Superplastic flow behavior in 1 mol%GeO
2 and 1 mol%NdO
1.5 co-doped tetragonal ZrO
2-3 mol%Y
2O
3 polycrystal (3Y-TZP) was examined at 1400°C under an initial strain rate of 1.3×10
-4 s
-1. 1 mol%GeO
2 or NdO
1.5-doping slightly enhances high-temperature ductility in 3Y-TZP, but 1 mol%GeO
2 and 1 mol%NdO
1.5 co-doped TZP exhibits large elongation to failure of more than 600%. The large ductility in TZP due to Ge
4+ and Nd
3+ co-doping can be explained by reduced flow stress. High-resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy (EDS) analysis revealed that Y
3+, Ge
4+ and Nd
3+ cations segregate in the vicinity of grain boundaries in the present materials. The segregation width of the dopant cation across the grain boundaries in GeO
2 and NdO
1.5 co-doped TZP is larger than that in GeO
2 or NdO
1.5 singly doped TZP. The reduction of the flow stress due to GeO
2 and NdO
1.5 co-doping is probably related to the increment in the segregation width.
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