Euro Superalloys 2010

Volume 278

doi: 10.4028/

Paper Title Page

Authors: De Xin Ma, Michael Mathes, Bin Zhou, Andreas Bührig-Polaczek
Abstract: Freckle occurrence is known to be dependent on the casting size and the superalloy components with large cross section are more prone to freckle formation. In our directional solidification experiment of superalloys freckles are observed in some thin samples, while some significantly thicker samples in the same shell mould cluster remain freckle free. The EBSD analysis shows that the samples with freckles have a good <001> axial orientation, while in freckle free samples the primary dendrite stems diverge from the sample axis. In an analogue system the flow behaviour through a mushy zone was simulated, in which the dendrite network was placed in different orientation. The <001> dendrite array shows the lowest resistance to the vertical melt flow. As the dendrite alignment diverges from <001> orientation, the flow speed slows down and reaches the minimum at the <111> orientation. This orientation effect of the interdendritic permeability explains well why the crystal orientation plays such an important role in the occurrence of freckles in superalloy castings.
Authors: Simona Hutařová, Marta Kianicová, Tomás Vlasák, Pavel Hutař, Tomáš Podrábský, Jan Hakl
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.
Authors: Katsushi Tanaka, Wataro Hashimoto, Toru Inoue, Haruyuki Inui
Abstract: The effect of elastic driving force on the microstructural change of superalloys in the secondary creep stage is evaluated by elastic energy calculations with the concept of effective eigen strain where both lattice mismatch and creep strain are taken into account The elastic energy calculations indicates that the elastic state in the secondary creep stage is totally different to that in the initial one where the lattice misfit between γ and γ' phases is over accommodated along the [100] and [010] directions by creep deformation in the γ phase. The excess creep dislocations for the over accommodation are required so as to develop an internal stress field to prevent further creep deformations. The planer raft structure with the plane normal oriented to the [001] direction is unstable in the over accommodated state. The γ/γ' lamellar interfaces will be inclined to make a wavy raft structure of which elastic energy is lower than the ideal 001 planer raft structure.
Authors: N. Miura, Y. Kondo, Keiji Kubushiro, Satoshi Takahashi
Abstract: The changes in the morphology of ’ precipitates and the dislocation substructures at the vicinity of grain boundaries with creep deformation were investigated on a polycrystalline nickel-based superalloy, IN-100. The experiments were done at 1273K in the stress range of 70-180MPa with one set of samples deformed until rupture and another interrupted after reaching the minimum creep rate. The cuboidal ’ precipitates were regularly arrayed at the vicinity of grain boundaries and eutectic ’ precipitates and carbides were observed at the grain boundaries on as-heat treated IN-100. However, on both the creep interrupted and ruptured samples at all stress conditions, plate - like shaped ’ precipitates covered the grain boundaries. The width of the plate - like shaped ’ precipitates increased with decreasing the stress and is wider in the creep ruptured than in the creep ruptured samples. There were few dislocations in the grains and in the plate - like shaped ’ precipitates, but a large number of the dislocations were observed around the interface of the plate - like shaped ’ precipitates. TEM observations on the specimen creep ruptured at 180MPa indicated that strong constant was observed at the vicinity of grain boundaries, but weak contrast in the grain interior. These results suggested that drastic plastic deformation occurred only at the vicinity of grain boundaries. Consequently, the plate - like shaped ’ precipitates at the grain boundaries inhibited the dislocations movement and acted as the creep strengthener.
Authors: Y. Kondo, N. Miura
Abstract: The single crystal nickel-based superalloy, CMSX-4, creep interrupted at the creep strain of 0.01 at 1273K-400MPa was aged at 1273K without stress to make clear the /' rafting mechanism. And the changes in the ' morphology and the dislocation substructures at the /' interface with simple aging time were studied and compared with the previous work at 1273K-250MPa. The cuboidal ' precipitates were regularly arrayed when the creep test was interrupted, but changed to the rafted /' structures normal to the pre-creep stress axis with simple aging time. The aspect ratio of the ' precipitates increased with increasing simple aging time up to 3.60x106s, attained to the maximum value and then decreased. The maximum value of the aspect ratio at 400MPa was lower than one at 250MPa. A number of dislocations were tangled with each other at the /' interfaces of the creep interrupted specimen. These tangled dislocations were only rearranged by simple aging and the dislocation density at the /' interfaces was almost constant, independent of simple aging time. All dislocations of the creep interrupted specimen at the /' interfaces with six kinds of the <110> Burgers vectors were detected equally. The proportion of the six kinds of the <110> Burgers vectors didn’t change with simple aging time. Consequently, dislocations at the /' interfaces were considered as not the misfit ones, but the traces of mobile dislocations in the  channels and the formation of the rafted /' structures might be closely correlated with the rearrangement of tangled dislocations at the /' interfaces.
Authors: Benedict M.B. Grant, Elisabeth Knoche, Michael Preuss, Joao Quinta da Fonseca, Mark R. Daymond
