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Whitmore L.,Christian Doppler Laboratory | Ahmadi M.R.,Christian Doppler Laboratory | Guetaz L.,CEA Grenoble | Leitner H.,Christian Doppler Laboratory | And 3 more authors.
Materials Science and Engineering A | Year: 2014

The microstructure of thermally aged nickel-based 718Plus superalloy is investigated using transmission electron microscopy (TEM). Solution annealing at 980. °C for 30. min is followed by either the standard quenching to room temperature or quenching directly to 788. °C, before isothermal aging at 788. °C for four hours. Micro-hardness and yield strength are measured to compare the effects of the two variations. The size and phase fraction of γ' precipitates are measured using dark-field TEM and related to the hardness and yield strength through a theoretical model based on coherency and antiphase boundary effects. A population of very small sub-precipitates is observed and the larger γ' precipitates are investigated in detail using high resolution scanning TEM to reveal information about the chemical ordering. © 2014 Elsevier B.V.


Mitsche S.,University of Graz | Sommitsch C.,Christian Doppler Laboratory | Huber D.,Bohler Schmiedetechnik GmbH Co KG | Stockinger M.,Bohler Schmiedetechnik GmbH Co KG | Poelt P.,University of Graz
Materials Science and Engineering A | Year: 2011

The nickel-based superalloy Allvac® 718Plus™ is a future candidate for turbine disc applications, as this new material combines the formability and cost advantages of Alloy 718 with the higher temperature capability of Waspaloy. Due to the strong influence of the microstructure on the final mechanical properties, a fundamental knowledge of the dynamic recrystallization mechanism of Allvac® 718Plus™ is of great importance in order to develop precise microstructure evolution models for this material. Compression tests were performed at temperatures of 900°C to 1050°C and strain rates between 0.1s-1 and 10s-1. The microstructures obtained were analyzed by electron backscatter diffraction (EBSD) to evaluate the influence of the different hot forming parameters on the dynamic softening processes of Allvac® 718Plus™. © 2011 Elsevier B.V.


Whitmore L.,Christian Doppler Laboratory | Leitner H.,Christian Doppler Laboratory | Povoden-Karadeniz E.,Christian Doppler Laboratory | Radis R.,Christian Doppler Laboratory | And 2 more authors.
Materials Science and Engineering A | Year: 2012

The effects of single and double aging upon the ATI 718Plus Nickel superalloy are investigated. Double aging is found to produce an increase in hardness as compared to single aging, thus an extensive micro-structural study is performed to identify and understand the physical reasons for this. Precipitation of the γ' phase is of particular interest in this case. Using transmission electron microscopy and energy dispersive X-ray spectroscopy, the γ' shape, size, number density and elemental constitution are investigated and correlated to the aging conditions. The γ/γ' lattice misfit is also measured. Only one main population of γ' precipitates is observed after double aging: simply the population produced during the first aging continues to evolve. Using a theoretical model for precipitation hardening, based on the shearing of precipitates, the hardness is related to the size and phase fraction of the γ' and a value for the anti-phase boundary energy of the γ' is calculated. Measurements are compared with thermo-kinetic simulation using the software package MatCalc. A dense dispersion of 1-2. nm size γ' phase clusters is additionally observed after both single and double aging, and this is correlated through simulation with the final cooling stage of the heat treatment. © 2011 Elsevier B.V..


Regener B.,Christian Doppler Laboratory | Krempaszky C.,Christian Doppler Laboratory | Werner E.,TU Munich | Berhuber E.,BOHLER Schmiedetechnik GmbH Co KG | Stockinger M.,BOHLER Schmiedetechnik GmbH Co KG
IOP Conference Series: Materials Science and Engineering | Year: 2014

Within the scope of this contribution a wire electric discharge machining assisted dissectioning method is presented, which combines high precision cutting, minimum generation of additional residual stresses during the cutting process and high precision measurement of the resulting distortion. As an example a forged component made of the titanium alloy Ti6Al4V is investigated in a multiple cut procedure. A finite element based mechanical model for the estimation of the residual stress distribution in the component from the distortion data is introduced and discussed. © 2010 IOP Publishing Ltd.


Oberwinkler B.,Bohler Schmiedetechnik GmbH & Co KG
International Journal of Fatigue | Year: 2016

This paper gives a critical overview of the literature available on the topic of the anomalously high mean stress sensitivity of Ti-6Al-4V. It summarizes key facts and offers insights gained by further experimental investigation. The pronounced mean stress dependence has to be considered adequately in fatigue lifetime analysis. Common models such as, for example, the Goodman relation are not sufficient and can lead to a significantly non-conservative or over-conservative estimation, depending on the considered cycles to failure. In this paper, the mean stress behavior of Ti-6Al-4V is analyzed in the high cycle fatigue region from 104 up to 108 load cycles. Explanations for the mean stress dependence for a wide range of stress ratios are given by introducing four fatigue limit determining mechanisms. One of them, the maximum stress controlled basal crack initiation, is identified as the underlying mechanism for anomalous mean stress sensitivity. New models are proposed to approximate the mean stress behavior in the high cycle fatigue region. Experimental validation confirms the suitability of the chosen approach. In addition, an anomalously low supporting effect is observed. © 2016 Elsevier Ltd


Repper J.,TU Munich | Repper J.,Paul Scherrer Institute | Hofmann M.,TU Munich | Krempaszky C.,TU Munich | And 4 more authors.
Journal of Applied Physics | Year: 2012

To know the residual stress state within a component is essential for predicting its service life time. Neutron diffraction is a reliable tool for non-destructive determination of macroscopic residual stresses, even deep inside a component. One standard procedure in neutron residual stress analysis is to cut out small macrostress free coupons from the strained component, which serve as reference samples. Redistribution of intergranular stresses due to relaxation when the coupon is cut out, however, may lead to spurious macroscopic stresses. A quantitative study of the change of intergranular stresses shows that, contrary to conventional thought, even the {311} Bragg reflection is affected strongly by this phenomenon. In this case, the use of reference values calculated using boundary conditions of mechanical equilibrium enables to extract reliable macroscopic residual stress values. © 2012 American Institute of Physics.


Whitmore L.,Christian Doppler Laboratory | Ahmadi M.R.,Christian Doppler Laboratory | Stockinger M.,Bohler Schmiedetechnik GmbH Co KG | Povoden-Karadeniz E.,Christian Doppler Laboratory | And 2 more authors.
Materials Science and Engineering A | Year: 2014

The effects of thermal aging upon the nickel-based 718Plus superalloy are investigated and modelled. Yield strength and micro-hardness measurements are made after solution annealing and after aging at 788. °C for 4. h. In order to explain the differences in strength and hardness, a detailed investigation of the microstructure is performed using transmission electron microscopy. The size and phase fraction of the γ' precipitates are measured and related to the measured hardness and yield strength using a theoretical model of precipitation strengthening based on the shearing of precipitates in terms of coherency strengthening and the formation of an antiphase boundary. © 2013 Elsevier B.V.

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