Microstructure and Phase Stability Studies on Heusler Phase Ni 2 AlHf and G-phase Ni 16 Hf 6 Si 7 in Directionally Solid
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Q. Liu Laboratory of Atomic Imaging of Solids, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015, People’s Republic of China, and Institute of Materials and Technology, Dalian Maritime University, Dalian 116026, People’s Republic of China
L.L. He, J.T. Guo, and D.X. Li Laboratory of Atomic Imaging of Solids, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015, People’s Republic of China (Received 15 September 1999; accepted 14 March 2000)
Small additions of Hf to directionally solidified NiAl–Cr(Mo) eutectic resulted in precipitation of a high density of Heusler phase Ni2AlHf along with fine G-phase Ni16Hf6Si7. The Heusler phase was mainly located on the grain boundary region. The fine G-phase formed in the presence of Si, which was a contamination resulting from contact with ceramic shell molds during directional solidification of the alloy. These fine G-phases were cuboidal in shape and coherent with the NiAl matrix. After hot isostatic pressing and aging treatment, the fine G-phases completely disappeared. The density of the Heusler phase was partially reduced, and the Heusler particles precipitated preferentially on the NiAl/Cr(Mo) interfaces and grain boundaries of the NiAl matrix. Some Heusler particles precipitated locally within the NiAl matrix, and small amounts of them precipitated within the Cr(Mo) phase. The structures of the NiAl/Ni2AlHf and NiAl/Ni16Hf6Si7 interfaces were investigated by high-resolution electron microscopy. The habit plane of the fine G-phase was {001}NiAl. This result was in good agreement with calculation based on the linear elastic theory. The misfit dislocation network on the NiAl/Ni2AlHf (110) interface was calculated from the O-lattice model and compared with the observation, which showed good agreement. I. INTRODUCTION
Directionally solidified NiAl–Cr(Mo) eutectic alloy has been studied for many years1,2 and renewed recently3 for its much higher fracture toughness than that of polycrystalline NiAl. Its microstructure was characterized by a lamellar Cr(Mo) phase embedded within a NiAl matrix. However, its low creep strength restricted its hightemperature application. Small additions of refractory metals from Group IVB, in particular Hf, were found to be very effective in improving the high-temperature creep strength of NiAl single crystal.4 These elements provide strengthening by solid-solution effects and precipitate hardening. The latter is believed to be due to nucleation of the G-phase Ni16Hf6Si7, which, after prolonged aging at high temperatures, may be replaced by Heusler phase Ni2AlHf. It should be noted that Si is not an intentional alloying addition in these materials but enters the melt through reaction with the ceramic shell molds during directional solidification of the alloy inJ. Mater. Res., Vol. 15, No. 6, Jun 2000
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gots. Extensive studies on mechanical properties of the Heusler single-phase and the NiAl–Ni2AlHf two-phase alloys have been reported,5–10 and the tw
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