No abstract

R-curves, process zones, and shielding stresses of barium titanate (BaTiO 3 ) and partially stabilized zirconia (PSZ) have been studied using compact-tension (CT) specimens. BaTiO 3 and PSZ exhibited pronounced R-curves that rose over similar crack lengths and showed steady-state toughnesses of 0.7 and 6.4 MPa⅐m 1/2 , respectively. Both steady-state toughnesses were ϳ80% larger than the initial fracture toughnesses. Ferroelastic domain switching was the main toughening mechanism in BaTiO 3 , whereas, in PSZ, transformation toughening was the main toughening mechanism. The crack process zone and crack-opening-displacement (COD) profile of each material was studied in detail using atomic force microscopy. Crackclosure-stress distributions were extracted from the COD profiles, using weight-function methods. The resulting stress profiles indicated that compressive residual stresses of 40 MPa in BaTiO 3 and 400 MPa in PSZ acted in a limited region behind the crack tip. In the PSZ, crack bridging seemed to be a competing mechanism to transformation toughening.

Fully dense disks, each consisting of a single crystal (111) plate of the relaxor‐based ferroelectric Pb(Mg1/3Nb2/3)O3‐35 mol% PbTiO3 (PMN‐35PT) embedded in a 0.48 ± 0.05 µm grain size polycrystalline matrix of the same composition, were formed by hot‐pressing at 950°C for 30 min under 20 MPa. Specimens were subsequently annealed to promote migration of the single‐crystal boundary through the polycrystal (a process termed seeded polycrystal conversion). An anneal of 10 h at 1150°C using PMN‐35PT packing powder resulted in minimal single‐crystal boundary migration, and was accompanied by matrix grain coarsening to 1.86 ± 0.20 µm. In contrast, an anneal of 10 h at 1150°C using PbZrO3 (PZ) sacrificial powder resulted in significant migration of the single‐crystal boundary through the polycrystal, accompanied by matrix grain coarsening to 13.3 ± 0.3 µm. The shape of the grown crystal relative to the seed plate was consistent with the <111> direction being the fastest growth direction. Based on the grown crystal dimensions, a lower bound <111> growth velocity of 0.14 mm/h was calculated. The increased boundary mobility in the specimen that was annealed using PZ sacrificial powder is attributed to a boundary‐wetting liquid PbO‐based second phase that formed during the anneal. This phase is believed to have formed via PbO absorption from the surrounding vapor phase due to a higher equilibrium PbO vapor pressure above PZ than above PMN‐35PT. The grown single crystal exhibited a promising <100> strain of 0.5% at an applied electric field of 4 MV/m.