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Knobbe H.,IWT - Foundation Institute of Materials Engineering | Koster P.,Institute Fr Mechanik und Regelungstechnik Mechatronik | Christ H.-J.,IWT - Foundation Institute of Materials Engineering | Fritzen C.-P.,Institute Fr Mechanik und Regelungstechnik Mechatronik | Riedler M.,Bohler Schmiedetechnik GmbH and Co KG
Procedia Engineering | Year: 2010

Forged Ti6Al4V alloy in two different microstructures was used for investigations on fatigue behaviour with special focus on crack initiation and short crack propagation characteristics. Both microstructures are in the bi-modal condition containing different amounts and sizes of primary alpha grains. Interrupted fatigue experiments were carried out using a servohydraulic test facility. Different stress levels were imposed at a constant R ratio of -1 and a frequency of 20Hz with a sinusoidal command signal. SEM together with the EBSD technique was applied for the crack observation as well as for the determination of local crystallographic orientation data with the objective of linking initiation sites and crack paths to microstructural features. It was found that most of the cracks initiate on boundaries between two lamellae in favourably oriented colonies. These cracks propagate on prismatic glide planes with high Schmid factors until they reach another boundary. In some cases crack splitting was observed leading to crack propagation in different directions on different slip planes resulting in a reduction of crack growth rate. The crack paths can be attributed either to prismatic slip planes or the basal slip plane, whereas basal slip was found inside the lamellae or in primary alpha grains. Crack deflection at boundaries is also a common feature, and is probably related to high tilt and twist angles between the grains involved. © 2010 Published by Elsevier Ltd.


Warchomicka F.,Vienna University of Technology | Poletti C.,University of Graz | Stockinger M.,Bohler Schmiedetechnik GmbH and Co KG
Materials Science and Engineering A | Year: 2011

Structural applications of near beta titanium alloys are gradually increasing in the aerospace industry because of their high specific mechanical properties and good corrosion resistance. Furthermore, a wide range of microstructures can be obtained by thermomechanical processes. This work determines by the use of EBSD technique the mechanism of restoration active in the near beta titanium alloy Ti-5Al-5Mo-5V-3Cr-1Zr for deformations in both α+β and β field near to the β transus temperature (T β=803°C). Hot compression tests are carried out up to 0.7 true strain by means of a Gleeble ® 1500 machine at strain rates of 0.01, 0.1 and 1s -1. Dynamic recovery of β phase and rotation of the α grains take place predominantly in the α+β field. Further deformation produces continuous dynamic recrystallization of the β phase influenced by the strain rate. Dynamic recovery is observed during deformation above the T β, where the misorientation is increasing towards the grain boundaries forming new small grains with a substructure at high strain rates and larger deformation. The stress exponent and the apparent activation energy for the sinh constitutive equations are determined and the microstructural features are correlated with the Zener-Hollomon parameter. © 2011 Elsevier B.V.


Radis R.,Christian Doppler Laboratory | Radis R.,University of Graz | Zickler G.A.,Christian Doppler Laboratory | Stockinger M.,Bohler Schmiedetechnik GmbH and Co. KG | And 4 more authors.
Materials Science Forum | Year: 2010

In this paper, the precipitation behaviour of δ (Ni 3(Nb,Al)) and γ' (Ni3(Al,Ti,Nb)) phases in the nickel-base superalloy ATI Allvac® 718Plus™, as well as their kinetic interactions are discussed. Important parameters such as volume fraction, mean radius and number density of precipitates are experimentally determined and numerically simulated as a function of the heat treatment parameters time and temperature. To match the experimentally observed kinetics, the predicted interfacial energy of the precipitates, as calculated for a sharp, planar phase boundary, is adjusted to take into account the interfacial curvature and entropic effects of a diffuse interface. Correction functions for the interfacial energies of δ as well as γ' precipitates are presented. Using these modified interfacial energies, the calculated results show excellent agreement with the experimental measurements. © (2010) Trans Tech Publications.


