Thin-foil phase transformations of tetragonal ZrO/sub 2/ in a ZrO/sub 2/-8 wt% Y/sub 2/O/sub 3/ alloy

Tetragonal zirconia (t-ZrO/sub 2/) grains in an annealed ZrO/sub 2/-8 wt% Y/sub 2/O/sub 3/ alloy transformed to orthorhombic (o) or mono-clinic (m) symmetry by stresses induced by localized electron beam heating in the transmission electron microscope. Different transformation mechanisms were observed, depending on foil thickness and orientation of individual grains. In thicker grains (≥150 nm), the transformation proceeded by a burst-like growth of m laths, and this is believed to approximate bulk behavior. In thinner grains near the edge of the foil, usually those with a [100]/sub t/ orientation perpendicular to the thin-foil surface, continuous growth of an o or m phase with an antiphase-boundary-containing microstructure was observed. The o phase is believed to be a high-pressure polymorph of ZrO/sub 2/, which forms (paradoxically) as a thin-foil artifact because it is less dense than t-ZrO/sub 2/, but more dense than m-ZrO/sub 2/. In some very thin grains, the t → m transformation was thermoelastic. Furthermore, a mottled structure often occurred just before the t → m or t → o transformation, which is attributed to surface transformation. Aside from the lath formation, the observed transformation modes are a result of the reduced constraints in thin foils