Application of Transmission Electron Microscopy to Engineering Practice in Ceramics: Introduction, Fundamentals, and Application to Precipitation Phenomena

Abstract

This paper, the first of eight in this issue of the Journal devoted to transmission electron microscopy (TEM) of ceramics, provides a brief introduction to the physics of electron optics as it relates to the study of microstructural defects in crystalline solids. The particular application of TEM discussed in this paper, the study of subsolidus precipitation in star sapphire (Ti‐doped AI2O3) and in magnesia‐partially stabilized zirconia (Mg‐PSZ), illustrates the usefulness of this technique in providing detailed crystallographic and microstructural information on precipitation reactions. In star sapphire, TEM was used to identify unambiguously the needle‐like precipitate phase responsible for the asterism in gem‐quality crystals as rutile, the stable tetragonal form of TO2. The observed orientation relation between precipitate and matrix allows good lattice matching between the two phases. The work on PSZ described in the present paper consists of “deciphering” the precipitation history of a commercial, sintered, polycrystalline ceramic showing, in particular, that precipitation occurred in three distinguishable stages. Although tetragonal ZrO2 is the precipitate phase in all three stages, a polymorphic transformation to monoclinic symmetry occurs in two of the three types of precipitates; the tetragonal symmetry is retained metastably in the third. Furthermore, the internal structure of the precipitates and the precipitate/cubic zirconia host microstructural relations are different in each case; these differences can profoundly affect mechanical properties.