Surface Porous Structure and Microhardness of Intermetallic NiAl Compound
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THE intermetallic NiAl compound is a promising high-temperature structural material owing to its low density, high melting point, and excellent oxidation resistance.[1,2] The practical structural applications of this material, however, are limited by its low ductility and toughness at room temperature and low creep resistance at an elevated temperature.[3,4] Alloying,[5] composited with other materials,[6] and directional solidification (DS)[7] are normally applied to improve the low ductility and toughness of NiAl. The porous NiAl has excellent oxidation and corrosion resistances at high temperatures.[8,9] The NiAl intermetallic compound has satisfactory anticorrosion in NaCl solution at room temperature.[10] Meanwhile, the porous Ni-Al alloys show satisfactory corrosion resistance to the acid/alkaline solution,[11] and the porous Ni-Al alloys may be applied in severe environment due to their good physical and chemical properties. Given its excellent chemical stability, porous NiAl with regular nanopores can be used as the template for the deposition of rare-metal nanodots.[12–14] Two techniques have been used to produce porous NiAl structures. One process is the reactive synthesis of Ni and Al powders to
JIANJUN GAO, ZHILONG ZHAO, LUFENG WEI, KAI CUI, SHAOYI WANG, NING LI, JINGYING GUO, SEN CHEN, ZHIRONG HU, and YALONG LIU are with the School of Mechanical Engineering, Northwestern Polytechnical University, 710072 Xi’an, China. Contact e-mail: [email protected] Manuscript submitted November 15, 2017. Article published online June 14, 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A
sinter bulk porous NiAl, and the bending strength of this material depends on the addition of Al.[15] Nevertheless, the volume fraction of pores in this material increases with prolonged preheating treatment duration.[16] Another method by Hassel et al. combines DS and selective etching (SE) to prepare a well-ordered porous structure on the surface of NiAl.[17–19] Both the W and Mo elements, as a combined second-phase strengthening, can improve the room-temperature toughness and the high-temperature strength of the directionally solidified NiAl alloys.[20,21] In the current work, the system NiAl-W as a substrate is used to prepare the porous NiAl. Compared with the NiAl-Mo eutectic alloys, the advantage of this system is that the W fibers with different section shape can be obtained at different growth rates,[22,23] whereas the shape of Mo fibers is kept at a rectangle in directionally solidified NiAl-Mo eutectic alloys grown at different growth rates[24] and the rectangular NiAl pores are obtained after SE of Mo fibers.[18] After an SE of W fibers in directionally solidified NiAl-W eutectic alloys, the NiAl pores with different shape are obtained. Many studies have focused on the structure of DS eutectic alloy.[25,26] Little work has been carried out on the influence of growth rate and the etching condition on the morphology of porous NiAl structure. In addition, the relationship between the porous NiAl structure and its microhardness
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