MICROSTRUCTURE EVOLUTION AND ORIENTATION ANALYSIS OF HYPEREUTECTIC Al-Al2Cu ALLOY UNDER DIRECTIONAL SOLIDIFICATION
GAO Ka, LI Shuangming(), FU Hengzhi
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi′an 710072
Cite this article:
GAO Ka, LI Shuangming, FU Hengzhi. MICROSTRUCTURE EVOLUTION AND ORIENTATION ANALYSIS OF HYPEREUTECTIC Al-Al2Cu ALLOY UNDER DIRECTIONAL SOLIDIFICATION. Acta Metall Sin, 2014, 50(8): 962-970.
Dendrite is a fundamental growth pattern in alloy solidification. Normally, dendrites with a specific growth orientation which can remarkably influence casting properties have attracted many great interests. The previous investigations mainly focus on the growth of simple monophase dendrites solid solution. For complex intermetallics in solidification, the correlation between processing parameters and microstructure morphologies with the preferred growth directions has not been shed a light. In this work, considering intermetallic Al2Cu phase has crystalline anisotropy and can exhibit colorfully complicated growth morphologies with specific growth direction in different solidification conditions, the primary Al2Cu phase growth behavior of Al-40%Cu (mass fraction) hypereutectic alloy was investigated by using a high thermal gradient directional solidification apparatus. The Al2Cu phase growth behavior in the experiments included its change in growth morphology and orientation transition. With solidification distance increasing, due to the alloy liquid composition ahead of the solid/liquid interface approaching the eutectic point, the primary Al2Cu dendrite transited from regular faceted V-shaped morphology to the entirely coupled eutectic at 10 μm/s. Its growth direction changed from [110] direction to the normal direction of (121) plane. The EBSD result indicated that the [001] direction of Al2Cu phase was along the direction of heat flux. As the growth rate was changed abruptly from 10 to 2 μm/s, the alloy liquid composition ahead of the solid/liquid interface increased firstly and then decreased to the eutectic point. It caused that the primary Al2Cu dendrite transited from regular faceted V-shaped morphology to long smooth lath morphology, and finally disappeared to form the entirely coupled eutectic. Moreover, the growth direction of Al2Cu phase changed from [110] direction to [001] direction. The experimental results show that the directional solidification process parameter is the dominant factor affecting the final morphology and growth direction of dendrites.
Fund: Supported by National Natural Science Foundation of China (Nos.50971101 and 51074127) and Fund of State Key Laboratory of Solidification Processing in NWPU (No.34-TP-2009)
Fig.1 Directionally solidified microstructures of Al-40%Cu hypereutectic alloy in longitudinal sections with solidification rate of 10 μm/s
(a) V-shaped morphology at the solidified distance of 35 mm
(b) non-faceted dendrite at the solidified distance of 70 mm
(c) eutectic at the solidified distance of 80 mm
Fig.2 Variation of mass fraction of Cu with solidification fractions at 10 μm/s in Al-40%Cu hypereutectic alloy
Fig.3 Directionally solidified microstructures of Al-40%Cu hypereutectic alloy in longitudinal sections after abrupt change in growth rate from 10 to 2 μm/s
(a) V-shaped morphology at the solidified distance of 35 mm
(b) non-faceted dendrite at the solidified distance of 42 mm
(c) eutectic at the solidified distance of 50 mm
Fig.4 Directionally solidified microstructures of Al-40%Cu hypereutectic alloy in transverse sections at growth rate of 10 μm/s (a), the standard inverse pole figure of Al and Al2Cu phase (b), {100} pole figure (c) and inverse pole figure (d)
Fig.5 Inverse pole figures of directionally solidified Al-40%Cu hypereutectic alloy in longitudinal sections at 10 μm/s at various solidified distances
(a) schematic of directionally solidified microstructure
(b) inverse pole figure of V-shaped dendrite
(c) inverse pole figure of complex morphology dendrite
Fig.6 Inverse pole figures of directionally solidified Al-40%Cu hypereutectic alloy in longitudinal sections after abrupt change in growth rates from 10 to 2 μm/s at various solidified distances
(a) schematic of directionally solidified microstructure
(b) inverse pole figure of V-shaped dendrite
(c) inverse pole figure of big lath dendrite
(d) inverse pole figure of thin lath dendrite
Fig.7 Schematic drawings of morphology forming in the changing growth velocity of graphite (a), Al2Cu phase with faceted characteristics (b) and Al2Cu phase with non-faceted characteristics (c) (V[0001] and indicate the growth rates along the [0001] and directions of graphite, respectively; , , , , , , and indicate the growth rates along the normal directions of (001), (110), (112), (211), (202), (310), and planes of Al2Cu phase, respectively.)
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