Synthesis, microstructure, physical and mechanical properties of phase-pure entropy-enhanced (Nb0.8Ti0.05Ta0.05V0.05M0.05)4AlC3 (M = Hf, Zr) ceramics

The first 413-phase entropy-enhanced (Nb0.8Ti0.05Ta0.05V0.05M0.05)4AlC3 (M = Hf, Zr) (EEMAXHf and EEMAXZr) ceramics were successfully consolidated by spark plasma sintering (SPS) using Nb, Ti, Ta, V, Zr, Hf, Al and graphite as initial materials. The formation of solid solution with five transition metals at the M sites of hexagonal M4AlC3 unit cell was confirmed by elemental analyses. Compared with pure Nb4AlC3, both the electrical and thermal conductivities of the entropy-enhanced ceramics showed a slight decrease, which is attributed to the lattice distortion and the increasing lattice defects that prevents the transfer of electrons and phonons. On the other hand, the mechanical properties of entropy-enhanced ceramics were greatly enhanced compared to pure Nb4AlC3. The measured fracture toughness of EEMAXHf and EEMAXZr ceramics were 8.2 MPa·m1/2 and 10.0 MPa·m1/2, respectively, which were increased by 18.8% and 44.9% compared to Nb4AlC3. The compressive strength of EEMAXHf and EEMAXZr ceramics were 987 MPa and 1187 MPa, respectively, being 92.0% and 130.9% higher than that of Nb4AlC3, respectively. EEMAXHf and EEMAXZr ceramics also possessed the higher Vickers hardness of 6.8 GPa and 7.4 GPa, respectively.