Papers by Keyword: Rafting

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Authors: Mustapha Jouiad, J. Ghighi, Jonathan Cormier, E. Ostoja-Kuczynski, G. Lubineau, J. Mendez
Abstract: An unprecedented investigation consisting of the association of X-Ray tomography and Scanning Electron Microscopy combined with Focus Ion Beam (SEM-FIB) is conducted to perform a 3D reconstruction imaging. These techniques are applied to study the non-isothermal creep behavior of close (111) oriented samples of MC2 nickel base superalloys single crystal. The issue here is to develop a strategy to come out with the 3D rafting of γ’ particles and its interaction whether with dislocation structures or/and with the preexisting voids. This characterization is uncommonly performed away from the conventional studied orientation [001] in order to feed the viscoplastic modeling leading to its improvement by taking into account the crystal anisotropy. The creep tests were performed at two different conditions: classical isothermal tests at 1050°C under 140 MPa and a non isothermal creep test consisting of one overheating at 1200°C and 30 seconds dwell time during the isothermal creep life. The X-Ray tomography shows a great deformation heterogeneity that is pronounced for the non-isothermal tested samples. This deformation localization seems to be linked to the preexisting voids. Nevertheless, for both tested samples, the voids coalescence is the precursor of the observed damage leading to failure. SEM-FIB investigation by means of slice and view technique gives 3D views of the rafted γ’ particles and shows that γ corridors evolution seems to be the main creep rate controlling parameter.
Authors: Jie Shan Hou, Jian Ting Guo, Chao Yuan, Yong An Guo, Gu Song Li, Lan Zhang Zhou
Abstract: The effects of the selective addition of Hafnium (Hf) on the grain boundary, phase, carbides and creep properties of experimented nickel superalloy after standard heat treatment and long-term exposure were investigated. Predicted by the Bayesian neural network, the creep life is prolonged with Hf content of 0-0.6 mass%, which is more effective at low stresses. The decrease of creep life of Hf free alloy after long term exposure was pronounced. Comparative study showed that the mainly small, coherent, blocky and closely spaced MC(2) and M23C6 carbides precipitated on the grain boundaries in the 0.4wt% Hf contained alloy, and that relatively larger, incoherent MC(1) carbides precipitated on the grain boundaries in the Hf free alloy. During long term thermal exposure, fine discrete M23C6 carbides decomposed from primary carbide, inducing a layer along the grain boundary, and the coarsening of grain boundary in Hf free alloy is more pronounced. At high stresses, the Hf-free alloy exhibited a stronger tendency of rafting than the 0.4Hf alloy, while the tendency of appearance of rafting was very similar at low stresses. However, Hf can render the alloy prone to the formation of σ phase, according to D-electrons method. Thus, the Hf content needs to be controlled to a suitable level.
Authors: Jiří Buršík
Abstract: Two-phase microstructure of ordered cube-shaped precipitates in the disordered matrix is characteristic of Ni-base superalloys. This microstructure degrades under the applied stress: depending on the stress direction, lattice misfit and elastic parameters of both constituent phases, the precipitates coalesce and change their overall shape. Various atomic configurations were modeled in this work representing various morphologies of precipitates developed under applied stress. A model Ni-base alloy containing six alloying elements typical of advanced Ni-based superalloys was used. Generated configurations were further subject to study of elastic parameters by means of computer straining experiments. Relaxation of atomic positions in the strained crystal blocks was implemented using molecular dynamics calculations with phenomenological Lennard-Jones pair potentials and interactions involving three coordination spheres. Changes of elastic parameters due to varying precipitates morphology are discussed.
Authors: Alexander Epishin, Udo Brückner, Thomas Link, Pedro D. Portella
Authors: I.S. Kim, Baig Gyu Choi, Seong Moon Seo, Chang Yong Jo
Abstract: Microstructural evolution during high temperature exposure and its effects on tensile and stress rupture properties of the Ni-base superalloy B1900 have been studied. Tensile deformation of the as-cast specimen was concentrated in the localized slip bands in general. Stacking faults and deformation twins were observed in the as-cast tensile specimen tested at 871°C where the alloy exhibited the lowest ductility. Dense dislocation network formed at γ/γ´ interface during thermal exposure caused homogenous deformation in the thermally exposed tensile specimen. Thermal exposure did not have significant effect on the stress rupture lives of the alloy at 760°C and at and above 871°C but it reduced stress rupture life of the alloy at 816°C γ´ coarsening and coherency loss at the γ/γ´ interface during thermal exposure were primarily responsible for the deterioration of mechanical properties and characteristic deformation behavior of the alloy.
Authors: Jin Jiang Yu, Xiao Feng Sun, Tao Jin, Heng Rong Guan, Zhuang Qi Hu
Abstract: Stress rupture and tensile properties of a single crystal superalloy DD32 are investigated comparing with the alloy SRR99. It is shown that the alloy DD32 offers an improved creep temperature capability of more than 60°C at higher stresses. The g¢ precipitates in the stress ruptured samples were rafted to P-N type directional coarsening. The fracture mechanism of the stress ruptured samples was initiated from the micropores.
Authors: Akiyuki Takahashi, Yutaka Kobayashi, Masanori Kikuchi
Abstract: This paper describes phase field simulations of the rafting behavior of γ’ phase with a simple interfacial dislocation network model. The interfacial dislocation network model accounts for the effect of the network on the lattice misfit between γ and γ’ phases and the subsequent rafting behavior. The model is implemented into the phase field simulation to see the dependence of the rafting behavior of γ’ phases on the interfacial dislocation network. Without the dislocation network model, the amount of the rafting was negligibly small. On the other hand, with the dislocation network model, the γ’ phases shows a large amount of rafting, which is in good agreement with the results of the experimental observations. Therefore, the combination of the phase field method and the simple interfacial dislocation network model developed in this work is appropriate for the simulation of the rafting of γ’ phases.
Authors: Toshiya Tanimoto, Yuhki Tsukada, Yoshinori Murata, Toshiyuki Koyama
Abstract: Microstructural evolution in single crystal Ni-based superalloys is investigated by the phase field simulation. During creep, the morphology of the γ phase changed from the cuboidal shape to the rafted one, and the rafted structure was collapsed in the late stage of creep. The simulation on the microstructural evolution is based on thermodynamic information, diffusion equation, elastic anisotropy and a homogeneous lattice misfit. It is found that caused by external stress result in the morphological change of the γ phase to the rafted structure, and this rafted structure is collapsed by inhomogeneous lattice misfit. These morphological changes can be explained by the change in stable morphology of the γ phase.
Authors: Armand Coujou, Mustafa Benyoucef, M. Legros, Nicole Clément
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