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 pct Nb-wire mesh reinforcements (Nb-m/MoSi2); results are compared with monolithic MoSi2 and MoSi2 reinforced with 20 vol pct spherical Nb particles (Nb-p/MoSi2). It is found that the high aspect ratio wire reinforcements induce significant toughening in MoSi2, both under monotonic and cyclic fatigue loading conditions. Specifically, the Nb-m/MoSi2 composite exhibits R-curve behavior with a steady-state fracture toughness of similar to 13 MPa root m, compared to unstable fracture at K-c values below 5 MPa root 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-p/MoSi2. 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)(5)Si-3 reaction layer interface.