by mechanical alloying (MA) and spark plasma sintering (SPS). Simple FCC and BCC phases evolved after MA, while Cu-rich FCC and sigma (σ) phases evolved along with FCC and BCC phases after SPS. Further, NiCoCuFe, NiCoCrFe and NiCoFe equi-atomic alloys were investigated to confirm the formation of Cu-rich FCC, and σ phases. The hardness was observed to be 770 ± 10 HV for AlCoCrCuFe and 400 ± 10 HV for NiCoCrCuFe. Phase evolution after MA and SPS indicate that configurational entropy is not sufficient enough to suppress the formation of Cu-rich FCC, and σ phases, and enthalpy of mixing appears to play an important role in determining the phase formation in high entropy alloys after sintering.

► At least two phases formed in high entropy alloys of AlCoCrCuFe and NiCoCrCuFe.
► These alloys exhibit high hardness after densification by spark plasma sintering.
► Cu separates during sintering, possibly due to high ΔHmix with other elements.
► Cr is responsible for sigma phase formation in Cr containing alloys.
► Solid solution criteria for HEAs are insufficient to explain results in our study