Effect of Al on structure and mechanical properties of Fe-Mn-Cr-Ni-Al non-equiatomic high entropy alloys with high Fe content

Microstructure and mechanical properties of the Fe-Mn-Cr-Ni-Al system non-equiatomic high entropy alloys with a different Al content (x = 0-14 at.%) were studied in the present work. The Fe40Mn25Cr20Ni15 alloy was composed of the face-centered cubic (fcc) matrix phase with a small amount of coarse body-centered cubic (bcc) particles. Addition of a small amount of Al (x = 2-6) resulted in an increase in the fraction of the bcc phase to 26% and the formation of fine B2 precipitates within the bcc phase. At higher amounts of Al (x = 10 and x = 14) the microstructure consisted of coarse bcc matrix grains with the B2 precipitates inside. The alloys tend to become stronger with an increase in the Al content from 0 to 10 at.%; further increase in Al concentration did not influence strength considerably. The alloys exhibited pronounced softening with an increase in testing temperature from 25 to 400 °C-600 °C. Ductility of the alloys was high enough (>50%) at all temperatures. A quasi-binary Fe40Mn25Cr20Ni15-Al phase diagram was constructed using a ThermoCalc software and a TCHEA2 database; reasonable agreement between the experimental and predicted phase compositions of the alloys was obtained. It was suggested that an addition of the strong bcc-stabilizing and compound-forming Al to a bcc-prone Fe40Mn25Cr20Ni15 alloy is beneficial for the development of the alloys with the disordered bcc matrix and the embedded B2 precipitates having attractive mechanical properties.