| 其他摘要 | In this paper, a gradient NiCrAlYRe coating was prepared by combining Arc ion plating (AIP) with the low-pressure gas phase deposition aluminizing. Microstructure and high temperature oxidation behavior of the gradient NiCrAlYRe coating have been studied by using thermogravimetry analysis (TGA), optical photomicrograph (OM), scanning electron microscopy (SEM) equipped with energy dispersive X-ray analysis (EDX), electron probe microanalysis (EPMA) and X-ray diffraction (XRD). The main contents are summarized as following.
1. Preparation of the gradient NiCrAlYRe coating and its microstructure analysis
Arc ion plating (AIP) and the low-pressure gas phase deposition aluminizing was combined together to produce a gradient NiCrAlYRe coating on nickel-based single crystal superalloy DD32. Elements in this coating were chemically-graded with Al enrichment in the outer zone and Cr enrichment in the intermediate zone. Aluminizing was achieved at 950℃ for 4 h using Fe-50wt.%Al alloy powder as reagent and NH4Cl as activator. Gas pressure was controlled in the range of 0.02 and 0.06MPa. The aluminizing process was based on an intermediate temperature with an intermediate aluminum activity. The gradient NiCrAlYRe coating had a three-zone structure: the outer zone consisted of single β-NiAl; the intermediate zone was composed of γ, γ΄, β-NiAl and α-Cr; the inner zone was IDZ which was composed of γ, γ΄, α-Cr and a small amount of TCP phases. Al was gradiently distributed in the coating and Cr concentrated in the intermediate zone. During the aluminizing process, both the outward diffusion of Ni and the inward diffusion of Al happened in the coating, however, the outward diffusion of Ni dominated.
2. Cyclic oxidation of the gradient NiCrAlYRe coating
During cyclic oxidation at 1100℃, the gradient NiCrAlYRe coating showed lower mass gain than the conventional NiCrAlYRe coating. It also showed better resistance to oxide scale spallation. The sufficient β-NiAl phase in the near surface region of the gradient coating not only facilitated the quick formation of continuous Al2O3 scales during the initial oxidation stage, but also ensured the stability and the continuity of this protective oxide scales when exposure time increased and Al was losing unceasingly. By comparing the thickness of oxide scales formed on two coatings, we observed that the oxide scale formed on the gradient coating was thinner than that on the conventional coating. A thinner oxide scale usually suffers smaller thermal stress and growth stress, and less likelihood of cavity formation beneath the oxide scales, thus it is more resistant to crack and spallation. Besides, α-Cr phases existing in the interdiffusion zone could serve as a “diffusion barrier” which to some degree could inhibit the outward diffusion of Ni and the inward diffusion of Al. Re partitions almost exclusively to the α-Cr phases and two kinds of α-Cr phases with different Re contents were observed in the as-oxidized coatings.
3. Isothermal oxidation of the gradient NiCrAlYRe coating
The gradient NiCrAlYRe coating showed a lower oxidation rate than the conventional NiCrAlYRe coating during the early stage, whether oxidized at 900, 1000 or 1100℃. Two coatings showed similar mass gain after long time oxidation at 900℃, however, a much lower mass gain was observed on the gradient coating during the second oxidation stage when oxidized at 1000 or 1100℃. Thus, the gradient NiCrAlYRe coating showed better long-term oxidation resistance than the conventional NiCrAlYRe coating. During oxidation at 900℃, θ-Al2O3 was observed to grow on both coatings, however, the gradient coating provided conditions more favorable for θ-Al2O3 to grow. This might be due to different phase compositions in the near surface region of the two coatings. The outer zone of the gradient coating consisted of single phase β-NiAl. θ-Al2O3 usually forms when β-NiAl is oxidized at around 900℃. The conventional coating consisted of γ, γ΄ and α-Cr. Cr and Re in the conventional coating may inhibit the growth of θ-Al2O3. Besides, for the gradient NiCrAlYRe coating, the mass gain after long time oxidation at 1000℃ was conversely lower than that at 900℃. This might be related to the θ-Al2O3→α-Al2O3 phase transformation. The oxide scale formed during oxidation at 900℃ was a mixture of θ-Al2O3 and α-Al2O3. While the oxide scale formed during oxidation at 1000℃ was almost complete α-Al2O3. Compared with α-Al2O3, θ-Al2O3 has a much higher growth rate and is less protective. So after 60 hours oxidation, the mass gain on the gradient coating at 1000℃ was lower than that at 900℃ |
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