Effect of Ag Content on the Microstructure and Crystallization of Coupled Eutectic Growth in Directionally Solidified Al
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utectic growth in a ternary alloy, where two phases exchange two components and reject an impurity into the liquid, results in the formation of a boundary layer in the ternary alloy ahead of the solidification front and thus a planar-to-cellular transition at a certain growth velocity.[1–5] Similar combinations of morphological regions have been observed along the univariant lines between the primary crystallization regions of Pb-Sn-Cd by Bao and Durand[6] and by Holder and Oliver,[7] between Al-Al2Cu in Al-Cu-Mg by Garmong,[8] between Cu-Cu2Mg in Cu-Mg-Ni by Fehrenbach et al.[9], and between Al-Si in Al-Cu-Si by This morphology SchnakeandVarschavsky.[10]
DAFAN DU, ANPING DONG, DA SHU, GUOLIANG ZHU, and BAODE SUN are with the Shanghai Key Lab of Advanced High-temperature Materials and Precision Forming, Shanghai Jiao Tong University, Shanghai 200240, P.R. China. Contact e-mails: [email protected]; [email protected] YVES FAUTRELLE is with the SIMAP-EPM-Madylam/G-INP/CNRS, PHELMA, BP 75, 38402 St Martin d’Heres Cedex, France. XI LI is with the SIMAP-EPMMadylam/G-INP/CNRS, PHELMA and also with the State Key Laboratory of Advanced Special Steel, Shanghai University, Shanghai 200072, P.R. China. Contact e-mail: [email protected] Manuscript submitted January 1, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
transformation bears some similarities to that observed at the solid/liquid interface of single-phase solids that follows the Mullins–Sekerka instability.[11] Recently, due to moderate melting temperatures and nearly equal phase fractions in the eutectic solid, Al-Al2Cu eutectics in Al-Cu and univariant Al-Cu-Ag alloys have become interesting candidates for the investigation of coupled growth patterns. Both experimental work[12–15] and numerical simulations[16,17] were performed for the Al-Cu-Ag alloys with different compositions and under different solidification conditions. Moreover, crystallographic ORs between Al and Al2Cu in Al-Cu-Ag alloys with low Ag content were studied by Hecht.[5] The results showed that the regular and elongated eutectic cells correspond to the ‘‘Beta 6’’ and ‘‘Alpha 4’’ OR, respectively. In addition, molecular dynamics simulations supported the experimental results.[18] However, to date, the influence of the Ag content on the morphology of coupled eutectic growth has mainly been studied theoretically.[3,4,24–26] The experimental study on the influence of Ag content on the coupled eutectic morphology and preferred orientation relationship (OR) between eutectic phases in ternary Al-Cu-Ag alloys has not yet been studied. In the present work, we chose four alloys with different compositions along the univariant eutectic groove in a ternary Al-Cu-Ag alloy
Fig. 1—(a) Liquidus projection of the Al-Cu-Ag system: the investigated alloy compositions selected in this study are along the univariant eutectic reactions: L fi a(Al) + Al2Cu. (b) Schematic illustration of the Bridgman-type directional solidification furnace.
Table I. Alloy 1 2 3 4
Al-Cu-Ag Alloy Compositions Used Throughout This Study
Alloy C
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