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Mesure et Evolution des gradients de propriétés mécaniques dans le système superalliage à base de nickel MC2 revêtus McrAlY

Abstract : Monocrystalline nickel based superalloys are the most suitable materials for the design of high performance turbine blades. In service, these mechanical parts undergo both harsh mechanical and thermal solicitations. The chemical composition and also microstructure of these superalloys have been optimized through different generations to resist creep deformation at temperature higher than 1373K. To protect these materials from environment stresses (isothermal and cyclic oxidation at high temperature, intermediate temperature corrosion) blades are generally coated with a NiAlPt or MCrAlY layer (M=Ni, Co). Both deposition process and thermomechanical treatment generate an interdiffusion zone (IDZ) between the coating and the substrate. Moreover, the microstructure of the different strata (superalloy, coating and interdiffusion zone) evolve in service due to the combine effect of high temperature and stress. Such evolutions may locally affect the mechanical properties of turbine blades. MCrAlY coatings have been studied extensively from an oxidation and hot corrosion point of view. The drop of mechanical properties of superalloys because of the presence of coating has been investigated on full system, so that to say the coated superalloy. These study aims to estimate a none bearing thickness of the superalloy independently from the size of the interdiffusion zone. Refinement of predictive models on mechanical behaviour and lifetime of materials require data relative to local properties and their evolution when ageing. These data were not available until now. Hence, this work deals with microstructural and mechanical characterisation of the interdiffusion zone of the system MC2 coated with NiCoCrAlYTa by electrochemical codeposition. Tensile specimens were extracted from the different zones of this functionally graded material on “as received” state and after ageing with and without applied stress. Then, these specimens were mechanically tested at high temperature in optimised controlled atmosphere in order to prevent ultrathin specimens from oxidation effects on the mechanical behaviour. These specimens in a range of 20 to 400μm thick were obtained by use of a lapping machine in order to limit the impact of gritting on residual stress. Two mechanical test rigs were especially designed for this study to perform creep and tensile testing on these ultrathin specimens under controlled atmosphere at temperature up to 1473K. Results obtained from these ultrathin specimens were compared with testing conducted on bulk materials. The “size effect” was explored in parallel so that to conclude on the feasibility of using these data to quantify the behaviour gradient of this system.
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Submitted on : Wednesday, June 27, 2018 - 10:12:57 AM
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Damien Texier. Mesure et Evolution des gradients de propriétés mécaniques dans le système superalliage à base de nickel MC2 revêtus McrAlY. Mécanique des matériaux [physics.class-ph]. Institut National Polytechnique de Toulouse, 2013. Français. ⟨tel-01824322⟩



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