(111)-specific coalescence twinning and martensitic transformation of tetragonal ZrO2 condensates

ZrO2 nanoparticles (10-400 nm) were produced by laser ablation deposition and studied by transmission electron microscopy. The particles were predominantly tetragonal (t-) phase with spherical shape. Relatively large t-ZrO2 particles, however, showed {111} and {100} faces and tended to attach over well-developed {111}, hence forming twinned bicrystals. Upon electron irradiation, the t-ZrO2 particles transformed to monoclinic (m-) phase, at a smaller critical size than that reported in matrix systems, by nucleating from particle surface, in particular at the outcrop of planar defect. Further growth caused {100} or {110} type twin boundary but not closure twins, in contrast with that under the influence of matrix constraint. In situ observations of t-m transformation for spherical ZrO2 nanoparticles also enabled shape-change determination, typically dilation and contraction along orthogonal <110> directions and invariant plane strain along (001) twin plane, for this martensitic-type transformation.