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Measurement of Mechanical Coherency Temperature and Solid Volume Fraction in Al-Zn Alloys Using In Situ X-ray Diffraction During Casting

Drezet, Jean-Marie ; Mireux, Bastien ; Kurtuldu, Güven ; Magdysyuk, Oxana ; Drakopoulos, Michael

In: Metallurgical and Materials Transactions A, 2015, vol. 46, no. 9, p. 4183-4190

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    Titel
    • Measurement of Mechanical Coherency Temperature and Solid Volume Fraction in Al-Zn Alloys Using In Situ X-ray Diffraction During Casting
    Autor
    • Drezet, Jean-Marie. Computational Materials Laboratory, Ecole Polytechnique Federale Lausanne, Station 12, 1015, Lausanne, Switzerland
    • Mireux, Bastien. Computational Materials Laboratory, Ecole Polytechnique Federale Lausanne, Station 12, 1015, Lausanne, Switzerland
    • Kurtuldu, Güven. Computational Materials Laboratory, Ecole Polytechnique Federale Lausanne, Station 12, 1015, Lausanne, Switzerland
    • Magdysyuk, Oxana. RAL, Diamond Light Source, Didcot, UK
    • Drakopoulos, Michael. RAL, Diamond Light Source, Didcot, UK
    Dokumententyp
    • Postprint
    Sprache
    • Englisch
    Veröffentlicht in
    • Metallurgical and Materials Transactions A, 2015, vol. 46, no. 9, p. 4183-4190. Springer US; http://www.springer-ny.com
    Andere elektronische Ausgabe
    Klassifikation
    • Technik, Ingenieurwissenschaften
    OAI-PMH-ID
    • oai:doc.rero.ch:333079
    Summary
    During solidification of metallic alloys, coalescence leads to the formation of solid bridges between grains or grain clusters when both solid and liquid phases are percolated. As such, it represents a key transition with respect to the mechanical behavior of solidifying alloys and to the prediction of solidification cracking. Coalescence starts at the coherency point when the grains begin to touch each other, but are unable to sustain any tensile loads. It ends up at mechanical coherency when the solid phase is sufficiently coalesced to transmit macroscopic tensile strains and stresses. Temperature at mechanical coherency is a major input parameter in numerical modeling of solidification processes as it defines the point at which thermally induced deformations start to generate internal stresses in a casting. This temperature has been determined for Al-Zn alloys using in situ X-ray diffraction during casting in a dog-bone-shaped mold. This setup allows the sample to build up internal stress naturally as its contraction is prevented. The cooling on both extremities of the mold induces a hot spot at the middle of the sample which is irradiated by X-ray. Diffraction patterns were recorded every 0.5seconds using a detector covering a 426×426mm2 area. The change of diffraction angles allowed measuring the general decrease of the lattice parameter of the fcc aluminum phase. At high solid volume fraction, a succession of strain/stress build up and release is explained by the formation of hot tears. Mechanical coherency temperatures, 829K to 866K (556°C to 593°C), and solid volume fractions, ca. 98pct, are shown to depend on solidification time for grain refined Al-6.2wtpct Zn alloys.