"Smaller is stronger" is a paradigm in materials development. [1] In the case of metallic materials, the smaller the crystals which make up the material, the more obstacles there are to plastic deformation due to dislocation motion. The utmost limit is found in bulk metallic glasses, which have strengths of up to E/40 (E being the elastic modulus), closer than any other bulk material to the theoretical strength, given by ≈E/20. [2] In ceramic polycrystals, dislocation motion is impeded at ambient temperature, and the strength is controlled by the dimensions of the critical defects. Assuming a semi-circular surface flaw of diameter d, the strength is given byin which K C stands for the fracture toughness; improvements in strength are associated with an increase of the toughness or a reduction of the defect size, which is normally related to the characteristic dimension of the microstructure. Higher toughness in bulk ceramics has been achieved by transformationtoughening of ZrO 2 -based materials [3] or by the self-reinforcement of silicon nitride materials with elongated single-crystal whiskers which led to crack bridging and deflection upon crack propagation.[4] The strength, however, increases linearly with toughness, and the maximum strength of these bulk ceramics was slightly over 1 GPa. Higher strengths could be achieved only by reducing the flaw sizes below the micron size, but the synthesis of bulk nanoceramics with a homogeneous and defect-free (porosity, impurities) microstructure and uniform grain size distribution is still a daunting task, regardless of the recent developments to process nanostructured materials.[5] For instance, pressureless sintering of Al 2 O 3 -based ceramics always leads to grain growth, which limits mechanical properties, and extremely high pressures (as high as 1 GPa) are needed for low temperature sintering. Ultrahard nanoceramics with equiaxed grains were recently produced by devitrification of eutectic corundum-based glasses, [6] and the authors reported a homogeneous microstructure which could be tailored by phase assemblage and heat treatments to a maximum toughness of 4.2 MPam 1/2 , which seems to be the limit for Al 2 O 3 -based nanoceramics.Directionally-solidified eutectics (DSE) are self-organized materials in which phase segregation is driven thermodynamically to produce homogeneous and coherent microstructures. The competition between the flux perpendicular to the growth front with a scale length d C = 2D/m, where D is the diffusion coefficient in the melt and m the growth rate, and the lateral diffusion with a scale length k, the domain size, produces the eutectic morphology that can be controlled by the processing conditions. In particular, k decreases with m according to the equation k 2 m = C (C is a material-dependent constant), [7] and bulk nanostructured ceramics can be obtained in principle by rapid directional solidification from the melt. Under the ideal conditions of coupled eutectic growth, regular eutectic structures can be obtained made up of stacking l...