Krumphals A.,Christian Doppler Laboratory | Poletti C.,Christian Doppler Laboratory | Warchomicka F.,Vienna University of Technology | Stockinger M.,Bohler Schmiedetechnik GmbH and Co KG | Sommitsch C.,Christian Doppler Laboratory
International Journal for Multiscale Computational Engineering | Year: 2014

The static coarsening behavior of the alpha-beta titanium alloy Ti-6Al-4V during heat treatments is modeled using a probabilistic cellular automata model (CA). For this purpose the kinetics of grain growth is described via transformation probabilities which are determined by diffusion mechanisms at grain and phase boundaries. For temperature changes an algorithm is implemented which adjusts the fraction of alpha and beta phase to reach equilibrium phase values. Hence, the CA is capable of calculating grain coarsening as well as grain dissolution in the two-phase area during heating and isothermal treatments at forging temperature. For these calculations, an initial microstructure is used as input and it can be imported from either virtual created microstructures, real micrographs, or electron backscatter diffraction (EBSD) maps. The model output includes mean diameter, grain size distribution, and virtually simulated microstructures which can be easily compared with experimental micrographs. Examples showing a good correlation between the predicted microstructures and experimental results, as well as data from literature, are presented in this work. The successful implementation of this model will lead to predictions of behavior in other dual-phase alloys. © 2014 by Begell House, Inc.


Zickler G.A.,Christian Doppler Laboratory | Radis R.,Christian Doppler Laboratory | Radis R.,University of Graz | Schnitzer R.,Christian Doppler Laboratory | And 3 more authors.
Advanced Engineering Materials | Year: 2010

ATI Allvac 718Plus is a novel nickel-based superalloy, which was designed for heavy-duty applications in aerospace gas turbines. The precipitation kinetics of the intermetallicδ(Ni3Nb) and γ′ (Ni3(Al,Ti)) phases in this alloy are of scientific as well as technological interest because of their significant influence on the mechanical properties. Important parameters like grain size are controlled by coarse d precipitates located at grain boundaries, whereas small γ′ precipitates are responsible for strengthening by precipitation hardening. In the present study, the microstructure is investigated by threedimensional atom probe tomography and simulated by computer modeling using the thermo-kinetic software MatCalc. The results of numerical simulations and experimental data are compared and critically discussed. It is shown that the chemical compositions of the phases change during isothermal aging, and the precipitation kinetics ofδand γ ′ phases interact with each other as shown in a time temperature precipitation (TTP) plot. The TTP plot sh ws C-shaped curves with characteristic discontinuities in the temperature range, where simultaneous and concurrent precipitation of the d and γ′ phases occurs. This leads to a competition in the diffusion of Nb and Al, which are partly present in both phases. Thus, the present study gives important information on heat treatments for ATI Allvac 718Plus in order to achieve the desired microstructure and mechanical properties. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Maderbacher H.,University of Leoben | Ganser H.-P.,Materials Center Leoben Forschung | Riedler M.,Bohler Schmiedetechnik GmbH and Co KG | Stoschka M.,University of Leoben | And 2 more authors.
Advanced Materials Research | Year: 2011

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. © (2011) Trans Tech Publications, Switzerland.


Koster P.,University of Siegen | Knobbe H.,University of Siegen | Fritzen C.-P.,University of Siegen | Christ H.-J.,University of Siegen | Riedler M.,Bohler Schmiedetechnik GmbH and Co KG
Procedia Engineering | Year: 2010

The propagation of stage I-cracks is simulated in a mill-annealed microstructure of forged Ti6Al4V, consisting of primary alpha grains and lamellar alpha/beta colonies. The crack growth mechanisms are investigated experimentally by means of fatigue tests yielding the following results: Within primary alpha grains cracks usually grow on the basal plane or a prismatic plane. In the lamellar colonies crack propagation often occurs parallel to the orientation of the lamellae. The misorientation between active slip bands in neighbouring grains is measured by electron backscatter diffraction. These findings have been implemented into a two-dimensional, mechanism-based short-crack model, which describes crack propagation as a partially irreversible dislocation glide on a crystallographic slip plane. The model is solved numerically using dislocation dipole boundary elements. The nonuniform propagation kinetics of short cracks is considered by defining grain boundaries as obstacles to plastic slip and crack propagation. The described model is used to simulate crack propagation in virtual microstructures, which are based on Voronoidiagrams. Statistical parameters such as grain size and volume fraction can be adjusted to agree with the real microstructure. © 2010 Published by Elsevier Ltd.


