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 different thermomechanical properties (i.e. Youngʹs modulus, Poisson ratio, as well as thermal expansion coefficient), giving rise to different residual stresses in the matrix. The R-curve behavior of these composites was first studied by stable-crack propagation experiments as a function of the volume fraction of dispersoid. The optimum fraction for toughening was different in the two composites: 25 and 15 vol% addition led to maximum toughness in the 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 bridging sites in the crack wake, but only a few of them were plastically stretched. Residual stresses in the Al2O3 matrix, after sintering and microscopic bridging tractions during crack propagation, were quantitatively assessed using microprobe fluorescence spectroscopy. Bridging microstresses were assessed in situ by a linear map along the crack profile, at the critical condition for fracture propagation. Experimentally collected residual stresses and bridging stresses were discussed to explain the different fracture behavior of the composites.