Microstructures, mechanical properties, and strengthening mechanisms of the (NbMoTa)_100−xC_x refractory medium-entropy alloys

Refractory high/medium-entropy alloys (RH/MEAs) are known for their outstanding performance at elevated temperatures; however, they usually exhibit poor room-temperature plasticity, which can be attributed to the non-uniform deformation that occurs at room temperature. Once cracks nucleate, they will rapidly propagate into vertical splitting cracks. Here, we introduce multiple phases including FCC and HCP phases into the NbMoTa RMEA via appropriate addition of carbon. The results show that multiple-phase synergy effectively suppresses non-uniform deformation, thereby delaying the onset of vertical splitting cracks. An optimal combination of compressive strength-plasticity is achieved by the (NbMoTa)_92.5C_7.5 alloy. The significant improvement in room-temperature mechanical properties can be attributed to its hierarchical microstructure: in the mesoscale, the BCC matrix is divided by eutectic structures; while at the microscale, the BCC matrix is further refined by abundant lath-like FCC precipitates. The FCC precipitates contain high-density stacking faults, acting as a dislocation source under compressive loading. The HCP phase in the eutectic microstructures, in turn, acts as a strong barrier to dislocation movement and simultaneously increases the dislocation storage capacity. These findings open a new route to tailor the microstructure and mechanical properties of RH/MEAs.