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Effect of Current Intensity on Microstructure of Ni3Al Intermetallics Prepared by Directional Solidification Electromagnetic Cold Crucible Technique |
Guotian WANG, Hongsheng DING( ), Ruirun CHEN, Jingjie GUO, Hengzhi FU |
National Key Laboratory for Precision Hot Processing of Metals, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China |
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Cite this article:
Guotian WANG, Hongsheng DING, Ruirun CHEN, Jingjie GUO, Hengzhi FU. Effect of Current Intensity on Microstructure of Ni3Al Intermetallics Prepared by Directional Solidification Electromagnetic Cold Crucible Technique. Acta Metall Sin, 2017, 53(11): 1461-1468.
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Abstract Due to their excellent high-temperature strength, and good oxidation resistance, Ni3Al-based alloys have long attracted considerable interest as a class of high-temperature structural material. These properties, combined with their unique high thermal conductivity, make them ideal for special applications, such as blades in gas turbines and jet engines. However, polycrystalline Ni3Al alloys show almost no ductility and extremely low fracture resistance at ambient temperatures. Ni3Al alloys with the high ductility at room-temperature can be adjusted by the microstructure through directional solidification (DS) and matching. It has been shown that the electric field can refine the solidification structure, reduce the dendrite spacing, promote the diffusion and change the solute redistribution in the solidification process. In order to improve the room temperature ductility of Ni3Al alloy, the effect of current intensity on microstructure of DS Ni-25Al alloy is investigated. In this work, the effects of constant current intensity and NiAl phase on the microstructure are researched. The results show that in the DC electric field, as the result of the aggregation of current along dendrite tip and the Joule heat at the tip of dendrite, the primary dendritic spacing (λ) decreases with the increasing of current intensity. And the solid-liquid interface tends to be straight resulting from the Joule heat and Peltier effect caused by the segregation of current and the difference in conductivity between solid and liquid interface. When no direct current is applied the DS samples contain the L12 structure of Ni3Al matrix and B2 structure of NiAl precipitate phase. The microstructure is a duplex structure which consist of gray Ni3Al matrix and black NiAl precipitates. NiAl precipitates with regular shape and has obvious orientation along with the growth direction. When the DC current is applied, NiAl precipitates is irregular and dispersion and has no obvious directionality, due to Joule heat effect generated by the current effect, the undercooling increased and the precipitated NiAl phase transformed into thin martensite NiAl phase with twin crystal symmetry from the NiAl-B2 type structure.
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Received: 27 March 2017
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Fund: Supported by National Natural Science Foundation of China (No.51471062) |
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