The influence of trace impurities on the mechanical characteristics of a superplastic 2 mol% yttria stabilized zirconia

Abstract

In contrast to metallic alloys, the mechanical characteristics of superplastic ceramics are very sensitive to minor changes in levels of trace impurities. In the present study, the mechanical behavior of a 2 mol% yttria stabilized tetragonal zirconia was studied in tension and compression in two batches of material, with small variations in levels of trace impurities, to examine the influence of stress axis and impurity content on the deformation behavior. The mechanical properties of the material were characterized in terms of the expression: ε ̇ ∝σ n where ε ̇ is the strain rate, σ is the stress and n is termed the stress exponent. The mechanical behavior of the ceramic was identical in tension and compression, for a material with a given level of impurity. The high purity specimens exhibited a transition from a stress exponent of ∼3 to ∼2 with an increase in stress, whereas the low purity material displayed only n∼2 behavior over the entire stress range studied. Detailed high resolution and analytical electron microscopy studies revealed that there was no amorphous phase at interfaces in both batches of material; however, segregation of Al at interfaces was detected only in the low purity material. The observed transition in stress exponents can be rationalized in terms of two sequential mechanisms: grain boundary sliding with n∼2 and interface reaction controlled grain boundary sliding with n∼3. The transition from n∼3 to ∼2 occurred at lower stresses with an increase in the grain size and a decrease in the purity level.