New insights into the thermal aging mechanism of yttria stabilized zirconia: A phase field study

Abstract

In this paper, a novel phase-field (PF) model is proposed to study the thermal aging mechanism of single crystalline t'-YSZ. The influences of the initial compositional content of yttria and the initial twin structure of the t' phase on the aging process are systematically discussed. The PF model can recover the modulated structure and nano/micro hybrid structure observed in experiments. The PF simulation results indicate that the initial compositional content of yttria is the most important influential factor of the thermal aging process. Besides that, the transformation strain, the initial twin structure and the anti-phase boundaries (APBs) of the t' phase can also have significant influences on the thermal aging kinetics. The typical spinodal region is more suitable to predict the thermal aging behavior of single domain YSZ. For multi-domain YSZ with initial twin structures and APBs, the spinodal region should be further divided into the kernel region and marginal region. In the kernel region, the thermal aging occurs by spinodal decomposition with the formation of a modulated structure, which is followed by merging and coarsening. In the marginal region and outside the spinodal region, the phase decomposition leads to a hybrid structure with coarse grained cubic phase and fine grained tetragonal phase, which exhibits the characteristics of nucleation and growth. The hybrid structure is consistent with previous experimental observations. It is revealed that the boundaries of the nano sized tetragonal grains evolve from the twin boundaries and APBs. The nucleation-growth mechanism should be properly understood when it is applied to illustrate the evolution process of the hybrid structure. The PF model and the new insights obtained in this study are helpful to understand the thermal aging mechanisms of t'-YSZ.