Mise en ordre magnétique dans de nouveaux oxychlorures de baryum-cobalt à valences mixtes

National audienceLes structures magnétiques de deux nouveaux oxychlorures de baryum cobalt à valence mixte: Ba6Co6ClO16 (P-6m2, a=5,676(1) Å et c=14,457(2) Å) et Ba5Co5ClO13 (P63/mmc, a=5,698(1) Å et c=24,469(5) Å) ont été résolues par diffraction des neutrons. Les structures nucléaires de ces composés sont particulièrement similaires avec la présence de blocs oligomères Co4O15 ou Co3O12 connectés entre eux via des dimères tétraédriques Co2O7. Pour chacun des composés, des mesures de susceptibilité magnétique en fonction de la température ont montré l'existence d'une transition paramagnétique antiferromagnétique à basse température. Cette transition trouve son origine dans la mise en ordre antiferromagnétique des unités tétraédriques Co2O7

Tribology and high temperature friction wear behavior of MCrAlY laser cladding coatings on stainless steel

Temperature can have a significant effect on the extent of wear damage of metallic components. Thermal barrier coatings can improve the high temperature tribological and friction wear behavior. In this work the dry friction and wear behavior at low and high temperature of NiCoCrAlY and CoNiCrAlY laser cladding coatings were evaluated, as well as for the austenitic stainless steel AISI 304 used as substrate. Dense coatings, with good bonding to the substrate was obtained by coaxial laser cladding tracks (40% overlapping), with previously optimized laser parameters. Tribological wear tests were performed by sliding wear at room temperature and 500 ºC, with an Al2O3 ball on disk configuration tribometer. The wear scar surface was evaluated by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) microanalysis. The 3D topography of the wear track was determined by inductive contact profilometer which enabled the wear rate calculation. The microstructure of the coatings consists of γNi/β-NiAl or γCo/β-(Co,Ni)Al phases depending on the chemical composition of the alloy, as confirmed by X-ray diffraction (XRD) analysis. The wear test results show a reduction in wear rate at high temperature for all materials tested. For the NiCoCrAlY coating, the high temperature also reduces the friction coefficient, while it significantly increases the friction coefficient of CoNiCrAlY coating. The main damage mode is abrasion and adhesion, caused by the oxide and partially-oxidized particles in the contact surface. The coatings and substrate results were compared, resulting in an improved wear behavior.The authors acknowledge the financial support of the Ministry of Science and Innovation of the Government of Spain through research project MAT2011-28492-C03 and the Generalitat Valenciana through ACOMP/2013/114 support. Professor Juan Carlos Pereira Falcon thanks the University of Carabobo (Grant CD3997-2011) for the financial support to pursue his doctoral studies at the UPV. Furthermore, the authors thank Dr. Jose Luis Jorda for the XRD analysis.Pereira, JC.; Zambrano, J.; Licausi, M.; Tobar, M.; Amigó Borrás, V. (2015). Tribology and high temperature friction wear behavior of MCrAlY laser cladding coatings on stainless steel. Wear. 330:280-287. https://doi.org/10.1016/j.wear.2015.01.048S28028733

Laser Cladding of MCrAlY coatings on stainless steel

MCrAlY alloys (where M stands for Ni,Co or both) are commonly used as overlay protective coatings in gas turbine engine components against high temperature oxidation and corrosion. The protective effect of these alloys is due to the formation of a continuous thermally stable oxide layer on the coating surface. In this work several types of MCrAlY alloys, differing in their elemental composition, have been deposited on austenitic stainless steel by means of laser cladding. The microstructure of the coatings have been characterized by SEM-EDS and XRD. As expected, elemental composition compatible with γ-Ni/γ'-Ni3Al, β-Ni/β'-NiAl or γCo/β-(Co,Ni)Al phases are observed in hypoeutectic or hypereutectic microstructures depending on the alloy composition. The high temperature oxidation behavior of the coatings was evaluated by air furnace oxidation tests at 1100ºC for 200 h. The oxidized surface of the samples was examined by SEM-EDS and the oxides present identified by theoretical stoichiometric calculations. Results indicate the formation of a uniform Al3O2 protective oxide scale with NiO, CoO, Y2O3/YAlO3 and Cr2O3 oxide inclusions.Tobar, M.; Amado, J.; Yáñez, A.; Pereira Falcón, JC.; Amigó Borrás, V. (2014). Laser Cladding of MCrAlY coatings on stainless steel. Physics Procedia. 56(C):276-283. doi:10.1016/j.phpro.2014.08.172S27628356

