Birgit Skrotzki

Precipitation of T1 and θ′ Phase in Al-4Cu-1Li-0.25Mn During Age Hardening: Microstructural Investigation and Phase-Field Simulation

Experimental and phase field studies of age hardening response of a high purity Al-4Cu-1Li-0.25Mn-alloy (mass %) during isothermal aging are conducted. In the experiments, two hardening phases are identified: the tetragonal θ′ (Al2Cu) phase and the hexagonal T1 (Al2CuLi) phase. Both are plate shaped and of nm size. They are analyzed with respect to the development of their size, number density and volume fraction during aging by applying different analysis techniques in TEM in combination with quantitative microstructural analysis. 3D phase-field simulations of formation and growth of θ′ phase are performed in which the full interfacial, chemical and elastic energy contributions are taken into account. 2D simulations of T1 phase are also investigated using multi-component diffusion without elasticity. This is a first step toward a complex phase-field study of T1 phase in the ternary alloy. The comparison between experimental and simulated data shows similar trends. The still unsaturated volume fraction indicates that the precipitates are in the growth stage and that the coarsening/ripening stage has not yet been reached.

Nature of Precipitates in Peakaged and in Subsequently Crept Al-Ge-Si Alloy

Transmission electron microscopy (TEM) and X-ray diffraction (XRD) have been used to study the morphology and the lattice parameters, respectively of the diamond cubic SiGe precipitates that are present in peak aged, and in subsequently crept samples of a ternary Al-Ge-Si alloy. TEM reveals that there exist variations in the morphology of the SiGe precipitates within the microstructures under all conditions, and that nucleation of SiGe precipitates occurs in the alloy during creep. X-ray diffraction studies reveal that the lattice parameter of the SiGe precipitates present in the peak aged alloy does not change due to either over ageing or subsequent creep deformations, whilst, formation of SiGe precipitates having a different lattice parameter is detected under certain creep conditions.

Quantitative Charakterisierung der Gefüge-Anisotropie einer stranggepressten TiAl-Legierung

Kurzfassung In dieser Arbeit werden quantitative mikrostrukturelle Untersuchungen am Duplex-Gefüge einer stranggepressten intermetallischen γ-Titanaluminidlegierung der 3. Generation (TNBV5) diskutiert. Die Mikrostruktur wurde in raster- und transmissionselektronenmikro-skopischen Untersuchungen von senkrecht und parallel zur Strangpressrichtung entnommenem Probenmaterial charakterisiert. Es erfolgten eine Bestimmung der Kornflächen lamellarer und globularer Körner, eine Bestimmung des Anteils lamellarer Kolonien sowie eine Analyse des Anteils der verzwillingten γ-Körner. Zudem wurde eine Zeiligkeit der lamellaren Kolonien in Strangpressrichtung nachgewiesen und der mittlere Grenzflächenabstand der lamellaren Bereiche bestimmt. In ersten Kriechversuchen an unterschiedlich orientierten Proben wurde der Einfluss der Gefüge-Anisotropie auf das Kriechverhalten dieser Legierung dokumentiert. Die hier ausführlich beschriebenen metallographischen Methoden ermöglichen eine quantitative Analyse des Einflusses mikrostruktureller Anisotropien auf die mechanischen Eigenschaften und das Kriechverhalten von Titan-aluminiden.

Multi-Axial Thermo-Mechanical Fatigue of a Near-Gamma TiAl-Alloy

A material family to replace the current superalloys in aeronautical gas turbine engines is considered to be that of gamma Titanium Aluminide (-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 thermo-mechanical fatigue (TMF) behaviour of this -TiAl alloy have been examined at 400 – 800oC with strain amplitudes from 0.15% to 0.7%. The tests were conducted at both in-phase (IP) and out-of-phase (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.

Evaluation of high spatial resolution imaging of magnetic stray fields for early damage detection

The paper discusses the evaluation of elastic and plastic strain states in two low-carbon steels of the same steel group with high spatial resolution GMR (giant magneto resistance) sensors. The residual stress distributions of tungsten inert gas welded plates were determined by means of neutron diffraction as a reference. The normal component of local residual magnetic stray fields arise in the vicinity of the positions of maximum stress. The experiments performed on flat tensile specimen indicate that the boundaries of plastic deformations are a source of stray fields. The spatial variations of magnetic stray fields for both the weld and the tensile samples are in the order of the earths magnetic field.

