Synthesis Mechanism and Strengthening Effects of Laminated NiAl by Reaction Annealing
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TRODUCTION
NIAL intermetallics have been studied extensively as an attractive material for high-temperature applications attributed to their high melting point, low density, good thermal conductivity, and oxidation resistance.[1–3] However, it is hard to fabricate NiAl sheet using traditional methods due to their poor ductility at room temperature. Researchers have fabricated a series of Ni/NixAly laminated composite sheets in a near-net shaping way by reaction synthesis of stacked Ni and Al foils,[4–7] which provides a new way to fabricate NiAl sheets. For example, Konieczny[7] studied the microstructure and mechanical properties of Ni-(Ni2Al3+NiAl3) and Ni-(Ni3Al+NiAl) composites fabricated by reaction synthesis. It turned out that the reaction sequence of NixAly during the process depended on the reaction temperature. However, it usually takes very long time to obtain stoichiometric NiAl using this solid-solid synthesis method. Moreover, many voids form in the Ni/ NixAly laminated materials as a result of Kirkendall effect and the difference in molar volumes between the products. Therefore, extra annealing treatments at high temperature are usually needed to eliminate voids.[6,7] Recently, combustion synthesis of NiAl has been studied,[8,9] which takes less time to synthesize NiAl.
YAN DU, Ph.D. Student, GUOHUA FAN, Associate Professor, QINGWEI WANG, Assistant Researcher, and LIN GENG, Professor, are with the School of Materials Science and Engineering, Harbin Institute of Technology, 150001, Harbin, China. Contact e-mail: [email protected] Manuscript submitted July 6, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
According to the thermal explosion mode,[8] the reaction starts at the melting point of Al and progresses rapidly due to an intense exothermic reaction. Therefore, it will be more quickly to obtain NiAl (if Ni and Al react completely when the atom ratio of Ni to Al is 1 to 1) by solid-liquid reaction than by solid-solid reaction. Our previous study[10] shows that a novel laminated NiAl sheet with bimodal grain size distribution can be produced quickly by the solid-liquid reaction using alternate layers of Ni and Al foils. However, the synthesis mechanism and the strengthening effect of the laminated NiAl have not been investigated in detail. The influences on bimodal and multimodal intermetallic precipitate distribution have been studied for nickel alloys as high-temperature structural materials.[11,12] It shows that heat treatment parameters strongly affect the size and distribution of intermetallic precipitates.[12] Moreover, it has been reported that metal materials, which have microscopic hierarchical structures[13] or bimodal grain size distribution structures,[14,15] have advantages in enhancing high strength and ductility combination. In addition, fracture stresses of NiAl at room temperature and high temperature are dependent on grain size and significantly improvement occurs below a critical grain size.[16–18] As mentioned above, materials with heterogeneous microstructures exhibit enhanced stren
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