FRACTURE AND FATIGUE-CRACK GROWTH-BEHAVIOR IN DUCTILE-PHASE TOUGHENEDMOLYBDENUM DISILICIDE - EFFECTS OF NIOBIUM WIRE VS PARTICULATE REINFORCEMENTS

A study has been made of the fracture toughness/resistance-curve (R-cu rve) and cyclic fatigue-crack propagation behavior in a molybdenum dis ilicide composite, ductile-phase toughened with nominally 20 vol pct N b-wire mesh reinforcements (Nb-m/MoSi2); results are compared with mon olithic MoSi2 and MoSi2 reinforced with 20 vol pct spherical Nb partic les (Nb-p/MoSi2). It is found that the high aspect ratio wire reinforc ements induce significant toughening in MoSi2, both under monotonic an d cyclic fatigue loading conditions. Specifically, the Nb-m/MoSi2 comp osite exhibits R-curve behavior with a steady-state fracture toughness of similar to 13 MPa root m, compared to unstable fracture at K-c val ues below 5 MPa root m in unreinforced MoSi2 or Nb-p/MoSi2. Such behav ior is seen to be associated with extensive crack deflection within th e reaction layer between Nb and the matrix, which leads to crack bridg ing by the unbroken ductile phase. Similarly, resistance to fatigue-cr ack growth is found to be far superior in the wire-reinforced composit e over pure MoSi2 and Nb-p/MoSi2. Although crack paths are again chara cterized by extensive deflection along the Nb/matrix reaction layer, t he role of crack bridging is diminished under cyclic loading due to fa tigue failure of the Nb. Instead, the superior fatigue properties of t he 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.