INTERNAL-STRESSES AND THE MARTENSITE START TEMPERATURE IN ALUMINA-ZIRCONIA COMPOSITES - EFFECTS OF COMPOSITION AND MICROSTRUCTURE

In alumina-based composites containing ceria-stabilized tetragonal zirconia, the martensite start temperature (M(s)) of the tetragonal-to-monoclinic zirconia phase transformation exhibits a grain size dependence that becomes increasingly pronounced as the zirconia content decreases. Neutron diffraction experiments confirm earlier dilatometry measurements of M(s) in composites containing greater than or equal to 20 vol% ZrO2 and were instrumental in obtaining M(s) values in lower zirconia content (i.e., 10 vol%) composites. The dependence of M(s) on zirconia content is related to the internal stresses that arise from differences in thermal expansion coefficients between the two phases. Neutron diffraction measurements show that the internal tensile stresses in the zirconia grains increase with decreasing zirconia content. The measured internal stresses are in quantitative agreement with predictions based on models assuming isolated ZrO2 particles at low zirconia contents and a continuous ZrO2 ''matrix'' phase at higher zirconia contents. This assumption is consistent with the observed microstructural development in which the low zirconia contents result in isolated zirconia grains, whereas higher zirconia contents result in more interconnected zirconia grains.