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  • Eggeler, G. (17)
  • Pfetzing, J. (6)
  • Skrotzki, Birgit (6)
  • Brookes, Stephen-Peter (5)
  • Klingelhöffer, Hellmuth (5)
  • Kühn, Hans-Joachim (5)
  • Olbricht, Jürgen (5)
  • Peter, D. (5)
  • Sievert, Rainer (5)
  • Agudo Jácome, Leonardo (4)
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Erscheinungsjahr

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Schlagworte

  • Creep (3)
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  • Pseudoelasticity (3)
  • R-phase (3)
  • Axial-torsional Loading (2)
  • Gamma titanium aluminide (2)
  • Grain boundary sliding (2)
  • Micromechanical model (2)
  • NiTi (2)
  • Stress-induced phase transformations (2)
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Organisationseinheit der BAM

  • 5 Werkstofftechnik (5)
  • 5.1 Materialographie, Fraktographie und Alterung technischer Werkstoffe (3)
  • 5.2 Experimentelle und modellbasierte Werkstoffmechanik (3)

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Localization events and microstructural evolution in ultra-fine grained NiTi shape memory alloys during thermo-mechanical loading (2010)
Schaefer, A. ; Wagner, M.F.-X. ; Pelegrina, J.L. ; Olbricht, Jürgen ; Eggeler, G.
Subjecting a thin NiTi specimen to uniaxial tension often leads to a localized martensitic transformation: macroscopic transformation bands form and propagate through the specimen, separating it into regions of fully transformed martensite and original austenite. In the present study, the alternating current potential drop (ACPD) technique is used to analyze the change in electrical resistance of ultra-fine grained NiTi wires subjected to a broad range of thermo-mechanical load cases: (i) uniaxial tensile straining at constant temperatures (pseudoelastic deformation); (ii) cooling and heating through the transformation range at constant load (actuator load case); (iii) a combination of mechanical and thermal loading. We monitor the ACPD signals in several zones along the gauge length of specimens, and we demonstrate that a localized type of transformation is a generic feature of pseudoelastic as well as of shape memory deformation. Moreover, the ACPD signals allow to differentiate between temperature-induced martensite (formed during cooling at no or relatively small loads), stress-induced martensite, and reoriented martensite (formed under load at low temperatures).
On the stress-induced formation of R-phase in ultra-fine-grained Ni-Rich NiTi shape memory alloys (2011)
Olbricht, Jürgen ; Yawny, A. ; Pelegrina, J.L. ; Dlouhy, A. ; Eggeler, G.
Phase transformations in binary ultra-fine-grained (UFG) pseudoelastic NiTi wires were studied in a wide temperature range using mechanical loading/unloading experiments, resistance measurements, differential scanning calorimetry (DSC), thermal infrared imaging, and transmission electron microscopy (TEM). The formation of R-phase can be detected in the mechanical experiments. It is shown that the stress-strain response of the R-phase can be isolated from the overall stress-strain data. The R-phase always forms prior to B19¢ when good pseudoelastic properties are observed. The stress-induced B2 to R-phase transition occurs in a homogeneous manner, contrary to the localized character of the B2/R to B19' transformations. The temperature dependence of the critical stress values for the formation of the martensitic phases shows a Clausius Clapeyron type of behavior with constants close to 6 MPa/K (B19') and 18 MPa/K (R-phase). A stress-temperature map is suggested that summarizes the experimentally observed sequences of elementary transformation/deformation processes.
Axial-torsional thermo-mechanical fatigue of a near-gamma tial-alloy (2012)
Skrotzki, Birgit ; Brookes, Stephen-Peter ; Kühn, Hans-Joachim ; Sievert, Rainer ; Klingelhöffer, Hellmuth ; Pfetzing, J. ; Peter, D. ; Eggeler, G.
Axial-torsional thermomechanical fatigue of a near-gamma TiAl-alloy (2010)
Brookes, Stephen-Peter ; Kühn, Hans-Joachim ; Skrotzki, Birgit ; Klingelhöffer, Hellmuth ; Sievert, Rainer ; Pfetzing, J. ; Peter, D. ; Eggeler, G.
The uniaxial, torsional and axial-torsional thermomechanical fatigue (TMF) behavior of the near-γ TiAl-alloy TNB-V5 was investigated. TMF tests were performed at 400-800 °C with mechanical strain amplitudes ranging from 0.15% to 0.7%. The tests were conducted thermomechanically in-phase (IP) and out-of-phase (OP). For the same lifetimes, uniaxial IP tests required the highest strain amplitudes, while OP test conditions were most damaging and needed the lowest strain amplitudes. The Mises equivalent mechanical strain amplitudes of pure torsional tests were found in between uniaxial in-phase and out-of-phase tests for the same lifetimes. The non-proportional multiaxial out-of-phase test showed a lower lifetime at the same equivalent mechanical strain amplitude compared to the other types of tests. The microstructure has been characterized applying electron microscopy and microstructural parameters such as fraction of twinned grains, grain size, lamellar distance and dislocation density have been quantified.
