Revealing a correlation between microstructure, stoichiometry, coherency strains and mechanical behavior of bulk polycrystalline oxide-based age hardened/toughened ceramic alloys

Abstract

The processing challenges and microstructural inhomogeneity associated with ceramic (nano)composites render it important to develop hardened/toughened oxide-oxide ceramic alloys, possessing good mechanical properties, via a facile solid-state precipitation-based approach. However, detailed understanding of processing-microstructure-property correlations in age-hardened/toughened polycrystalline ceramics is still lacking. The present research, therefore, focuses on understanding the possible effects of variation of dimension/stoichiometry of coherent second-phase precipitate particles in such ceramic alloys on the coherency strains, associated residual stresses and mechanical properties. Polycrystalline MgO-based ceramic alloys have been developed via controlled aging treatments of partially supersaturated Fe(oxide)-containing MgO solid solution. Air-quenching post sintering-cum-solution treatment results in the formation of ultra-fine (∼5–20 nm) coherent intragranular Mg-ferrite precipitates; with subsequent aging treatments causing increases in their size and amount. The intragranular precipitate particles remain coherent with the MgO matrix even upon aging at 1000 °C for 20 h, despite coarsening to ∼130 nm. Detailed compositional analysis using EDS (with HAADF-STEM) and 3D APT reveal progressive increment in Fe-content of the precipitates with aging duration, such that the ultra-fine particles obtained right after solution treatment correspond to non-stoichiometric Fe-deficient Mg-ferrite (with ∼6 at.% Fe), whereas the relatively coarser particles obtained after 20 h of aging correspond to near-stoichiometric MgFe2O4 (with ∼27 at.% Fe). Such increment in Fe-content results in increase in lattice misfit/coherency strains, which dominates the hardening response and improves the hardness despite loss of solution hardening. Furthermore, increase in compressive residual stress in the matrix due to increment in misfit/coherency strains results in improved resistance to crack propagation.