Can Y2O3-MgO eutectics be new promising structural and optical ceramics?

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The manuscript addresses the solidification of Y2O3-MgO eutectic composites for the first time, and the study of their microstructure, mechanical properties and infrared transmittance. The composites with eutectic composition (20 % vol MgO) have been solidified using the laser floating zone method, at solidification rates between 25 mm/h and 750 mm/h. Their microstructure has been quantified and the conditions for coupled or cellular regime determined. They are dense, with elastic modulus, as determined from nanoindentation tests of around 200 GPa. The highest 3-point flexural strength (256±25 MPa) values and highest infrared optical transmittance has been observed for the material solidified with the finest, still homogeneous microstructure that could be achieved. This microstructure, which is achieved at a solidification rate of 50mm/h, consists of MgO fibers of 600 nm diameter embedded in the Y2O3 matrix. The thin MgO rods and the associated residual stress field contribute to the material strength as they deviate the propagating crack. The Vickers Hardness ranged from 9.5 to 11.5 GPa, with mild increase with the decrease of the microstructural size which has been rationalized as the MgO rods blocking the movement of dislocations in the matrix, as in other studied composites. Indentation fracture toughness values around 3 MPa.m1/2, independent of the sample microstructure, have been measured. In the longitudinal direction transmittance values higher than 80 % in the wavelength range from 3.5 to 7 μm, for 1 mm thick slices have been measured. Light scattering limits the short wavelength cut-off to around 2 μm for light travelling in the longitudinal direction, while a transmittance gap is observed in the transverse direction, of photonic crystal like nature.