PLASTIC-DEFORMATION OF ZIRCONIA SINGLE-CRYSTALS - A REVIEW

The high-temperature deformation behaviour of zirconia single crystals stabilised with yttria is reviewed. Cubic or fully stabilised zirconi a (FSZ), which is considered the matrix of high-strength partially sta bilised zirconia (PSZ), deforms plastically down to 400 degrees C with out confining hydrostatic pressure. The relevant deformation behaviour above about 1200 degrees C is characterised by athermal dislocation m otion mainly on cube slip planes and diffusion-controlled recovery. Te tragonal polydomain zirconia or t' zirconia consists of plate-like tet ragonal domains alternately stacked to form large colonies. These colo nies are arranged in a characteristic way to fill the whole crystal vo lume. t' zirconia shows ferroelastic behaviour preceding dislocation p lasticity. In tension, a tetragonal single crystal forms containing re sidual defects. The data available indicate that the coercive stress s trongly depends on temperature. In situ straining experiments in a hig h-voltage electron microscope show an instantaneous switching of the i ndividual tetragonal domains with the domain boundaries moving sidewis e through the domains. Dislocations moving after the ferroelastic defo rmation are strongly bowed. The nature of the pinning agents is not cl ear yet. PSZ crystals are thought to consist of a cubic matrix and pre cipitates of the tetragonal phase of a structure similar to that of th e colonies in t' zirconia. Complete dislocations in the cubic matrix m oving on cube planes are partial dislocations in two of the three tetr agonal variants of the precipitates. They have then to produce a stack ing fault or antiphase boundary like defect. The strong age-hardening and overageing experimentally observed can be explained by a decreasin g width of the matrix channels between the precipitates and an increas ing width of the domains within the colonies. Recent in situ studies i n a high-voltage electron microscope have shown that the precipitates in PSZ may undergo ferroelastic deformation, too. Besides, in a number of cases the matrix of PSZ crystals turned out to be tetragonal rathe r than cubic. Thus, the formation of a tetragonal single crystal conta ining residual defects as during the ferroelastic deformation in t' zi rconia should affect the subsequent dislocation plasticity. The stacki ng fault or antiphase boundary like defects have experimentally been o bserved on {110} planes, however, not on the usual {100} slip planes. Thus, in spite of numerous experimental results a number of questions are still open particularly concerning the plastic deformation of PSZ crystals. (C) 1997 Elsevier Science S.A.