Microstructure and improved mechanical properties of ultrafine-grained Mo-12Si-8.5B (at.%) alloys with addition of ZrB2

Multi-hierarchical Mo-12Si-8.5B-xZrB2 (x = 0, 0.5, 1.0, 1.5, 2.5 wt%) alloys consisting of three ultrafine-grained (UFG, 0.47-0.81 µm) phases of Mo5SiB2 (T2), Mo3Si and Mo solid solution (α-Mo) were prepared by mechanical alloying following hot pressing. Microstructure observations showed that the intermetallic phases (Mo3Si and T2) distributed dispersedly in the continuous α-Mo matrix associated with the homogeneously embedded nanoscaled particles (10-225 nm) in the grain interiors and at the grain boundaries. The Mo-12Si-8.5B-xZrB2 alloys exhibited monotonically increasing compressive strength to 3.13 GPa with increasing content of ZrB2, and the fracture toughness increased about 27% and reached at 11.5 MPa m1/2 at 1.0 wt% ZrB2, rendering the Mo-12Si-8.5B-1.0 wt% ZrB2 alloy possessing the best combined mechanical properties of high strength and high toughness. The underlying reason for the superior mechanical properties of the Mo-12Si-8.5B-xZrB2 alloys is that the dispersedly distributed nanosized particles in the UFG multi-phased-matrix can not only effectively block the dislocation motion to increase the strength but also store the dislocations to increase the strain hardening ability during mechanical deformation.