Depth-Sensing Indentation on REBa2Cu3O(7-\delta) Single Crystals obtained from Xenotime Mineral

A natural mixture of heavy rare earths oxides extracted from xenotime mineral have been used to prepare large single crystals of high-temperature REBa2Cu3O(7-\delta) superconductor grown using the CuO-BaO self-flux method. Its mechanical properties along the ab-plane were characterized using instrumented indentation. Hardness and elastic modulus were obtained by the Oliver and Pharr method and corresponds to 7.4 \pm 0.2 GPa and in range 135-175 GPa at small depths, respectively. Increasing the load promotes the nucleation of lateral cracks that causes a decrease in hardness and the measured elastic modulus by instrumented indentation at higher loads. The indentation fracture toughness was estimated by measuring the radial crack length from cube-corner indentations at various loads and was 0.8 \pm 0.2 MPa.m1/2. The observed slip systems of REBa2Cu3O(7-\delta) single crystals were [100](001) and [010](001), the same as for YBa2Cu3O(7-\delta) single crystals. The initial stages of deformation and fracture in the indentation process were investigated. The hardness and elastic modulus were not strongly modified by the crystallographic orientation in the ab-plane. This was interpreted in terms of the resolved shear stresses in the active slip systems. Evidence of cracking along the {100} and {110} planes on the ab-plane was observed. As a conclusion, the mechanical properties of REBa2Cu3O(7-\delta) single crystals prepared from xenotime are equivalent to those of YBa2Cu3O(7-\delta) single crystals produced by conventional rare earths oxides.Comment: The paper will appear in Volume 42 (2012) of the Brazilian Journal of Physic

Creep resistance of Fe–Ni–Cr heat resistant alloys for reformer tube applications

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Influence of microporosity and macroporosity on the mechanical properties of biphasic calcium phosphate bioceramics: Modelling and experiment

Macroporous biphasic calcium phosphate (BCP) bioceramics, for bone substitution applications, have been synthesized, cold isostatically pressed and pressureless sintered, using naphthalene particles as a porogen to produce macropores. The resulting materials are mixtures of P-tricalcium phosphate and hydroxyapatite with various microporosities and macroporosities. Mechanical properties (Young's modulus, compressive strength and fracture toughness) were measured on specimens over the widest attainable ranges of porosities, and compared to previously proposed analytical models and hypotheses. These models describe the evolution of the mechanical properties as functions of macroporosity and microporosity separately, the strength model considering macropores as critical flaws in the ceramic. Results show that the presence of macropores strongly influences the critical flaw size, but the latter appears to increase with macroporosity. This phenomenon can be explained by the presence of clusters of macropores, acting as critical flaws, becoming larger as macroporosity increases. (C) 2009 Elsevier Ltd. All rights reserved

Fabrication and mechanical properties of calcium phosphate cements (CPC) for bone substitution

Calcium phosphate cements have been used in medical and dental applications for many years. However, their low strength and their high brittleness prohibit their use in many stress-bearing locations, which would require an improvement in mechanical properties. The influence of microstructural parameters on the latter has nevertheless barely been investigated in a systematic manner. To this aim, apatite cements have been fabricated through alpha-TCP (alpha-tricalcium phosphate) hydrolysis, and their mechanical properties have been measured (Young's modulus, fracture toughness and compressive strength), as a function of various parameters (particle size, liquid-to-powder ratio, amount and morphology of porosity, including macropores created by mannitol particles used as porogen). Five days following the mixing of phases, identification and microstructural observation indicated the presence of unreacted alpha-TCP particles, exhibiting very weak links with the apatite matrix and often surrounded by microcracks. The latter provoke a decrease in Young's modulus. The coarser the microstructure, the larger the critical flaw size causing fracture. In the case of macroporous materials, the critical flaw size increases with macroporosity. The knowledge gained should allow, in the end, to improve mechanical properties by controlling the microstructure, and to find a better compromise between strength and biological behaviour. (C) 2010 Elsevier B.V. All rights reserved

Surfactant effect of impurity sulphur in ductility dip cracking of a high-chromium nickel model alloy

International audienceThe sensitivity to ductility dip cracking was measured in two model high chromium nickel alloys, with the same composition, apart from different sulphur contents. The newly developed Refusion Cracking Test was used, that consists in repetitive refusion lines conducted at the specimen surface. Grain boundary cracks develop in the heat affected zone near the refusion line. Cracking is much more pronounced in the sulphur-enriched alloy, which demonstrates a strong deleterious effect of sulphur. However post-mortem analyses using WDS and STEM-EDS revealed no segregation of sulphur at grain boundaries. In contrast, grain boundary fracture surfaces are covered with sulphur. This suggests a dynamic type of grain boundary embrittlement where sulphur acts as a surfactant, facilitating crack opening. Sulphur is efficiently provided to the crack tip as it propagates, due to accelerated diffusion by plastic deformation. This allows crack growth rates higher than 10 µm/s

Microstructure and properties of oxygen controlled melt textured NdBaCuO superconductive ceramics

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Preparation and characterization of melt textured NdBa

NdBaCuO superconductors have been textured by the oxygen controlled melt growth method. The addition of platinum or Nd422 does not refine the 422 phase, but the latter improves the microstructural quality. High purity precursors are necessary to achieve high Tc and Jc (up to 56 000 A/cm2). The flux pinning mechanisms are discussed in terms of oxygen deficiency, Nd-Ba substitution and softening of the vortex lattice