Ultra-high hard and fracture-resistant multi-phase nanocrystalline AlCrFeMoNbNi multi-principal element alloy

Abstract

Multi-phase nanocrystalline AlCrFeMoNbNi multi-principal element alloy was developed using mechanical alloying followed by spark plasma sintering. Mechanical alloying resulted in a two-phase microstructure (BCC and FCC), whereas subsequent sintering resulted in a multi-phase microstructure consisting of FCC, Rhombohedral and B2 phases. Extremely high hardness values in the range of 35.79–16.05 GPa were measured as a function of indentation load between 25 and 5000 g. A highly pronounced indentation size effect was observed. From classic Nix-Gao analysis, a load independent hardness of 13.82 GPa was estimated. The high hardness arises from a combination of finer grain size and solid solution strengthening (FCC phase) and multi-phase structure consisting of B2 and Rhombohedral phases. The major phase being FCC (83%) offers nearly 85% of the total flow stress realized for this MPEA and the remaining 15% of the flow stress must be arising from both the B2 and Rhombohedral phases. It is to be noted that the phase fractions of B2 and Rhombohedral phases are 10% and 7% respectively. Indentation fracture toughness, evaluated using the cracks developed during Vickers indentation, yielded encouraging numbers in the range 12.28–13.85 MPam. The excellent combination of ultra-high hardness and indentation fracture toughness of this AlCrFeMoNbNi MPEA surpasses the reported data on several related harder materials, including silicides, nitrides, carbides, etc.