Ductile-to-brittle transition in superplastic silicon nitride ceramics

摘要

The ductile-to-brittle transition was observed in a superplastic silicon nitride nanoceramic. This transition depends on strain rates and deformation temperatures. Generally, the material exhibits ductility at low strain rates and high deformation temperatures. At 1600 deg C, the material is brittle when the strain rates are higher than 10~(-3)/s. At a fixed strain rate of 10~(-3)/s, the material exhibits brittleness when the temperatures are lower than 1550 deg C. Moreover, critical strain rate for the brittle to ductile transition depends on deformation temperature. The critical strain rates increase with increases in the deformation temperature. When the deformation temperature is 1700 deg C, the critical strain rates reach a maximum at 10~(-2)/s. The extent of superplastic deformation in the present material was found to be limited not by intergranular cavitation but by the initiation and growth of surface cracks.