Grain-size effect on cracking accumulation in yttria-doped zirconia ceramics during cyclic martensitic transformations

Bulk polycrystalline zirconia-based ceramics generally crack as they undergo martensitic transformations, largely due to the transformation mismatch stresses. Polycrystals subjected to cyclic transformations bod-ily lose individual grains and progressively comminute, but it is not yet clear how the grain size of a polycrystal affects such cyclic degradation. We explore this issue in 1.5 mol% yttria-doped zirconia by varying the grain size from 0.6 to 7.9 mu m in pellets of fixed composition and sample size and subjecting them to multiple thermal cycles through the transformation. A smaller grain size is found to increase the number of cycles required to disaggregate the pellet because of the larger amount of grain boundary area that must crack. Calorimetry analysis shows that the energy relieved through cracking decreases with in-creasing grain size and suggests an apparent material length scale of similar to 2 micrometers for the stress relief zone. For grain diameters below this critical length scale, complete stress relief is suggested, while at larger grain sizes, intergranular cracking apparently does not fully relieve the transformation mismatch stresses. Alternate accommodation mechanisms are required including the formation of multiple variants and even some transgranular fracture. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Automated summary

Bulk polycrystalline zirconia-based ceramics often crack during martensitic transformations due to transformation mismatch stresses. Smaller grain sizes require more cycles to disaggregate due to larger grain boundary area that must crack. The energy relieved through cracking decreases with increasing grain size, suggesting the need for alternate accommodation mechanisms at larger grain sizes to relieve transformation mismatch stresses.