RESIDUAL-STRESSES AND TRANSFORMATION TOUGHENING IN MOSI2, COMPOSITES REINFORCED WITH PARTIALLY-STABILIZED ZIRCONIA

The results of study of the effects of yttria stabilization (0-6 mol.% ) on the room-temperature fracture behavior and toughening mechanisms in zirconia-reinforced MoSi2 are presented in this paper. Transformati on toughening is shown to occur only in composites reinforced with zir conia particles stabilized with 2 mol.% yttria. However, the fracture toughness levels are comparable in the other composites with yttria le vels between 0 and 6 mol.%. Toughening in the other composites is attr ibuted to the combined effects of residual stress, microcrack shieldin g/anti-shielding and/or crack deflection. A rigorous micromechanics-ba sed model is presented for the estimation of residual stress levels in brittle materials reinforced with phases that can transform during co oling or under stress. The model is applied successfully to the ration alization of the observed fracture and toughening phenomena.