Regener B.,Christian Doppler Laboratory | Krempaszky C.,Christian Doppler Laboratory | Werner E.,TU Munich | Stockinger M.,BOHLER Schmiedetechnik GmbH and Co. KG
Computational Materials Science | Year: 2012

Due to finite thermal conductivity and the heterogeneous microstructure of Ti6Al4V, the temperature distribution within large components during thermal processing is highly heterogeneous on both, the macroscale and the microscale. To compute a spatial distribution of stresses at the microscale, a microdomain partition is prerequisite. By analysing representative micrographs, characteristic grain shapes are determined which serve as validation of numerically generated realistic microdomain partitions utilising the technique of spatial tessellations. By generalising the standard Voronoï tessellation, a more sophisticated tessellation, the Johnson-Mehl tessellation is introduced to capture these characteristics appropriately. The Johnson-Mehl cells grow isotropically around the kernels which result from an inhomogeneous Poisson point process, replicating the underlying phase evolution mechanism during thermal processing. In order to capture the anisotropy of the microstructure caused by preceding forging, a geometrical morphing is applied subsequently to the computation of the spatial tessellation. Comparison of the basic features of both, the experimentally derived micrographs and the numerically derived ones, reveals a good qualitative agreement. © 2011 Elsevier B.V. All rights reserved.


Huber D.,Bohler Schmiedetechnik GmbH and Co KG | Werner R.,University of Leoben | Clemens H.,University of Leoben | Stockinger M.,Bohler Schmiedetechnik GmbH and Co KG
Materials Characterization | Year: 2015

The effort of weight reduction in modern aircraft engines, while improving performance and efficiency, demands novel light-weight high temperature materials to replace heavy nickel-based superalloys for low pressure turbine blades. Low density, high specific stiffness, elevatederature strength retention as well as good environmental resistance make γ-TiAl based alloys a promising substitute. The requirement for high and balanced mechanical properties of low pressure turbine blades in next generation aircraft engines favors a hot working strategy. Thermo-mechanical processing, however, of γ-TiAl based alloys is a challenging task due to a narrow "processing window". Isothermal forging, state-of-the-art process for this material class, leads to high tooling and forging costs. With this in mind, Böhler Schmiedetechnik GmbH & Co KG has developed a "near conventional" thermo-mechanical processing technology. A die temperature about 400°C to 800°C below billet temperature and processing under standard atmosphere as well as usage of a conventional hydraulic press with high ram speed result in a more economical process. Subsequent heat treatment strategies can be used to tailor microstructure and, therefore, mechanical properties according to customer requirements. This paper summarizes industrial forging trials as well as mechanical testing and shows the effect of process variations on the final components' properties. © 2015 Elsevier Inc. All rights reserved.


Huber D.,Bohler Schmiedetechnik GmbH and Co KG | Clemens H.,University of Leoben | Stockinger M.,Bohler Schmiedetechnik GmbH and Co KG
Materials Research Society Symposium Proceedings | Year: 2013

Balanced mechanical properties are needed for TiAl low pressure turbine blades envisaged for use in new generation aircraft engines. However, thermomechanical processing of γ-TiAl based alloys is a challenging task due to a small "processing window". Isothermal forging, as state of the art process for this class of material, results in high productions costs and lower productivity. Due to these facts Bohler Schmiedetechnik GmbH & Co KG has developed a higher efficient "near conventional" thermomechanical processing technology. Lower die temperature and processing at standard atmosphere as well as the use of standard hydraulic presses with higher ram speed result in a highly economical process. Subsequent heat treatment strategies can be used to tailor microstructure and, therefore, mechanical properties according to customer needs. The paper summarizes our effort to establish a near conventional forging route for the fabrication of TiAl components for aerospace industry. © 2012 Materials Research Society.

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