Influence of residual and bridging stresses on the R-curve behavior of Mo-and FeAl-toughened alumina

Alumina matrix was toughened using either metal molybdenum or intermetallic FeAl particles. Mo and FeAl dispersoids were chosen because they have diff erent thermomechanical properties (i.e. Young's modulus, Poisson ratio, as well as thermal expansion coefficient), giving rise to different residual s tresses in the matrix. The R-curve behavior of these composites was first s tudied by stable-crack propagation experiments as a function of the volume fraction of dispersoid. The optimum fraction for toughening was different i n the two composites: 25 and 15 vol% addition led to maximum toughness in t he Mo- and FeAl added composite, respectively. This difference was ascribed to residual stresses. Microscopic observation of the crack path revealed, in both composites, the systematic presence of dispersoids acting as bridgi ng sites in the crack wake, but only a few of them were plastically stretch ed. Residual stresses in the Al2O3 matrix, after sintering and microscopic bridging tractions during crack propagation, were quantitatively assessed u sing microprobe fluorescence spectroscopy. Bridging microstresses were asse ssed in situ by a linear map along the crack profile, at the critical condi tion for fracture propagation. Experimentally collected residual stresses a nd bridging stresses were discussed to explain the different fracture behav ior of the composites. (C) 2000 Elsevier Science Ltd. All rights reserved.