| Popis: |
The metastable t-ZrO2 particles in both Y-TZP and Mg-PSZ can transform to m-ZrO2, and in the PSZ also to o-ZrO2, via a martensitic transformation. This research has demonstrated that the transformation is controlled by the thermally activated, stress-assisted nucleation of the martensite phase. The thermally activated character of the phase change was manifested as isothermal transformation kinetics in both materials: Residually stressed regions in indented Y-TZP's underwent isothermal t ⇒ m transformation during post-indentation anneals up to ~500°C. In Mg-PSZ's, slow isothermal t ⇒ m and t ⇒ o transformations occurred at room temperature after a short anneal at 1000°C, which reverses the room temperature aging. The phase transitions were accompanied by substantial transformation plasticity: Forward and reverse transformations during anneals in the residually stressed regions in the TZP caused large volume and shape changes, whose simulation with a finite element model was partially successful. In Mg-PSZ's the transformation was induced by internal stresses during room temperature aging as well as by external stresses applied during stress-relaxation tests; again, ample transformation plasticity ensued. Furthermore, the effect of the indentation-induced transformation in Mg-PSZ on the growth of indent cracks during four-point bending was investigated, and the crystallographically oriented and autocatalytic deformation zones around indents in Y-PSZ single crystals were analyzed. |
