MICROSTRUCTURE AND TENSILE CREEP MECHANISMS OF AN IN-SITU REINFORCED SILICON-NITRIDE

The creep behaviour of an acicular-grained Si3N4 in the temperature ra nge of 1260-1400 degrees C under 50-350 MPa tensile stress was investi gated. The viscous flow of an amorphous grain boundary phase and uninh ibited grain boundary sliding resulted in a fast initial creep regime characterized by a stress exponent = 1.8 and an activation energy = 50 9 kJ mol(-1), which increased to 3.2 and 1054 kJ mol(-1), respectively , in the secondary creep regime. Between these two regimes the creep r ate dropped rapidly due to grain interlocking and loading of the acicu lar grains parallel to the tensile axis. An enhanced dislocation activ ity in the parallel grains and cavitation in the grain pockets were ob served, but their roles in the creep deformation appear minor. It is p roposed that the mechanism controlling the secondary creep regime is t he nucleation and growth of surface steps on the atomically smooth (10 0) grain boundary plane through the solution/precipitation process. Co pyright (C) 1996 Acta Metallurgica Inc.