Asymmetric tensile and compressive creep deformation of hot-isostatically-pressed Y2O3-doped-Si3N4

The uniaxial tensile and compressive creep rates of an yttria-containing ho t-isostatically-pressed silicon nitride were examined at several temperatur es between 1316 and 1399 degrees C and found to have different stress depen dencies. Minimum creep rates were always faster in tension than compression for an equal magnitude of stress. An empirical model was formulated which represented the minimum creep rate as a function of temperature for both te nsile and compressive stresses. The model also depicted the asymmetric cree p deformation using exponential and linear dependence on tensile and compre ssive stress, respectively. Unlike other models which represent either tens ile or compressive creep deformation as a respective function of tensile or compressive stress, the model in the present study predicted creep deforma tion rate for both tensile and compressive stresses without conditional or a priori knowledge of the sign of stress. A statistical weight function was introduced to improve the correlation of the model's regressed fit to the experimental data. Post-testing TEM microstructural analysis revealed that differences in the amount of tensile- and compressive-stress-induced cavita tion accounted for the creep strain asymmetry between them, and that cavita tion initiated in tensile and compressively crept specimens for magnitudes of creep strain in excess of 0.1%. (C) 1998 Elsevier Science Limited. All r ights reserved.