INFLUENCE OF INTERNAL-STRESSES ON CRYSTALLIZATION OF INTERGRANULAR PHASES IN CERAMICS

Crystallization of the secondary phase in structural ceramics may lead to improved high temperature properties. Internal stresses due to both volume changes upon crystallization and cooling can result in microcrack formation or crack bridging and crack deflection processes affecting mechanical properties. Models for crystallization and stress evolution are developed which treat (i) the equilibrium degree of crystallization for non-relaxed volume mismatch stresses involving the influence of elastic constants, supercooling, volume change and amount of secondary phases on the crystallized volume fraction, (ii) the kinetics of the degree of crystallization leading to the equilibrium state, (iii) possible relaxation mechanisms of stresses involving (a) viscous flow of the intergranular phase and (b) solution-reprecipitation of primary grains and (iv) the crystallization kinetics controlled by simultaneous stress relaxation.