TRANSFORMATION PLASTICITY OF CEO2-STABILIZED TETRAGONAL ZIRCONIA POLYCRYSTALS .1. STRESS ASSISTANCE AND AUTO-CATALYSIS

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//2O//3, and is suppressed at higher temperature.