Papers by Keyword: Rafting

Abstract: Two kinds of representative volume elements (RVEs) are introduced to represent the experimental γ/γ′ morphology to study the influences of microstructures on the creep behavior of [0 0 1]-oriented single crystal nickel-based superalloys under tensile loading. One RVE (RVE1) is consisted with one cuboidal γ′ phase surrounded by γ phase. The other (RVE2) is constructed by two cuboidal γ′ phases and one rectangle γ′ phase. A raft criterion is implemented into a user subroutine to predict the rafting type. The misfit stress is considered by different thermal expansion coefficients of the two phases. The rafting type is correctly predicted. The evolutions of the stresses distributions are discussed.

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.

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.

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.

Abstract: An elasto-viscoplastic phase field model is proposed to study the microstructural evolutions under mechanical loadings in a Ni-base superalloy single crystal. Elastic anisotropy and inhomogeneity, as well as the description of the long range order in the γ’ phase are included in the model. Plastic activity is introduced using a continuum crystal plasticity framework. The coupled model is used to study microstructural evolutions in superalloys under creep at high temperature. Two loading orientations are investigated, [001] and [011]. Moreover simulations under alternate fatigue loading in the [001] orientation are performed. The viscoplasticity simulations are compared to purely elastic phase field simulations and experiments in order to show the influence of plastic activity on microstructural evolutions.

Abstract: Nickel-based creep resisting alloys (strengthened by γ´) are the basic materials for high-temperature constructional parts in aircraft engines and energy units. These parts are exposed to combined effects of mechanical stresses, high temperature and dioxide-corrosion conditions. The microstructure changes of cast polycrystalline Ni-based superalloy IN713LC after creep exposure were studied. Three specimens with three different diameters were used for creep tests. The degradation stage (damage parameter π) was determined for all parts of specimens. Individual parts of specimens were metallographic observed and analyzed by image analysis after rupture. The results were compared with model of stress distribution in the specimen with potential damage in the centre of the specimen.

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.

Abstract: Many investigations about superalloys and coatings have been done in the laboratory, but evaluating the degradation condition of hot section components during service is still important not only for repair and reuse but also for outage prevention. Time dependent degradation for second stage blades of gas turbine was investigated. The degradation analysis for used blades was divided into microstructure changes by position of the blade and mechanical tests of high temperature tensile test. In the microstructure analysis, the rafting and coarsening of γ', MC decomposition and TCP phase formation occurred and progressed with increasing service time, and especially the leading and trailing edge of top layer should be a check points for used blade. High temperature tensile results of 25,000 and 52,000 hrs used blades were also compared with serviced time and position in each blade.

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.

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.