The Influence of Pd on the Structure and Oxidation Performance of β-NiAl Diffusion Coatings

Pauletti, E.; d’ Oliveira, A.S.C.M

doi:10.1590/1980-5373-MR-2022-0384

E. Pauletti

Universidade Federal do Paraná (UFPR), Programa de pós-graduação em Engenharia e Ciência dos Materiais, Curitiba, PR, Brasil.

https://orcid.org/0000-0002-0565-2599

A.S.C.M d’ Oliveira ^**e-mail: sofmat@ufpr.br

Universidade Federal do Paraná (UFPR), Departamento de Engenharia Mecânica, Curitiba, PR, Brasil.

https://orcid.org/0000-0002-9736-6652

*e-mail: sofmat@ufpr.br

Figures (11)
Tables (5)

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Figure 1
Cross section and chemical profile for Ni, Al and Pd of aluminized coatings (SEM, BSE mode) processed at 1030 °C: a) unmodified; b) and c) modified with Pd layers measuring 8.0 and 9.6 µm, respectively (Ptt: precipitates; IDZ: interdiffusion zone).

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Figure 2
XRD of the surface of (unmodified and Pd-modified) aluminized coatings after processing at 1030 °C. Insert is a zoom-in view of the main peak region.

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Figure 3
Schematic representation highlighting the different microstructures for aluminized coatings: a) unmodified; b) Pd-modified.

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Figure 4
Mass gain of Pd-modified and unmodified aluminized coatings during isothermal oxidation at 1000 °C for 200 h.

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Figure 5
Morphology of Al₂O₃ scale at the surface of aluminized coatings after isothermal oxidation at 1000 °C for 200 h; a) Pd-modified coating (9.6 µm); b) unmodified coating.

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Figure 6
Confocal Raman mapping at the surface of the Pd-modified coating (9.6 µm) after isothermal oxidation at 1000 °C for 200 h: a) mapping in a spectral range from 14,308 to 14,472 cm^-1 (α-Al₂O₃); b) mapping in a spectral range from 14,506 to 14,663 cm^-1 (θ-Al₂O₃); c) mapping showing the distribution of polymorphic forms of Al₂O₃; d) and e) spectra in a specific region of the surface.

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Figure 7
Confocal Raman mapping at the surface of the unmodified aluminized coating after isothermal oxidation at 1000 °C for 200 h: a) mapping in a spectral range from 14,308 to 14,472 cm^-1 (α-Al₂O₃); b) mapping in a spectral range from 14,506 to 14,663 cm^-1 (θ-Al₂O₃); c) mapping showing the distribution of polymorphic forms of Al₂O₃; d) and e) spectra in a specific region of the surface.

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Figure 8
Confocal Raman analysis of Al₂O₃ oxide scale formed at the surface of a Pd-modified coating (9.6 µm) () and an unmodified aluminized coating () after isothermal oxidation at 1000 °C for 200 h.

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Figure 9
Average chemical composition (at. %) of aluminized coatings after isothermal oxidation at 1000 °C for 200 h: a) unmodified; b) Pd-modified (8.0 µm); Pd-modified (9.6 µm). The regions chosen for analysis are indicated by white squares.

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Figure 10
β-(NiAl) /Al₂O₃ and β-(Ni,Pd)Al /Al₂O₃ interfaces in aluminized coatings after isothermal oxidation at 1000 °C for 200 h: a) unmodified; b) Pd-modified (8.0 µm); c) Pd-modified (9.6 µm) (Ptt: precipitates).

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Figure 11
External layer of aluminized coatings after isothermal oxidation at 1000 °C for 200 h: a) Pd-modified; b) unmodified.

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Table 1
Chemical composition of the NI-183 alloy measured by EDX (wt. %).

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Table 2
Analytical reagents used for electroplating in the Pd bath.

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Table 3
Parameters used to Pd electroplate the surface of the Ni superalloy.

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Table 4
Average chemical composition (at. %) of layers for the unmodified and Pd-modified aluminized coatings after processing at 1030 °C.

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Table 5
Microstructural alterations due to modification of aluminized coatings with Pd and the proposed mechanisms.

Ni superalloy	Ni	Al	Cr	Co	Ti	W	Ta	Mo
NI-183	Bal.	3.2	14.5	9.2	2.7	2.9	2.1	1.9

Analytical reagent	Chemical formula	Concentration in the bath
Palladium chloride	PdCl₂	10 g/L
Ammonium chloride	NH₄Cl	50 g/L
Ammonium hydroxide	NH₃.H₂O (28%)	15 mL/L
Ammonium phosphate dibasic	(NH₄)₂HPO₄	50 g/L

Palladium salt	Pd concentration in bath (g/L)	Temperature (° C)	pH	Current density (mA/cm²)	Voltage (V)	Electroplating rate (µm/min)
PdCl₂	10	between 40 and 45	7.3	7.0	1.8 to 1.9	0.3

Thickness of electroplated Pd layer	Layer	Ni	Al	Cr	Ti	Pd
0 μm	External	23.9	62.5	5.9	2.4	0
0 μm	IDZ	50.3	12.7	17.8	6.3	0
8.0 µm	External	27.9	62	3.1	0.2	5.3
	Intermediate	23.9	62.1	5.3	2.2	1.7
	IDZ	29.3	33.8	17.7	8.3	0
9.6 µm	External	29.9	56.7	5.0	1.1	4.4
	Intermediate	29.9	54.9	5.2	2.7	2.0
	IDZ	30.5	30.7	17.7	9.8	0

Microstructural alteration due to modification of coating with Pd	Proposed mechanism
Reduction in thickness of intermetallic layer	Reduction of driving force for diffusion of Al
Increase in size of IDZ	Increase in outward diffusion of Ni from substrate to intermetallic layer
Absence of Cr and Ti precipitates in external layer	Reduction in diffusion of Cr and Ti to outer layer

[1] *e-mail: sofmat@ufpr.br

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The Influence of Pd on the Structure and Oxidation Performance of β-NiAl Diffusion Coatings

Abstract