Microstructure and Mechanical Properties of Nanostructured CoCrFeMoTi High-Entropy Alloy Fabricated by Mechanical Alloying and Spark Plasma Sintering

Abstract

CoCrFeMoTi high-entropy alloy (HEA) was synthesized using mechanical alloying and spark plasma sintering. The corresponding microstructural features and phase composition were compared with those of CoCrFeNi, CoCrFeNiTix (x = 0, 0.3 and 0.5) and CoCrFeNiMox (x = 0, 0.3, 0.5 and 0.85) HEAs. Co, Cr, Fe, Mo and Ti elemental powders were mixed in equiatomic ratio and mechanically alloyed in a planetary ball mill at 300 rpm for up to 80 h. The influence of the milling duration on the evolution of microstructure, constituent phases and morphology was studied. After 40 h of ball milling, two supersaturated BCC solid solution phases were obtained. Milling time increasing resulted in grain refinement and higher solid solution homogenization characterized by a high internal strain. A partial phase transformation from BCC to intermetallic phases when the temperature exceeds 660 °C was revealed. After SPS consolidation, the Vickers hardness was 778 ± 10 HV, in combination with an ultimate shear stress of 216 ± 20 MPa and a yield shear stress of 81 ± 15 MPa for the material sintered at 950 °C and 65 MPa. Further increasing in the sintering temperature resulted in enhanced hardness value of 1600 ± 5 but reduced yield shear stress. The failure analysis revealed a brittle fracture of the synthesized materials.