Epitaxial growth and oxidation behavior of an overlay coating on a Ni-base single-crystal superalloy by laser cladding

doi:10.1016/j.jmst.2018.10.011

Abstract

Abstract:

An overlay coating material was deposited on a single crystal superalloy SRR99 by laser cladding. The microstructure and oxidation behavior of this coating was investigated through scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results indicated that although the composition of the coating was chosen based on the γ' composition in René N5 superalloy, the primary solidification phase of this coating during laser cladding was γ-Ni. Furthermore, under the laser cladding condition, fine parallel dendrites grew epitaxially in the coating from the substrate, indicating the single crystal structure of the substrate was reproduced. When the single crystal MCrAlY coating was oxidized at 1000℃, both θ-Al₂O₃ and α-Al₂O₃ formed during initial oxidation process. As the oxidation time proceeded, the presence of θ-Al₂O₃ facilitated the formation of NiAl₂O₄ spinel oxide. Once the spinel was observed, it flourished and induced some porosity in the scale. When the scale thickness increased to 6-7 μm, large area spallation of the scale began.

Key words: Laser cladding, Single crystal growth, Epitaxy, Overlay coating

Jingjing Liang, Yongsheng Liu, Jinguo Li, Yizhou Zhou, Xiaofeng Sun. Epitaxial growth and oxidation behavior of an overlay coating on a Ni-base single-crystal superalloy by laser cladding[J]. J. Mater. Sci. Technol., 2019, 35(2): 344-350.

Figures/Tables 9

Table 1 Chemical compositions of the substrate and the coating powder.

	Cr	Co	W	Mo	Ta	Al	Hf	Ti	C	Y	Ni
TMBC-1	4.06	6.13	4.62	1.02	9.91	8.10	0.30	-	-	0.031	Bal.
SRR99	8.50	5.01	9.52	-	2.81	5.50	-	2.17	0.015		Bal.

Fig. 1. Optical microstructure of the one-layer coating deposited on a SRR99 substrate by laser cladding: (a) whole coating; (b) bottom part; (c) top part.

Fig. 2. Optical microstructure of the two-layer coating deposited on a SRR99 substrate by laser cladding: (a) periphery part; (b) middle part.

Fig. 3. (a) Isopleth and (b) Scheil-Gulliver solidification process map in the coating system calculated with Thermo-Calc software and TTNi8 database.

Fig. 4. XRD patterns of (a) TMBC-1 powder and (b) its as-deposited coating by laser cladding.

Table 2 Phase constituents of oxide products on the oxidized coatings at 1000 °C.

Time	Oxide products
20 min	α-Al₂O₃ (vw)
5 h	α-Al₂O₃ (m)
10 h	α-Al₂O₃ (s), NiAl₂O₄ (vw)
155 h	α-Al₂O₃ (vs), Spinel (NiAl₂O₄ (m), NiCr₂O₄ (s), CoCr₂O₄ (w))
355 h	α-Al₂O₃ (vs), NiO (s), Spinel (NiAl₂O₄ (s), NiCr₂O₄ (s), CoCr₂O₄ (w))
506 h	NiO (vs), α-Al₂O₃ (s), Spinel (NiAl₂O₄ (s) CoAl₂O₄ (m) NiCr₂O₄ (m) CoCr₂O₄ (w))

Fig. 5. Surface morphologies of the coating after oxidation at 1000 °C in air for (a) 20 min, (b) 1 h, (c) 10 h, (d) 155 h, (e) 355 h, (f) 506 h.

Fig. 6. EDS result of the oxidized scale marked by red rectangle in Fig. 5(a).

Fig. 7. Cross-section morphologies of the coating after oxidation at 1000 °C in air for (a) 10 h, (b) 155 h, (c) 355 h, (d) 506 h.

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