Based on the SEM microstructure of the quenched Ni-Al-CuO sample shown in Fig.1, the region near the com-bustion wave front can be divided into three parts: an initial zone (raw material),a reacting zone, and a product zone(completely reacted and quenched). Since addition of CuO did not affect the reaction of Ni and Al, the reactions between Ni-Al and Al-CuO are discussed separately.
Fig. 1 Low-magnification micrograph of Ni–Al–CuO sample after quenching
Figure 2 shows the typical microstructure of the reacting zone in the quenched Ni-Al-CuO coating sample. As shown in Fig.2(a), the reaction of Ni and Al was triggered by the appearance of liquid Al that reacted with suspended Ni to form eutectic Al, Ni-Al at the front end of the reacting zone. Figure 2(b) and(c), taken from theadjacent zone, indicate that the dissolution of Ni into molten Al increased the contact surface between the two phases and facilitated formation of NiAl, intermetalliccompounds. NiAl, can be observed on the outer surface of Ni particles. Near the boundary between the reacting and product zones, NiAl crystals began to precipitate from the saturated Al-Ni solution, as demonstrated in Fig.2(d).This reaction process is consistent with the dissolution-precipitation mechanism. During this process, the energy released by the reaction of Ni and Al caused the reaction temperature to reach that of the Al-CuO reaction.This is why the reaction between Al-CuO appeared at 740℃ in the Ni-Al-CuO system but did not appear in the Al-CuO system.
Fig. 2 Microstructural evolution of Ni–Al–CuO; (a) initial reaction of Ni–Al; (b) appearance of large amount of regular Al3Ni; (c) more Al3Ni2 precipitated out from the Al–Ni liquid; (d) NiAl precipitated as final product of Ni–Al reaction.
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