EFFECT OF GRAIN SIZE ON THE MARTENSITIC TRANSFORMATION AND NUCLEATION MECHANISM IN CERIA-DOPED TETRAGONAL ZIRCONIA POLYCRYSTALS

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Effect of grain size/particle size was studied on the martensitic transformation temperature(Ms) using both bulk and pulverized specimens of ceria-doped TZP(tetragonal Zirconia Polycrystals). In bulk specimens, the Ms markedly decreased with grain refinement with concurrent hardening of the matrix. In powder specimens, the Ms varied only moderately with the particles size independently of the grain size. The Ms of the bulk specimen was lower than that of powders of the same grain size.From these results, it was suggested that the constraining stress in a bulk specimen was an essential factor for the grain size dependence of Ms. Although the exact nature of the constraining stress could not be known, the increase of the average yield stress resulting from the grain refinement was conjectured. This would naturally suppress the nucleation of martensite since the large transformation strain associated with the nucleation must be elastically and plastically accommodated by the surrounding matrix. This model successfully explains the present experimental observations. On the other hand, the existing theory in which stress concentration at the grain boundary owing to the anisotropic thermal expansion coefficient would fail to account for the lower Ms for a bulk than powders.