POSSIBILITY OF SPINODAL DECOMPOSITION IN ZRO2-Y2O3 ALLOYS - A THEORETICAL INVESTIGATION

The thermodynamics and kinetics of cubic --> tetragonal phase transformations in ZrO2-Y2O3 alloys were investigated by using thermodynamic stability analysis and kinetic computer simulations to explore the possibility of a spinodal mechanism during decomposition, Based on a simple free energy model, it is shown that, depending on the alloy composition, a cubic phase aged within the t + c two-phase field may result in three different sequences of phase transformations: (1) direct nucleation and growth of the equilibrium t-phase from the c-phase matrix; (2) formation of a metastable t'-phase followed by nucleation and growth of the equilibrium c- and t-phases; and (3) formation of a transient t'-phase followed by its spinodal decomposition into two tetragonal phases with one of the tetragonal phases eventually transforming to the equilibrium c-phase. The temporal microstructure evolutions for different compositions mere studied by using computer simulations based on the time-dependent Ginzburg-Landau (TDGL) model which incorporates the long-range elastic interactions.