Influence of Mo/Cr ratio on the lamellar microstructure and mechanical properties of as-cast Al0.75CoCrFeNi compositionally complex alloys

The Al0.75CoCrFeNi alloy (Al16Co21Cr21Fe21Ni21 in at.%) presents a lamellar microstructure in the as-cast state consisting of a spinodally-decomposed B2/BCC matrix and Widmanstätten-type FCC plates. In this study, to retain the lamellar microstructure and improve tensile strength, Al16Co21Cr21-xFe21Ni21Mox alloys with x ≤ 10 at.% were investigated. For x = 2 at.%, the Widmanstätten microstructure changed into a vermicular one due to the stabilization of the BCC phase. With increasing the Mo/Cr ratio, the BCC phase transformed into topologically close-packed (TCP) phases, i.e., σ phase for x = 4 at.% and R phase for x ≥ 6 at.%, whose volume fractions increases with x. The as-cast alloys with x = 10 and 4 at.% presented the largest microhardness of ~600 HV0.5. The former had the highest volume fraction in TCP phases, which are hard and brittle while the latter presented the finest microstructure (enhanced phase boundary strengthening). While the alloys with x > 4 at.% were too brittle to machine tensile specimens, the others were tested between 20 and 700 °C. The ultimate tensile strength increased with increasing x up to ~1460 MPa for x = 4 at.% at 400 °C. At 700 °C, the strength of all alloys significantly decreased due to the softening of the B2 phase. Most of them had limited ductility and showed intergranular fracture except for x = 4 at.% presenting pronounced necking with ~38% ductility. The latter effect was attributed to the occurrence of interfacial sliding resulting in cavitation at grain boundaries and interphase boundaries.