Transformation Plasticity of CeO2‐Stabilized Tetragonal Zirconia Polycrystals: I, Stress Assistance and Autocatalysis

Abstract

Transformation plasticity in CeO2‐stabilized tetragonal zirconia polycrystals due to the tetragonal‐to‐monoclinic transformation was studied by inducing volumetric and shear deformation under compression and bending between the burst temperature of martensite (monoclinic) formation (Mb) and the burst temperature of austenite (tetragonal) formation (Ab). The stress‐strain curve features a load drop, a perfect plastic regime, and an extended strain‐hardening regime before the exhaustion of transformation. Macroscopic shear bands formed in the perfect plastic regime. The yield stress has a strong, positive pressure and temperature sensitivity but is strain‐rate sensitive only in the last stage of deformation. These results are rationalized in terms of stress assistance to the transformation which, in a homogeneous tetragonal polycrystal, may propagate autocatalytically. Autocatalysis can be impeded by a second phase, such as monoclinic ZrO2 or Al2O3, and is suppressed at higher temperature. Flow localization is found to precede and precipitate crack formation. As a result, the actual fracture energy is much less than the total plastic work. The implications of stress‐assisted, autocatalytic transformation on strength and toughness are explored.