"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...

This paper reports on a comparative study of the mechanical performance at different temperatures of a commercial Portland cement, used as a control, and a new cementitious material made from an 8M-NaOH activated fly ash and containing no OPC. Two types of mechanical tests were conducted: (i) high temperature mechanical tests, to determine the strength and fracture toughness of the two materials between 251 and 6001C, and (ii) post-thermal treatment tests, to evaluate the residual strength after 1 h of exposure to different temperatures (2001, 4001, 6001, 8001, and 10001C). In both cases, the results showed that the new cementitious material performed significantly better at high temperatures than the Portland cement control. Differential thermogravimetry (DTG)/TG, Fourier transform infrared (FTIR), and X-ray diffraction analyses were also conducted to analyze the mineralogical and microstructural variations taking place in the material as a result of high temperature exposure. The results of these tests were correlated with the mechanical behaviour observed.

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The effect of Y2O3 content on the flexure strength of melt‐grown Al2O3–ZrO2 eutectics was studied in a temperature range of 25°–1427°C. The processing conditions were carefully controlled to obtain a constant microstructure independent of Y2O3 content. The rod microstructure was made up of alternating bands of fine and coarse dispersions of irregular ZrO2 platelets oriented along the growth axis and embedded in the continuous Al2O3 matrix. The highest flexure strength at ambient temperature was found in the material with 3 mol% Y2O3 in relation to ZrO2(Y2O3). Higher Y2O3 content did not substantially modify the mechanical response; however, materials with 0.5 mol% presented a significant degradation in the flexure strength because of the presence of large defects. They were nucleated at the Al2O3–ZrO2 interface during the martensitic transformation of ZrO2 on cooling and propagated into the Al2O3 matrix driven by the tensile residual stresses generated by the transformation. The material with 3 mol% Y2O3 retained 80% of the flexure strength at 1427°C, whereas the mechanical properties of the eutectic with 0.5 mol% Y2O3 dropped rapidly with temperature as a result of extensive microcracking.