Investigation on strengthening and toughening mechanisms of Nb-Ti-ZrB2 metal matrix ceramic composites reinforced with in situ niobium and titanium boride

Nb-Ti-ZrB₂ metal matrix ceramic composites with a fixed atomic ratio Nb/Ti = 2/1 and ZrB₂ volume fraction changing from 0, 11 vol%, 23 vol% to 36 vol% were hot pressed at 1600 °C under 30 MPa. The influence of ZrB₂ content and Ti addition on the phase constitution, microstructure evolution, toughening mechanisms and strengthening mechanisms were investigated. It was shown that the formation of in situ Nb-rich (Ti,Nb)B and Ti-rich (Nb,Ti)B was attributed to a high mutual solubility of monoborides and the amount of niobium and titanium borides increased with increasing ZrB₂ content. The needle-shaped (Ti,Nb)B phase weakened the damage to fracture toughness caused by ZrB₂ particle fracture due to crack bridging, crack defection and the pull-out toughening mechanisms. The highest fracture toughness of the Nb-Ti-ZrB₂ composites was 12.0 MPa·m^1/2. The stiff (Nb,Ti)B phase acted as a strong obstacle to the dislocation motion, leading to dislocation pile-up and enhancing the strength of the Nb-Ti-ZrB₂ composites during compression tests. However, stress concentration around the needle-shaped (Ti,Nb)B phase easily leads to crack initiation and extension, resulting in decreased strength. The yield strength of Nb-Ti-ZrB₂ composites ranged from 657.3 MPa to 1783.0 MPa owing to the combined influence of the strenghening mechanism caused by (Nb,Ti)B and the weakening mechanism caused by (Ti,Nb)B. The compressive deformation and failure process were also discussed in detail in this study.