The mechanism of thermal transformation hysteresis in ZrO₂-CeO₂ shape-memory ceramics

Hysteresis is an important phase transformation property to understand and control in shape-memory materials. While a detailed understanding of hysteresis has been developed in shape-memory alloys (SMAs), it is far less understood in emerging shape-memory ceramics (SMCs). Here, we systematically study thermal hysteresis of the tetragonal-monoclinic martensitic transformation in ZrO₂-CeO₂ SMCs. We present calorimetry measurements of thermal hysteresis, which reveal a non-monotonic trend with CeO₂ content as well as significant rate dependence, and correlate these findings with the transformation crystallography measured by in situ X-ray diffraction. We quantitatively analyze these results in the framework of martensite nucleation theory and conclude that the initial decrease in hysteresis with CeO₂ content can be attributed to improving interface compatibility (λ₂) for coherent nucleation whereas the sharp increase in hysteresis at higher CeO₂ contents (below temperatures of ~773 K) is a result of interface friction consistent with thermally-activated interactions with the Peierls barrier of the crystal. Our analysis gives an activation energy of ~136 kJ/mol, which is consistent with previously reported activation energies of isothermal transformation in these materials. These findings clarify contradicting reports in the ZrO₂ literature and confirm that the same controlling factors of hysteresis in SMAs, namely λ₂ and thermal friction, are also operative in SMCs.