Intermetallic compound NiAl with B2 crystal structure is regard as a potential candidate of high-temperature structural materials, due to its high melting point, low density, good thermal conductivity and good oxidation resistance. However, its poor high-temperature strength and serious lack in fracture toughness and ductility at room temperature have prevented its commercial applications. Recent researches have found that these shortcomings can be improved significantly by the fabrication of NiAl-based eutectic composites involving refractory metal phases, and the NiAl/Cr(Mo) alloys display attractive mechanical properties. Furthermore, mechanical properties at elevated temperatures of NiAl/Cr(Mo) eutectic alloy can be improved effectively by minor addition of Hafnium (Hf). But the addition of Hf results in the increase of brittleness and almost no compressive strain at room temperature. The reason is that Heusler (Ni2AlHf) particles prefer to precipitate at NiAl/Cr(Mo) interface and eutectic cell boundary. Therefore, for the NiAl–Cr(Mo)/Hf lamellar eutectic alloys strengthened by Heusler precipitates, a feasible way to improve its high-temperature strength and room temperature ductility is to refine the Ni2AlHf precipitates and optimize their distribution and morphology.
As a method to improve materials properties, magnetic fieldhas been applied in many fields. The studies on synthesis of ceramic materials find that magnetic field can control crystalline orientation and grain orientation. And moreover magnetic field still can refine microstructure and improve the distribution and morphology of precipitates. Therefore, in the present investigation, the Ni–33Al–28Cr–6Mo–0.2Hf (at.%) alloy was chosen and treated by strong magnetic field (SMF), and its microstructural evolution and RT compressive properties were investigated simultaneously.
After the treatment of strong magnetic field, Heusler phase (Ni2AlHf) in the NiAl–Cr(Mo)–0.2Hf eutectic alloy is transformed into Hf solid solution; moreover the Hf solid solution becomes fine and distributes uniformly. The microstructure of the NiAl–Cr(Mo)–0.2Hf alloy with 10T 1173K treatment has an obvious disintegration. The Cr(Mo) phase trends to be spheroidizing and many eutectic cells are substituted by massive NiAl and Cr(Mo). When the strong magnetic field treatment temperature increases to 1273K, the microstructure of the alloy aligns to the direction of strong magnetic field. Compared with the NiAl–Cr(Mo)–0.2Hf alloy with heat treatment, The room temperature compressive ductility of strong magnetic field treated alloy improve significantly, which can be attributed to microstructure improvement resulted by the strong magnetic field treatment.
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