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- Computed tomography (2)
- Neutron diffraction (2)
- Absorptionskante (1)
- Aluminum alloys (1)
- Internal stress (1)
- LPSO Phase (1)
- Legierung (1)
- Load partition (1)
- Metal matrix composite (1)
- Mg-Y-Zn Alloy (1)
Organisationseinheit der BAM
Room temperature mechanical behavior of extruded Mg–Y–Zn alloys with varying fractions of LPSO phase was studied in tension and compression along the extrusion direction. The microstructure is characterised by elongated LPSO fibers along the extrusion direction within the magnesium matrix. Moreover, the magnesium matrix presents a bimodal grain structure with dynamically-recrystallized grains and deformed, elongated grains with the basal plane parallel to the extrusion direction. The beginning of plasticity depends on the volume fraction of deformed and DRX grains. Alloys with low volume fraction of LPSO phase (<10 vol%), with a high volume fraction of deformed grains, show the typical behavior of extruded magnesium alloys where yield stress in tension is higher than in compression. This effect is, however, reversed as the volume fraction of the LPSO phase increases since DRX grains are majority.
Stress-induced damage evolution in cast AlSi12CuMgNi alloy with one- and two ceramic reinforcements
(2017)
Two composites, consisting of an as-cast AlSi12CuMgNi alloy reinforced with 15%vol. Al2O3 short fibres and with 7%vol. Al2O3 short fibres + 15%vol. SiC particles were studied. Synchrotron computed tomography disclosed distribution, orientation, and volume fraction of the different phases. In-situ compression tests during neutron diffraction in direction parallel to the fibres plane revealed the load partition between phases. Internal damage (fragmentation) of the Si phase and Al2O3 fibres was directly observed in CT reconstructions. Significant debonding between Al-matrix and SiC particles was also found. Finally, based on the Maxwell scheme, a micro-mechanical model was utilized for the new composite with two ceramic reinforcements; it rationalizes the experimental data, and predicts the evolution of all internal stress components in each phase.
Absorptionskantentomographie, auch bekannt als differentielle Tomographie an Kanten, ist ein Verfahren, bei dem ausgenutzt wird, dass sich die Schwächung von Röntgenstrahlen beim Übergang der Photonenenergie über die Absorptionskanten sprunghaft ändert. Die Verwendung von Synchrotronstrahlung ermöglicht die Anwendung der Absorptionskantentomographie für nahezu jedes Element, da sie eine intensive, durchstimmbare Strahlenquelle mit kleiner Bandbreite darstellt. Der Mikrotomographieaufbau der BAM am Elektronenspeicherring BESSY II eignet sich mit einem Energiebereich von 5 keV bis über 60 keV und einer Ortsauflösung besser als 0.5 µm zur differentiellen Tomographie an den K-Kanten der Elemente von Chrom bis zu den Lanthaniden, bei Einbeziehung der L-Kante bis zu Uran.
In dieser Arbeit wird die Absorptionskantentomographie ausgenutzt, um das Gefüge von Legierungen dreidimensional und nichtdestruktiv zu vermessen. Als Beispiel wird eine Legierung zwischen Magnesium, Yttrium und Zink analysiert, die sich gegenüber unlegiertem Magnesium durch größere Festigkeit und geringere Korrosionsanfälligkeit auszeichnet. Der Grund dafür liegt in der Bildung einer kristallographisch hochgeordneten Phase (long period stacking ordered - LPSO) der Legierungsbestandteile, die die Matrix in Form von Fasern durchzieht. Anhand ihres Yttriumgehaltes wird die dreidimensionale Verteilung der LPSO-Phase in der Matrix für verschiedene Proben bestimmt und mit Schnittbildern und Korrosionstests verglichen. Die Absorptionskantentomographie erweist sich als eine geeignete Methode, um die dreidimensionale Mikrostruktur von Legierungen zu charakterisieren.
The evolution of the internal strains during in situ tension and compression tests has been
measured in an MgY2Zn1 alloy containing long-period stacking ordered (LPSO) phase using
neutron diffraction. The alloy was extruded at two different temperatures to study the influence
of the microstructure and texture of the magnesium and the LPSO phases on the deformation
mechanisms. The alloy extruded at 623 K (350 °C) exhibits a strong fiber texture with the basal
plane parallel to the extrusion direction due to the presence of areas of coarse non-recrystallised
grains. However, at 723 K (450 °C), the magnesium phase is fully recrystallised with grains
randomly oriented. On the other hand, at the two extrusion temperatures, the LPSO phase
orients their basal plane parallel to the extrusion direction. Yield stress is always slightly higher
in compression than in tension. Independently on the stress sign and the extrusion temperature,
the beginning of plasticity is controlled by the activation of the basal slip system in the dynamic
recrystallized grains. Therefore, the elongated fiber-shaped LPSO phase which behaves as the
reinforcement in a metal matrix composite is responsible for this tension–compression asymmetry.
Load partitioning between phases in a cast AlSi12CuMgNi alloy was investigated by in-situ compression test during neutron diffraction experiments. Computed tomography (CT) was used to determine volume fractions of eutectic Si and intermetallic (IM) phases, and to assess internal damage after ex-situ compression tests. The CT reconstructed volumes showed the interconnectivity of IM phases, which build a 3D network together with eutectic Si. Large stresses were found in IMs, revealing their significant role as a reinforcement for the alloy. An existing micromechanical model based on Maxwell scheme was extended to the present case, assuming the alloy as a three-phase composite (Al matrix, eutectic Si, IM phases). The model agrees well with the experimental data.
Moreover, it allows predicting the principal stresses in each phase, while experiments can only determine stress differences between the axial and radial sample directions. Finally, we showed that the addition of alloying elements not only allowed developing a 3D interconnected network, but also improved the strength of the Al matrix, and the ability of the alloy constituents to bear mechanical load.