Transformation-induced plasticity in zirconia-based ceramics at the nanoscale – NANOTRIP

Ceramics are materials with great potential for applications in harsh and corrosive environments or when a strong chemical stability is needed. However, the use of ceramics as structural materials remains limited due to their low ductility and high susceptibility to defects, which leads to a high variability in their fracture stress. A revolution occurred in the 1970s with the discovery in zirconia-based ceramics of transformation induced plasticity (or TRIP effect) rather than by dislocation propagation. However, the most studied alloy, yttrium-stabilized zirconia, only begins to plastically deform locally around propagating cracks, which slows down but not prevent their extension. This is not enough to allow for a widespread use of this ceramic. Another zirconia alloy, doped with cerium, has been identified more recently and transforms between a tetragonal and a monoclinic phase at lower stresses before cracks form. Initial experimental measurements are very encouraging, with pre-crack plastic strains within single grains reaching 7%. However, we do not understand the TRIP effect sufficiently well to produce macroscopic ceramics with more than 1% elongation to failure. Thus, the role of cerium on the stability of the tetragonal phase and its optimal concentration remain to be identified and the interactions between transformed zones and grain boundaries need to be studied in order to understand how the transformation can propagate between grains, a necessary condition to reach macroscopic ductility. The objective of the NANOTRIP project is to combine atomic-scale calculations (ab initio and molecular dynamics) with electron microscopy and synchrotron X-ray diffraction to study the nanoscopic processes at the origin of the TRIP effect in cerium-stabilized zirconia and answer the following questions: (1) what is the optimal concentration of cerium to maximize the TRIP effect, (2) what are the stress states, in terms of magnitude, orientation and heterogeneity, that favor the transformation, (3) what are the conditions for the growth of a transformed region across a grain boundary? To this end, we will perform ab initio stability analysis of strained and unstrained ceria and use these calculations to fit an empirical potential that will be used to perform larger scale molecular dynamics simulations and analyze the conditions for the growth of transformed nuclei in single and bi-crystals. In parallel, we will experimentally produce cerium-stabilized zirconia micropillars that will be deformed ex situ and in situ in a scanning microscope as well as in a synchrotron using several state-of-the-art techniques (Laue microdiffraction, coherent Bragg diffraction imaging, scanning X-ray diffraction). Simulations and experiments will be compared at the same scale, which will allow a synergetic dialogue: the simulations will use the same orientations and stress states as experimentally, and in return, the simulations will give access to the deformation mechanisms at the atomic scale, and the defects produced which will be used to analyze the X-ray spectra. Our objective will be to identify the rules for the selection of the monoclinic transformation variants and for the transmission across grain boundaries, a necessary step for the development of macroscopic plasticity models in TRIP ceramics. Furthermore, the platform we will develop by combining experiments and simulations will be applicable to other systems, such as co-doped or high-entropy ceramics.