Superplastic deformation in silicon nitride-silicon oxynitride in situ composites

Starting with a mixture of ultrafine beta-Si3N4 and a SiO2-containing addit ive, a superplastic Si3N4-based composite was developed, using the concept of a transient liquid phase. Significant deformation-induced phase and micr ostructure evolutions occurred in the nonequilibrium, fine-grained Si3N4 ma terial, which led to the in situ development of a Si3N4 22-vol%-Si2N2O comp osite and strong texture formation. The unusual ductility of the composites with elongated Si2N2O grains was attributed to the fine-grained microstruc ture, the presence of a transient liquid phase, and the alignment of the el ongated Si2N2O grains, The mechanical properties of the resultant composite were enhanced rather than impaired by superplastic deformation and subsequ ent heat treatment; the resultant composite exhibited both high strength (9 57 MPa) and high fracture toughness (4.8 MPa.m(1/2)).