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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.

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.

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.

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.

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