Fracture and Fatigue-Crack Growth Behavior in Ductile-Phase Toughened Molybdenum Disilicide : Effects of Niobium Wire vs Particulate Reinforcements

A study has been made of the fracture toughness/resistance-curve (R-curve) and cyclic fatigue-crack propagation behavior in a molybdenum disilicide composite, ductile-phase toughened with nominally 20 vol pet Nb-wire mesh reinforcements (Nb_m/MoSi₂); results are compared with monolithic MoSi₂ and MoSi₂ reinforced with 20 vol pet spherical Nb particles (Nb_p/MoSi₂). It is found that the high aspect ratio wire reinforcements induce significant toughening in MoSi₂) both under monotonie and cyclic fatigue loading conditions. Specifically, the Nb_m/MoSi₂ composite exhibits R-curve behavior with a steady-state fracture toughness of ∼13 MPa√m, compared to unstable fracture at K_c values below 5 MPa√m in unreinforced MoSi₂ or Nb_p/MoSi₂ Such behavior is seen to be associated with extensive crack deflection within the reaction layer between Nb and the matrix, which leads to crack bridging by the unbroken ductile phase. Similarly, resistance to fatigue-crack growth is found to be far superior in the wire-reinforced composite over pure MoSi₂ and Nb_pMoSi₂. Although crack paths are again characterized by extensive deflection along the Nb/matrix reaction layer, the role of crack bridging is diminished under cyclic loading due to fatigue failure of the Nb. Instead, the superior fatigue properties of the Nb_m/MoSi₂ composite are found to be associated with high levels of crack closure that result from highly deflected crack paths along the (Nb,Mo)₅Si₃ reaction layer interface.