Morphology Transition from Dendrites to Equiaxed Grains for AlCoCrFeNi High-Entropy Alloys by Copper Mold Casting and Br
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RECENTLY, high-entropy alloys (HEAs) have attracted increasing attention. As the entropy of solid solutions is usually higher than that of intermetallics, which can greatly lower the Gibbs free energy of the system. Thus, the alloys with high entropy of mixing mainly form face-centered-cubic or body-centered-cubic (bcc) solid solutions.[1–4] At present, the arc melting and copper mold casting are usually used for the preparation of HEAs,[5–15] during which the solidification process cannot be easily controlled, which leads to a changing microstructure from the surface to the center of the samples, e.g., inhomogeneous distribution of the dendrites. Furthermore, the undercooled liquid metals have to withstand tensile stress in the suction casting process, which causes the defects of the prepared samples, such as the pores due to the volume shrinking during solidification. Compared to the copper mold casting, the direction of the thermal conduction and extraction for rod-shape samples obtained by Bridgman solidification is mainly along the longitudinal direction, and the temperature gradient and growth rates can be precisely controlled by adjusting the heating power and withdrawal velocities. For instance, the Al-Cu alloys,[16] TiAl-based alloys,[17] and Ni-based superalloys[18] were prepared and optimized by Bridgman solidification. It was reported that dendrites/bulk-metallic-glass (BMG) composites were successfully tailored by Bridgman solidification, during
Y. ZHANG, Professor, and S.G. MA, Postdoctoral Candidate, are with the State Key Laboratory for Advanced Metals and Materials, University of Science & Technology Beijing, Beijing 100083, P.R. China. Contact e-mail: [email protected] J.W. QIAO, Associate Professor, is with the College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, P.R. China. Manuscript submitted April 28, 2011. Article published online November 15, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
which the dendrites are homogeneously distributed within the BMG matrix, and the dendrite spanning length has a strong dependence on the temperature gradient and growth rates.[19–21] In this study, the microstructure and mechanical properties of the AlCoCrFeNi HEAs were investigated by these two solidification techniques.
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EXPERIMENTAL PROCEDURE
Ingots with a nominal composition of AlCoCrFeNi (molar ratio) were prepared by arc melting the mixture of Al, Co, Cr, Fe, and Ni with purity higher than 99.5 wt pct under a Ti-gettered argon atmosphere. In order to ensure the compositional homogeneity, the alloys were remelted at least 4 times. Afterward, the ingots were divided into two parts. One part was remelted and then cast into cylindrical rods with diameters of 3 and 5 mm. The cross sections of the samples were etched with the aqua regia solution, and the microstructures were investigated by metallographic microscope. Another part was crashed into pieces and placed in an alumina tube with an internal diameter of 3 mm and a wall thickness of ~1 mm. The
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