Abstract: Understanding the relationship between deformation mechanisms and microstructure is essential if one wants to fully exploit the potential of advanced nickel base superalloys and develop future alloys. In the present work, the influence of the lattice misfit between  and ’ has been studied by means of in-situ loading experiments using neutron diffraction in combination with crystal plasticity modelling on RR1000 and Alloy 720Li. Both alloys were processed to generate three simplified uni-modal γ’ microstructures to allow determination of γ’ responses and experiments were carried out at 750°C. The results showed that a positive misfit strain increases the level of load partitioning from  to ’ during plastic deformation introduced by uniaxial tensile loading.
Authors: V.A. Vorontsov, R.E. Voskoboinikov, Catherine M.F. Rae
Abstract: The “Phase-Field Model of Dislocations” (PFMD) was used to simulate shearing of gamma-prime precipitate arrays in single crystal turbine blade superalloys. The focus of the work has been on the cutting of the L12 ordered precipitates by a<112>{111} dislocation ribbons during Primary Creep. The Phase Field Model presented incorporates specially developed Generalised Stacking Fault Energy (–surface) data obtained from atomistic simulations. The topography of this surface determines the shearing mechanisms observed in the model. The merit of the new –surface, is that it accounts for the formation of extrinsic stacking faults, making the model more relevant to creep deformation of superalloys at elevated temperatures.
Authors: Martin M. Franke, Michael Hilbinger, Astrid Heckl, Robert F. Singer
Abstract: This paper presents the results of an investigation on the interrelationship between thermophysical properties, processing conditions and primary dendrite arm spacing for a nickel-base superalloy. The research was realized for CMSX-4, directionally solidified in a Bridgman furnace. For a systematic, fast and cost-efficient investigation numerical finite element models were applied. The numerical model, composed of thermophysical material data, geometric data and boundary conditions, was calibrated and experimentally validated. Microstructural parameters of the castings were determined for a broad range of processing conditions and varying thermophysical properties in order to study general influences. Withdrawal speed, furnace temperature, enthalpy of fusion, solidification range, heat conductivity and specific heat were varied accordingly. The primary dendrite arm spacing is predominantly influenced by withdrawal speed and furnace temperature, but shows only a weak dependency on thermophysical properties.
Authors: Hermann Maderbacher, H.P. Gänser, Martin Riedler, Michael Stoschka, Martin Stockinger, Wilfried Eichlseder
Abstract: Heavy-duty aerospace components are frequently hot forged to satisfy the high requirements concerning their mechanical behaviour. Only the usage of high-performance materials together with a near-optimum manufacturing process enables the production of parts that are at the same time lightweight and mechanically extremely durable. Not only the static properties, but also the fatigue behaviour of Inconel718 is strongly influenced by the material’s microstructure resulting from the forging and heat treatment processes. Therefore, the static and fatigue properties may be controlled via the microstructural properties by suitably adjusting the parameters of the manufacturing processes. The present work links the complete forging and heat treatment process to the local distribution of the material’s fatigue strength within a component; the effect of the operating temperature is also considered. To this purpose, an empirical model is derived from fatigue tests on specimens with different microstructures at different temperatures. The resulting fatigue strength model is implemented, along with a microstructural evolution model from earlier work [1], into a finite element code in order to predict the local fatigue strength distribution in a component after being subjected to an arbitrary forging process. In a further step, the finite element code is linked to an optimization tool for determining the optimum set of manufacturing process parameters such that the component lifetime is maximized while taking process constraints into consideration.
Authors: Daniel Huber, Christof Sommitsch, Martin Stockinger
Abstract: Aerospace gas turbine disks operate in an environment of relatively high stresses caused by centrifugal forces and elevated temperatures. Because of the strong mechanical requirements and narrow specifications of such parts not only a correct, defect free final geometry is necessary, but also a defined microstructure. Even though the microstructure evolution during thermo-mechanical processing is well studied and understood for superalloys like IN718, the influences cannot easily be described analytically. Thus simulation tools are used to assure process stability and to optimize design parameters to meet the tough requirements in aerospace industries. Microstructure simulation of IN718 (and other materials) is well established at Bohler Schmiedetechnik GmbH & Co KG and appreciated by its customers. The advent of the newly developed nickel-base superalloy ATI Allvac® 718PlusTM led to extensive investigations and the development of an adapted microstructure model by Bohler Schmiedetechnik GmbH & Co KG and its research partners. Aim of this paper is a comparison of the microstructure evolution in IN718 and ATI Allvac® 718PlusTM during the thermo-mechanical treatment of turbine disks. Influences of process temperature, strain and strain rate on the final grain size are discussed by finite element simulations with a coupled grain structure model. Experimental results from trial forgings are compared with the outcome of the microstructure simulations.

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