Columnar-to-Equiaxed Transition and Equiaxed Grain Alignment in Directionally Solidified Ni 3 Al Alloy Under an Axial Ma
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CENTLY, due to the development of superconducting magnets, a high magnetic field is widely used in material processing.[1] Applying magnetic field can significantly affect the solidification process, such as damping the melt flows,[2,3] redistributing the solutes,[4,5] modifying the liquid/solid interface,[6,7] shifting the liquid-to-solid phase transformation temperature,[8,9] and aligning the crystal/grain orientation.[10,11] In previous works,[12–15] the high magnetic field induced columnar-to-equiaxed transition (CET) of dendritic array was reported. The application of a high magnetic field during directional solidification has caused the occurrence of CET in Pb-Sn, Zn-Cu, and Ni-based superalloys and types of Al-based alloys. The thermoelectric (TE) magnetic force acting on the dendrites is considered to be responsible for the phenomena.
HUAN LIU, WEIDONG XUAN, XINLIANG XIE, CHUANJUN LI, JIANG WANG, JIANBO YU, XI LI, YUNBO ZHONG, and ZHONGMING REN are with the State Key Laboratory of Advanced Special Steel & School of Materials Science and Engineering, Shanghai University, Shanghai 200072, P.R. China. Contact e-mails: [email protected], [email protected] Manuscript submitted February 13, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
However, the comprehensive understanding is far from achieved and no quantitative model has been proposed. The intermetallic compound Ni3Al alloy has attractive properties for structural use at elevated temperature and hostile environments. The yield strength of Ni3Al increases with increasing temperature up to about 973 K (700 C).[16] The oxidation and carburization resistance in both oxidizing and carburizing atmospheres is high, up to 1373 K (1100 C), due to the formation of a continuous alumina layer on the surface.[17,18] After Aoki and Izumi[19] found out that the addition of a small amount of boron could effectively reduce the brittleness of polycrystalline Ni3Al, application of Ni3Al-based alloys was widely investigated and several commercial Ni3Al-based alloys were developed.[17,20–23] Commercial Ni-rich Ni3Al-based alloys with two-phase c/c¢ structures were successfully used as structural materials in many industrial branches, such as forging dies, furnace assembly, and turbine blades. However, due to the lack of hot workability, the near-net-shape process by casting is used primarily in Ni3Al-based alloys.[23–25] The application of magnetic field during the solidification process may provide a new approach to control the solidification structure. The aim of the present work is twofold: (1) to investigate the effect of the magnetic field on the solidification structure in Ni-rich Ni3Al alloy; and (2)
by studying CET of the dendritic array, to propose a simple model that may enhance our understanding of the effect of the magnetic field on CET.
II.
EXPERIMENTAL PROCEDURES
Alloy ingots with the nominal composition of Ni-21.5Al-0.4Zr-0.1B (at. pct) (hereafter referred to as Ni-22Al) were prepared in a vacuum induction furnace. Cast samples were cut and enveloped in high-purity co
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