  Shape memory (SM) ceramics, such as yttria-stabilized tetragonal zirconia
(YSTZ), are a unique family of SM materials that offer unique properties
including ultra-high operating temperature, and high resistance to chemical
corrosion and oxidation. However, formation of defects is usually observed in
SM ceramics during manufacturing and/or by mechanical deformation. To fully
take advantage of the SM properties of these ceramics, it is necessary to fully
understand the nano-structural evolution of defects under external stimuli. In
this study, defect closure behaviors in YSTZ nanopillars are investigated by
atomistic simulations. Two characteristic orientations of [011-] and [001] are
selected to represent the dominant deformation mechanisms of phase
transformation and dislocation migration, respectively. With the presence of
crack and void, the strength and yield strain of nanopillars are noted to
decrease significantly, especially for [011-]-oriented YSTZ nanopillars. Volume
expansion associated with the tetragonal to monoclinic phase transformation is
observed to promote healing of crack and void. Atom stress analyses reveal
stress concentrations along the newly formed monoclinic phase bands. A critical
crack width is identified, less than which the crack can be fully closed in
compression. Size effect study reveals that an increase in nanopillar size has
a positive effect on crack self-healing behavior. For [001]-oriented YSTZ
nanopillars, dislocation migration leads to formations of an amorphous phase,
which also assist the crack and void closure process. The revealed crack/void
healing mechanisms may provide a path for mitigating internal defects that
influences the mechanical properties and deformation mechanisms of SM ceramics.