A Critical Analysis of the Conventionally Employed Creep Lifing Methods

The deformation of structural alloys presents problems for power plants and aerospace applications due to the demand for elevated temperatures for higher efficiencies and reductions in greenhouse gas emissions. The materials used in such applications experience harsh environments which may lead to deformation and failure of critical components. To avoid such catastrophic failures and also increase efficiency, future designs must utilise novel/improved alloy systems with enhanced temperature capability. In recognising this issue, a detailed understanding of creep is essential for the success of these designs by ensuring components do not experience excessive deformation which may ultimately lead to failure. To achieve this, a variety of parametric methods have been developed to quantify creep and creep fracture in high temperature applications. This study reviews a number of well-known traditionally employed creep lifing methods with some more recent approaches also included. The first section of this paper focuses on predicting the long-term creep rupture properties which is an area of interest for the power generation sector. The second section looks at pre-defined strains and the re-production of full creep curves based on available data which is pertinent to the aerospace industry where components are replaced before failure

Mechanical and microstructural characterization of MCrAlY coatings produced by laser cladding: The influence of the Ni, Co and Al content

[EN] Laser metal deposition (LMD) and laser cladding (LC) are alternative methods to thermal spraying processes to produce dense, high-quality coatings. In this work, two MCrAlY coatings (M=Ni+Co) have been prepared onto stainless steel substrate using a coaxial LC technique under two different Ni/Co and Al proportions. The mechanical properties were then evaluated with microhardness, nanoindentation, and three-point bending tests. The microstructure and composition of coatings were characterized by X-ray Diffraction (XRD) analysis and Field Emission Electron Microscopy (FESEM) coupled to an Energy Dispersive Spectroscopy (EDS) detector. The study revealed that the ¿/ß phases formed in the MCrAlY coating microstructure result in a lower elastic modulus than the austenitic stainless steel substrate, while an inverse behavior for hardness was observed due the presence of the aluminum-rich ß-phase. Under flexural loads, the failure of coatings showed plasticity and anisotropy characteristics depending on the two laser tracks orientations evaluated.The authors would like to acknowledge the financial support of the Ministry of Science and Innovation of the Government of Spain through research project MAT2011-28492-C03, and the support of the Generalitat Valenciana through ACOMP/2013/114Pereira, JC.; Zambrano, JC.; Rayón, E.; Yañez, A.; Amigó, V. (2018). Mechanical and microstructural characterization of MCrAlY coatings produced by laser cladding: The influence of the Ni, Co and Al content. Surface and Coatings Technology. 338:22-31. https://doi.org/10.1016/j.surfcoat.2018.01.073S223133

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

New cobaltite materials containing CdI2-type layers: Synthesis and structures of Ba2Co4ClO7 and Ba2Co4BrO7

Single crystals of the title compounds were prepared by solid-solid reaction using BaCl2 or BaBr2 flux at 1100 C. The structures of these two new cobaltites were solved and refined in the trigonal symmetry with space group R3m: a ¼ 5.716(2) A° , c ¼ 45.01(3) A° for Ba2Co4ClO7 and a ¼ 5.7434(5) A° , c ¼ 46.151(9) A° for Ba2Co4BrO7. The two compounds are isostructural and their structures can be considered as the intergrowth along [001] of hexagonal blocks (Ba2Co8O14)2 built from a close-packing of [O4] and [BaO3] layers with octahedral and tetrahedral cobalt, separated by fluorite-type double layers (Ba2Cl2)2þ or (Ba2Br2)2þ. The main difference between Ba2Co4ClO7 and Ba2Co4BrO7 is due to the fluorite-type layers: (Ba2Cl2)2þ double layers are perfectly ordered while (Ba2Br2)2þ blocks are affected by a structural disorder through the bromine atoms