Continuum thermodynamics of unusual domain evolution-induced toughening effect in nanocracked strontium titanate

[Display omitted] •Mechanical and electrical behaviors of nanoscale pre-cracked SrTiO3 are investigated.•A nonlinear thermodynamic model is constructed and employed for SrTiO3.•A large toughening effect is revealed and characterizes for nanoscale SrTiO3.•Unusual emergence of ferroelectricity due to strain is observed near crack tip.•Underlying mechanism of the large toughening effect is demonstrated. This work is concerned with the analysis of both unusual mechanical and electric behaviors of nanoscale pre-cracked incipient ferroelectric of SrTiO3 at room temperature. A nonlinear thermodynamic model based on the Ginzburg-Landau theory is thus constructed and employed, which takes into account the appropriate mechanical boundary conditions, the electromechanical coupling between the polarization and the mechanical strain, and the self-strains of the ferroelastic and ferroelectric phase transformations. A large toughening effect is explored as a consequence of a softening nonlinear mechanical behavior, which characterizes for nanoscale SrTiO3, despite of the brittleness of bulk SrTiO3 ceramic. Depending upon applied strain, the large toughening and nonlinear mechanical behavior are attributed to unusual domain evolution of strain-induced polarization from the crack tip, in which a local-to-global transition of ferroelectric phase takes place from polarization vortex to hybrid structure of vortex and stripe domains, and finally to stripe domain structure with Néel type domain walls. Here, we also show the great importance of cross-coupling between the ferroelectric polarization and mechanical strain as it intrinsically leads to the local-to-global transition of ferroelectric phase. More interestingly, such cross-coupling additionally scatters the concentrated stress near the crack tip to Néel type domain walls. The present findings shed light on the crucial role of mechanical strain in controlling both the size and topology of polarization structures in nanoscale SrTiO3. Our numerical results also provide an insight into the vital role of strain-induced polarization for the toughness and strength of SrTiO3 material. Based on our study, the applicability of the present results to other incipient ferroelectrics such as CaTiO3, EuTiO3, and KTaO3 with the same mechanism would be interesting and possible.