The role of Re in improving the oxidation-resistance of a Re modified PtAl coating on Mo-rich single crystal superalloy

doi:10.1016/j.jmst.2020.03.054

Abstract

Abstract:

A novel Re modified β-(Ni,Pt)Al coating was prepared on a Mo-rich Ni₃Al-based single crystal (SC) superalloy by electro-deposition of Re/Pt dual films and low-activity aluminizing. The isothermal oxidation behavior of the Re modified β-(Ni,Pt)Al coating and traditional β-(Ni,Pt)Al coating was comparatively studied at 1100 °C. Apparent spallation of oxide scale was found on the surface of traditional β-(Ni,Pt)Al coating after 300 h isothermal exposure, which mainly resulted from the evaporation of Mo-containing oxides. It is further found that the outward diffusion of Mo from the SC substrate was effectively inhibited in the Re modified β-(Ni,Pt)Al coating by forming the σ-MoRe phase both at the coating/superalloy interface and grain boundaries. It is also revealed that the addition of Re can stabilise Mo to form the σ-MoRe phase, allowing the improved oxidation resistance of the Re modified β-(Ni,Pt)Al coating.

Key words: Superalloy, PtAl coatings, Oxidation, Diffusion barrier

Jingchen Li, Liangliang Wei, Jian He, Hao Chen, Hongbo Guo. The role of Re in improving the oxidation-resistance of a Re modified PtAl coating on Mo-rich single crystal superalloy[J]. J. Mater. Sci. Technol., 2020, 58: 63-72.

Figures/Tables 18

Table 1 The chemical compositions (in at.%) of IC21 alloy substrate.

Al	Mo	Ta	Cr	Re	Y	Ni
16.5	8.7	0.8	2.3	0.2	0.03	Bal.

Fig. 1. Schematic illustration of the as-electroplated Ni/NiRe/Pt tri-layer coating.

Fig. 2. (a) BSE micrograph of the as-electroplated Ni/ NiRe/ Pt tri-layer coating; (b) higher magnification micrograph of the interface of electroplated layers.

Fig. 3. XRD patterns of as-prepared NiRe/Ni-Pt-Al coating and Ni-Pt-Al coating.

Fig. 4. (a) SEM cross-sectional images of the as-prepared Re modified β-(Ni,Pt)Al coating; (b) higher magnification of layer 1 in Fig. 4(a); (c) higher magnification of layer 2 in Fig. 4(a); (d) higher magnification of layer 3 in Fig. 4(a); (e) Re element mapping of layer 2 in Fig. 4(c); (f) Mo element mapping of layer 3 in Fig. 4(d).

Fig. 5. EDS composition profiles of: (a) Pt, Re and Mo; (b) Ni and Al measured across the Re modified β-(Ni,Pt)Al coating/substrate.

Fig. 6. XRD patterns of the β-(Ni,Pt)Al coating and the Re modified β-(Ni,Pt)Al coating after 300 h isothermal oxidation at 1100 °C.

Fig. 7. Photographs of: (a) β-(Ni,Pt)Al coating sample and (b) Re modified β-(Ni,Pt)Al coating sample after 300 h isothermal oxidation at 1100 °C.

Fig. 8. SEM micrographs of surfaces of the coatings after 300 h oxidation at 1100 °C: (a) β-(Ni,Pt)Al coating; (b) Re modified β-(Ni,Pt)Al coating; (c) High magnification (2000×) of the spallation area in Fig. 8(a); (d) High magnification (4000×) of the spallation area in Fig. 8(a).

Fig. 9. BSE micrographs of cross-sections of the coatings after 300 h oxidation at 1100 °C: (a) Re modified β-(Ni,Pt)Al coating; (b) β-(Ni,Pt)Al coating; (c) and (d) the local micrograph with high magnification of the oxide scales in (a) and (b), respectively.

Fig. 10. EDS composition profiles of: (a) Pt, Cr, Re and Mo; (b) Ni and Al measured across the oxidized Re modified β-(Ni,Pt)Al coating/IC21 substrate.

Table 2 Average chemical compositions of the precipitates in the β-(Ni,Pt)Al coating and the Re modified β-(Ni,Pt)Al coating after 300 h oxidation calculated from EPMA point probe spectra at six different points in each coating marked in Fig. 9(a) and (b) (in at.%).

Coating types	Ni	Mo	Re	Al	Cr
β-(Ni,Pt)Al coating	Bal.	40.3	0.0	8.0	0.0
Re modified β-(Ni,Pt)Al coating	Bal.	55.8	22.4	0.0	4.1

Fig. 11. (a) BSE micrograph of the boxed zone marked in Fig. 9(a) with higher magnification; (b) corresponding element mappings of Ni, Re, Mo and Cr.

Fig. 12. TEM results of the precipitate in the oxidized Re modified β-(Ni,Pt)Al coating: (a) SAED of the precipitate; (b) micrograph of the A: coating and B: the precipitate.

Fig. 13. Schematic model of the mechanism of Re blocking Mo outward diffusion in the Re modified β-(Ni,Pt)Al coating.

Fig. 14. EBSD patterns of the MoRe precipitate in the Re modified β-(Ni,Pt)Al coating after 300 h oxidation: (a) and (b) BSE images of the precipitate; (c) grain boundary map: red zone and yellow zone mainly containing Ni and Mo, respectively.

Table 3 Average chemical compositions of the boxed zone marked in Fig. 9(a) (in at.%).

Al	Ni	Cr	Mo	Pt	Re
23.2	Bal.	1.0	3.8	3.2	0.5

Fig. 15. Plots of phase fraction and elemental compositions of all phases in the Re modified β-(Ni,Pt)Al coating versus temperature with the certain composition in Table 3 obtained from Thermo-Calc using TCNi8 database: (a) volume fraction of all phases; (b) mole percentage of elements in γ′-NiAl phase (c) mole percentage of elements in β-NiAl; (d) mole percentage of elements in Mo(Re).

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