Transformation Plasticity of CeO 2 ‐Stabilized Tetragonal Zirconia Polycrystals: I, Stress Assistance and Autocatalysis

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Abstract
Transformation plasticity in CeO 2 ‐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 ( M b ) and the burst temperature of austenite (tetragonal) formation ( A b ). 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 ZrO 2 or Al 2 O 3 , 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.

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