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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

DOI: 10.1007/s11661-015-3041-y DOI Help

Authors: Jean-marie Drezet (EPFL) , Bastien Mireux (EPLF) , Guven Kurtuldu (Laboratory of Metal Physics and Technology, Department of Materials, Zurich, Switzerland.) , Oxana Magdysyuk (Diamond Light Source) , Michael Drakopoulos (Diamond Light Source)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Metallurgical And Materials Transactions A , VOL 46 (9) , PAGES 4183 - 4190

State: Published (Approved)
Published: September 2015

Abstract: 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 0.5 seconds using a detector covering a 426 9 426 mm2 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, 829 K to 866 K (556 C to 593 C), and solid volume fractions, ca. 98 pct, are shown to depend on solidification time for grain refined Al-6.2 wt pct Zn alloys.

Journal Keywords: Nanotechnology; Metallic Materials

Subject Areas: Materials

Instruments: I12-JEEP: Joint Engineering, Environmental and Processing

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