Thermoschock: Interpretation von makroskopischen Bruchmerkmalen

Kurzfassung Der vorliegende Beitrag beschreibt typische Bruchmuster, die an SSiC Proben nach Thermoschockexperimenten beobachtet wurden und korreliert diese qualitativ mit der aufgebrachten Belastung. Der Thermoschock wird durch eine schnelle, in der Probenmitte beginnende spiralförmige Aufheizung dünner Scheiben mit Laserstrahlung realisiert. Durch diese Versuchsführung wandert ein zunehmender Temperaturgradient von der Mitte beginnend in radialer Richtung durch die Probe. Dieser Temperaturgradient ist für die Ausbildung von Druckspannungen im Probeninneren und Zugspannungen im kalten Randbereich der Probe verantwortlich. Beim Erreichen einer kritischen, versagensrelevanten Spannung tritt Probenbruch auf. Dabei wird die bis zu diesem Zeitpunkt in der Probe gespeicherte elastische Energie teilweise in die Schaffung von Rissoberflächen umgewandelt. Der Zusammenhang zwischen gespeicherter elastischer Energie und Bruchmuster wird dargestellt.

Failure estimation of thermo-mechanically loaded hot parts in turbochargers

Components directly exposed to the exhaust-gas streams of combustion engines are increasingly thermo-mechanically loaded. A keystone for dimensioning is the calculated failure prediction. Within the FVV research project No. 916 (“Hot Parts”), a model for the calculated simulation of thermo-mechanical fatigue was developed for the cast iron material SiMo 4.05 in the division Mechanical Behaviour of Materials at the Federal Institute for Material Research and Testing (BAM). Using this model, the location and the time of the occurrence of first cracks in exhaust-gas turbocharger housings may be estimated in good approximation.

Artificial aging and creep of an Al–Ge–Si alloy

Abstract The microstructural stability of a peak-aged (24 h at 160°C) Al–Ge–Si alloy, containing incoherent GeSi precipitates with high interfacial energy, has been examined under different creep conditions, with temperatures varying from 120 to 140°C and initial stresses varying from 65 to 85 MPa. A combination of transmission electron microscopy (TEM) and X-ray diffraction revealed that under all creep conditions, fresh nucleation of second-phase precipitates occurs in the alloy. TEM and quantitative image analysis demonstrated that exposure at 120°C with and without stress has no observable effect on the precipitate size, while increasing the creep temperature to 140°C coarsens the precipitates. Furthermore, prolonging the isothermal aging time at 160°C coarsens the second-phase precipitates considerably. These results are discussed in terms of the known factors responsible for the coarsening of precipitates in agehardenable alloy systems.

The role of surface topography on deformation-induced magnetization under inhomogeneous elastic-plastic deformation

It is widely accepted that the magnetic state of a ferromagnetic material may be irreversibly altered by mechanical loading due to magnetoelastic effects. A novel standardized nondestructive testing (NDT) technique uses weak magnetic stray fields, which are assumed to arise from inhomogeneous deformation, for structural health monitoring (i.e., for detection and assessment of damage). However, the mechanical and microstructural complexity of damage has hitherto only been insufficiently considered. The aim of this study is to discuss the phenomenon of inhomogeneous “self-magnetization” of a polycrystalline ferromagnetic material under inhomogeneous deformation experimentally and with stronger material-mechanical focus. To this end, notched specimens were elastically and plastically deformed. Surface magnetic states were measured by a three-axis giant magnetoresistant (GMR) sensor and were compared with strain field (digital image correlation) and optical topography measurements. It is demonstrated that the stray fields do not solely form due to magnetoelastic effects. Instead, inhomogeneous plastic deformation causes topography, which is one of the main origins for the magnetic stray field formation. Additionally, if not considered, topography may falsify the magnetic signals due to variable lift-off values. The correlation of magnetic vector components with mechanical tensors, particularly for multiaxial stress/strain states and inhomogeneous elastic-plastic deformations remains an issue.

Influence of casting skin on the fatigue lifetime of ferritic ductile cast iron

Abstract The fatigue behavior of cast iron is usually investigated on machined specimens. Components of cast iron, however, have a casting skin. Therefore, the investigation of the influence of the casting skin on the lifetime is of interest. To study this influence, isothermal fatigue tests were carried out on heat resisting spheroidal graphite cast iron EN GJS SiMo 4.05 in 4-point bending setup at 400 °C. Specimens with and without casting skin were investigated comparatively. The number of cycles to failure was significantly lower for specimens with casting skin. Metallographic investigations underline the reduction of lifetime caused by casting skin.