Fracture mechanism of FeAl matrix composites with discontinuous ceramic reinforcements

The fracture mechanism of Fe-40at.%Al matrix composites with ceramic particles, whiskers and short fibers fabricated by reactive hot-pressing was investigated in air and in oil bath at similar to 300 K. The loading rate was varied in the range of 10(-2)-10 MPa m(1/2) s(-1) to characterize the sensitivity for environmental embrittlement. The fracture resistance of the composites basically depends on the stress relaxation by plastic deformation at a crack tip. The fracture toughness of the composites consequently decreases with the increasing volume fraction of the reinforcements due to the deterioration of ductility. Furthermore, the fracture resistance of the composites with a volume fraction of 5-10 vol.% is sensitively affected by the moisture induced hydrogen embrittlement. The environmental effect can be suppressed by the addition of boride particles and the micro-alloying of B into the matrix though the intrinsic toughness is not improved significantly. The effect of interaction between a growing crack and the discontinuous fibers on the fracture resistance of the composites was also examined. The toughening mechanisms such as crack bridging and fiber pull-out cannot be induced effectively because of the multiple-fracture of the fibers in the plastic deformation zone formed ahead of the crack tip before the matrix cracking. Therefore, the suppression of the environmental effect is an essential factor to improve the reliability of the composites. (C) 1999 Elsevier Science S.A. All rights reserved.