Axial-torsional thermo-mechanical fatigue of Ti-45 Al-5Nb-0.2B-0.2C (2007)
Brookes, Stephen-Peter ; Kühn, Hans-Joachim ; Skrotzki, Birgit ; Klingelhöffer, Hellmuth ; Sievert, Rainer ; Pfetzing, J. ; Eggeler, G.
Axial-torsional thermo-mechanical fatigue of Ti-45 Al-5Nb-0.2B-0.2C (2008)
Brookes, Stephen-Peter ; Kühn, Hans-Joachim ; Skrotzki, Birgit ; Klingelhöffer, Hellmuth ; Sievert, Rainer ; Pfetzing, J. ; Peter, D. ; Eggeler, G.
Multi-Axial Thermo-Mechanical Fatigue Of A Near-Gamma TiAl-Alloy (2005)
Brookes, Stephen-Peter ; Kühn, Hans-Joachim ; Skrotzki, Birgit ; Klingelhöffer, Hellmuth ; Sievert, Rainer ; Pfetzing, J. ; Peter, D. ; Eggeler, G.
A material family to replace the current superalloys in aeronautical gas turbine engines is considered to be that of gamma Titanium Aluminide (γ-TiAl) alloys. Structural components in aeronautical gas turbine engines typically experience large variations in temperatures and multiaxial states of stress under non-isothermal conditions. The uniaxial, torsional and bi-axial thermomechanical fatigue (TMF) behaviour of this γ-TiAl alloy have been examined at 400 – 800°C with strain amplitudes from 0.15% to 0.7%. The tests were conducted at both in-phase (IP) and out-ofphase (OP). The effects of TMF on the microstructure were also investigated. For the same equivalent mechanical strain amplitude uniaxial IP tests showed significantly longer lifetimes than pure torsional TMF tests. The non-proportional multiaxial OP test showed the lowest lifetimes at the same equivalent mechanical strain amplitude compared to the other types of tests.
Quantitative Charakterisierung der Gefüge-Anisotropie einer stranggepressten TiAl-Legierung / Quantitative Characterisation and Microstructural Anisotropy of a Hot-Extruded TiAl Alloy (2008)
Peter, D. ; Pfetzing, J. ; Wagner, M.F.-X. ; Somsen, C. ; Pesicka, J. ; Skrotzki, Birgit ; Eggeler, G.
Transformation activity in ultrafine grained pseudoelastic NiTi wires during small amplitude loading/unloading experiments (2015)
Pelegrina, J.L. ; Yawny, A. ; Olbricht, Jürgen ; Eggeler, G.
Martensitic phase transformations were studied in ultrafine grained Ni-rich pseudoelastic NiTi wires during cyclic deformation under small imposed strain amplitudes. Small strain variation tests were complemented by a thermographic analysis of the emerging temperature distributions in the specimens. The characteristics of the observed thermal profiles result from the specific phase transitions which take place at different stress levels. Homogeneous temperature changes, corresponding to a non-localized transformation activity along the specimen length, were observed throughout the whole range of applied stresses, starting from values as low as 100 MPa. This behavior is in line with previous literature reports for the stress-induced transformation from B2 austenite to R-phase as long as the critical stress for B19' martensite formation is not reached. In the present study, similar type of transformation activity could also be demonstrated at higher strains/stresses, even after the stress induced transformation to B19' was apparently completed. These findings suggest that transformation activity involving the B2 phase is present throughout the whole pseudoelastic stress–strain cycle; i.e., it is not restricted to the initial loading portion. Finally, non-localized transformation to or from B19' was identified during small amplitude strain variations in the plateau-like coexistence ranges of the pseudoelastic cycle.
Characteristics of the stress-induced formation of R-phase in ultrafine-grained NiTi shape memory wire (2013)
Olbricht, Jürgen ; Yawny, A. ; Pelegrina, J.L. ; Eggeler, G. ; Yardley, V. A.
The transformation between the cubic B2 and monoclinic B19' phases in ultrafine-grained pseudoelastic NiTi can occur as a two-step process involving the intermediate rhombohedral R-phase. Experimental work using differential scanning calorimetry, electrical resistance measurements and transmission electron microscopy has demonstrated the formation of this intermediate phase during thermal cycling and during mechanical loading. In the present paper, complementary mechanical and thermographic results are presented which allow to further assess the character of the stress-induced R-phase formation. The transformation from B2 to R-phase is demonstrated to occur homogeneously within the gauge length rather than via advancing Lüders-type transition regions as it is the case in the localized transformation from B2 or R-phase to B19'.
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