Ferroelastic domain nucleation in polycrystalline ceramics: A multiscale perspective

The nucleation behavior of ferroelastic domains in a tetragonal zirconia ceramic will be compared for single domain microcrystals (i.e. micropillars) and single domain polycrystals configurations. The microcrystal configuration is evaluated over a wide range of crystallographic orientations and stresses, the later of which is extensive, ranging between 94MPa to over 5GPa. Numerous deformation modes are observed for the microcrystals including, displacive twinning (fully reversible), dislocation mediated twinning, dislocation slip and combinations thereof. Ferroelastic domains are identified as those exhibiting a 90º reorientation of the c-axis across a twin boundary; yet, which of these mechanisms is responsible for the formation of such a twin boundary remains unclear. There are no apparent correlations between the active deformation mechanism and crystallographic orientation or slip system (i.e. Schmid factor). The polycrystalline specimens also show limited correlation between propensity for ferroelastic domain nucleation and crystallographic orientation relative to the loading axis—in this case deformation was performed via Vickers indentation rather than nanoindentation. Notably, deformation within the polycrystalline specimens resulted in large intragranular twins and limited dislocation activity, with a strong dependence on grain size for both the twinned grain (larger than average) and the surrounding grains (smaller than average). The combination of these observations suggest that constraint and localized stress transfer play a significant role in ferroelastic domain nucleation. It is not yet clear whether these factors also influence domain motion or reversibility, but ultimately this insight may lead to valuable strategies for microstructural engineering involving efficient toughening mechanisms such as ferroelastic switching.