TEXTURE FORMATION AND GRAIN BOUNDARY CHARACTERISTIC OF Al-4.5Cu ALLOYS DIRECTIONALLY SOLIDIFIED UNDER HIGH MAGNETIC FIELD
ZHONG Hua1(), REN Zhongming1, LI Chuanjun1, ZHONG Yunbo1, XUAN Weidong1, WANG Qiuliang2
1 State Key Laboratory of Advanced Special Steel, Shanghai University, Shanghai 200072 2 Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190
Cite this article:
ZHONG Hua, REN Zhongming, LI Chuanjun, ZHONG Yunbo, XUAN Weidong, WANG Qiuliang. TEXTURE FORMATION AND GRAIN BOUNDARY CHARACTERISTIC OF Al-4.5Cu ALLOYS DIRECTIONALLY SOLIDIFIED UNDER HIGH MAGNETIC FIELD. Acta Metall Sin, 2015, 51(4): 473-482.
Directional solidification of Al-4.5Cu alloy refined by adding Al-5Ti-1B has been carried out to investigate the texture formation and grain boundary characteristic of the paramagnetic crystal under a high magnetic field. OM and EBSD were applied to analyze the microstructures solidified at different temperature gradients (G) and magnetic field intensities (B). The results show that at the temperature gradient of 27 K/cm, the orientations of fcc a-Al grains without magnetic field are random. However, as a high magnetic field is imposed, the easy magnetization axes 〈310〉 of the a-Al grains are aligned parallel to the direction of the magnetic field leading to 〈310〉 texture. Meanwhile, the ratio of coincidence site lattice (CSL) grain boundaries increases with the increment of magnetic field intensity and reaches its maximum value at 4 T, but decreases as the magnetic field enhances further. On the other hand, when the temperature gradient is elevated, columnar dendrite morphology is exhibited without magnetic field; while a 6 T high magnetic field is introduced, the columnar dendrites are broken and equiaxed grains of random orientations are obtained. The alignment behavior of the free crystals in melt could be attributed to the magnetic crystalline anisotropy of a-Al. Moreover, the influence of fluid flow on the texture formation and CSL grain boundary development under magnetic field is discussed. The absence of convection is benefit for grain reorientation and CSL boundary formation. The application of high static magnetic field will inhibit the macro-scale convection. However, the interaction between thermoelectric current and magnetic field will cause micro-scale fluid flow, i.e., thermoelectric magnetic convection (TEMC). The TEMC will give rise to perturbation near the solid-liquid interface leading to the appearance of freckles as well as the decreasing of the ratio of CSL boundary. Moreover, it is proposed that the formation of CSL boundary is associated with the rotation of the free grains in melt along specific crystallographic axes by magnetic torque.
Fund: Supported by National Basic Research Program of China (No.2011CB010404), National Natural Science Foundation of China (Nos.51404148 and 51401116) and Ministry of Major Science & Technology of Shanghai (Nos.13DZ1108200, 13521101102 and 14521102900)
Fig.1 Quenched longitudinal microstructures near the solid/liquid interface of refined Al-4.5Cu alloys directionally solidified without (a) and with magnetic fields of B=2 T (b), B=4 T (c) and B=6 T (d) under temperature gradient of G=27 K/cm and pulling rate of 10 mm/s (The dashed lines show solid/liquid interfaces)
Fig.2 Quenched longitudinal microstructures near the solid/liquid interface of refined Al-4.5Cu alloys directionally solidified at B=0 T, G=65 K/cm (a), B=6 T, G=65 K/cm (b), B=0 T, G=101 K/cm (c) and B=6 T, G=101 K/cm (d)
Fig.3 EBSD false color maps of the transverse microstructures in the steady-state growth portion of refined Al-4.5Cu alloys directionally solidified without (a) and with magnetic fields of B=2 T (b), B=4 T (c) and B=6 T (d) under G=27 K/cm
Fig.4 EBSD false color maps of the transverse microstructures in the steady-state growth portion of refined Al-4.5Cu alloys directionally solidified at B=0 T, G=65 K/cm (a), B=6 T, G=65 K/cm (b), B=0 T, G=101 K/cm (c) and B=6 T, G=101 K/cm (d)
Fig.5 Inverse pole figures (IPFs) of transverse microstructures in the steady-state growth portion of refined Al-4.5Cu alloys directionally solidified without (a) and with magnetic fields of B=2 T (b), B=4 T (c) and B=6 T (d) under G=27 K/cm (The direction of the IPFs refers to the magnetic field direction and the colored spots in the region of the dashed semicircle indicats the grains orientated close to 〈310〉 direction)
Fig.6 IPFs of transverse microstructures in the steady-state growth portion of refined Al-4.5Cu alloys directionally solidified at B=0 T, G=65 K/cm (a), B=6 T, G=65 K/cm (b), B=0 T, G=101 K/cm (c) and B=6 T, G=101 K/cm (d) (The direction of the IPFs refers to the magnetic field direction and the colored spots in the region of the dashed semicircle indicates the grains orientated close to 〈310〉 direction)
Fig.7 Grain boundary characteristic maps of transverse microstructures in the steady-state growth portion of refined Al-4.5Cu alloys directionally solidified without (a) and with magnetic fields of B=2 T (b), B=4 T (c) and B=6 T (d) under G=27 K/cm (The colored lines represent the coincidence site lattice (CSL) boundaries)
Fig.8 Schematic of alignment of a-Al crystal under magnetic field
(a) magnetic torque caused by the magnetic anisotropy of the crystal
(b) process of rotation and re-orientation when the magnetic field is applied
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