HIGH-TEMPERATURE PRECIPITATION HARDENING OF Y2O3 PARTIALLY-STABILIZEDZRO2 (Y-PSZ) SINGLE-CRYSTALS .2. A QUANTITATIVE MODEL FOR THE HARDENING

Single crystals of 4.5 mol. % Y2O3 partially-stabilized ZrO2 were anne aled for 500 and 1000 h at 1600-degrees-C to coarsen low-solute conten t, coherent, internally-twinned (''colony'') tetragonal ZrO2 precipita tes, and were subsequently deformed in air at 1400-degrees-C. Although all samples maintained a very high flow stress, greater-than-or-equal -to 350 MPa at 1400-degrees-C, overaging occurs between 150 and 500 h. The high flow stresses in this system arise from significant obstacle hardening, and the fact that matrix dislocations are partials in two of the three precipitate variants (the ''hard variants''); thus, this system is the ceramic equivalent of gamma/gamma' Ni-based superalloys. The flow stress in the peak-aged condition can be modeled assuming (i ) that particle shearing occurs via the collapse of Orowan loops and c auses faulting of alternating lamella variants, and (ii) that Labusch statistics obtain. Overaging is not associated with loss of coherency but occurs at a critical twin spacing; matrix dislocations can then bo w out in alternate softer twin variants within the colony precipitates . This leads to the generation of Orowan loops around the ''hard varia nts'', the collapse of which occurs via the formation of superdislocat ions. The stacking fault energy in the tetragonal ZrO2 precipitates is between 0.2 and 0.4 J/m2.