Influence of yttrium on purification and carbide precipitation of superalloy K4169

doi:10.1016/j.jmst.2021.01.049

Abstract

Abstract:

The effect of reactive element yttrium on purification and carbide precipitation of superalloy K4169 has been studied through vacuum induction melting. The results show that yttrium has a strong deoxidizing and desulfurizing ability, and the impurity elements of sulfur and oxygen in melting K4169 are greatly reduced to 2-5 ppm and 4-5 ppm, respectively. The metallurgical reaction product is mainly Y₂O₃ and yttrium containing sulfides have not been observed. With the increase of the content of yttrium from 0.005 wt.% to 0.033 wt.%, the morphology and size of MC carbides in casting are significantly influenced due to that the yttrium changes the diffusion of the carbide forming elements and solid fraction during solidification. In the alloys without yttrium, only isolated blocky or strip carbides exist, while skeleton-like carbides are observed in the alloys with yttrium. Both the average size and the number density of skeleton carbides decrease monotonously with yttrium content increasing. After yttrium increasing to 0.033 wt.%, a phase of hexagonal Ni₁₇Y₂ is observed to precipitate in the form of symbiosis with MC carbide and matrix. The addition of yttrium is not beneficial for the properties of K4169 alloys at the room temperature, which is quite related to the MC carbides with a large skeleton-like morphology through the analysis of tensile fracture.

Key words: K4169, Rare earth element, Purification, Microstructure, Tensile properties

Shuting Cao, Yaqian Yang, Bo Chen, Kui Liu, Yingche Ma, Leilei Ding, Junjie Shi. Influence of yttrium on purification and carbide precipitation of superalloy K4169[J]. J. Mater. Sci. Technol., 2021, 86: 260-270.

Figures/Tables 24

Table 1 Amount of addition and analysis of element Y in different K4169 alloy ingots.

Alloy No.	0Y	50Y	70Y	330Y
Addition (wt.%)	0	0.01	0.02	0.1
Analysis of the content (wt.%)	0	0.005	0.007	0.033

Table 2 The impurity elements contents (ppm) in the K4169 alloy ingots. The stand errors are ±2 ppm for all the contents listed.

	0Y	50Y	70Y	330Y
S	9	2.5	3.5	5
O	6	4	5	4
N	14	14	13	13

Fig. 1. Impurity elements contents in K4169 alloy ingots with different Y additions. The variations of S (a), O (b) and N (c) contents are given with different Y additions, respectively. The error bars (±2 ppm) are omitted.

Table 3 Standard Gibbs free energies for the deoxidation, desulfurization, and deoxysulphuration reactions in Ni-based superalloys [26,28,29].

	Formula	ΔG^Θ (J mol^-1)	T =1695 K	Refs.
(1)	C + O = CO	-67742 - 39.75T	-135118	[28]
(2)	2Al + 3O = Al₂O₃	-1158310 + 364T	-540346	[28]
(3)	3CaO + 2Al + 3S = 3CaS + Al₂O₃	-741094 + 303T	-226559	[28]
(4)	2Y + 3O = Y₂O₃	-1792600 + 658T	-677290	[26]
(5)	2Y + 3S = Y₂S₃	-2344629 + 1067T	-534878	[27]
(6)	Y + S = YS	-899560 + 472T	-98384	[27]
(7)	2Y + 2O + S = Y₂O₂S	-634461 + 73T	-509506	[27]
(8)	3CaO + 2YS = 3CaS + Y₂O₃	-618783 + 173T	-323900	[27,28]

Table 4 Interaction coefficients of elements in Ni-based superalloys.

	Y	S	O
Y	$-\frac{364.8}{T}+0.2018=-0.016$	-5.83	-8.33
S	-2.1	$-\frac{1453}{T}+0.748=-0.120$	-0.178
O	-1.499	-0.08883	$\frac{305.84}{T}-0.1289=0.053$

Fig. 2. Dependence of the O (a) and S (b) concentrations on the Y concentration predicted by Eqs. (9) and (10), respectively. The error bars are ±2 ppm.

Fig. 3. Dominant zone diagram of oxide, oxysulfide and sulfide in the 330Y alloy calculated by the FactSage® software.

Fig. 4. TEM images and elemental mapping of the deoxidization or desulfurization products in the 330Y alloy.

Fig. 5. TEM analyses of yttrium oxide in the 330Y alloy: (a) morphology of yttrium oxide; (b) SADP of yttrium oxide [$\bar{7}8\bar{6}$]; (c) SADP of yttrium oxide [344].

Fig. 6. TEM images of different morphologies of Y2O3 particles in the 330Y alloy.

Fig. 7. (a) The SEM image of inclusions in the riser of the 330Y alloy. (b) Morphology of the inclusions and corresponding EDS mapping results with regard to the yellow box area.

Fig. 8. EPMA images of microstructure of samples with various Y additions: (a) alloy 0Y; (b) alloy 50Y; (c) alloy 70Y; (d) alloy 330Y.

Fig. 9. The OM images of the K4169 alloy in 0Y alloy (a1 to a3) and 330Y alloy (b1 to b3) at different isothermal temperatures, 1330 °C (a1 and b1), 1270 °C (a2 and b2), and 1200 °C (a3 and b3). The isolated blocky/strip MC carbides and the assembled skeleton-like MC carbides are marked by the red arrows and blue dashed circles, respectively.

Fig. 10. The solid fraction at different temperatures in the isothermal solidification experiments for the K4169 alloys with Y (330Y) and without Y (0Y).

Fig. 11. TEM analyses of carbides in the 330Y alloy: (a) morphology of the blocky MC carbide; (b) morphology of the strip MC carbide; (c) morphology of the symbiotic MC carbide; (d) SADP of the blocky MC carbide $\left[ 12\bar{1} \right]$; (e) SADP of the strip MC carbide [001]; (f) SADP of the symbiotic MC carbide [011].

Table 5 Spectral analysis of MC carbide compositions (at.%) in the 330Y alloy.

	Fe	Cr	Nb	Ti	Al	Ni	Mo	C
Isolated MC	0.43	0.35	44.90	5.18	0.02	1.33	0.44	47.31
Skeleton-like MC	0.65	0.45	43.69	7.49	0.01	1.67	0.47	45.56
Symbiotic MC	0.42	0.23	45.10	7.22	0.00	1.21	0.32	45.48

Table 6 Average size and number density of carbides.

Alloy no.	Average size (μm)	Std (μm)	Number density (10^-5 μm^-2)	Morphology
0Y				blocky/strip
50Y	46.96	20.05	6.414	skeleton-like
70Y	39.56	16.47	4.523	skeleton-like
330Y	34.02	13.36	5.191	skeleton-like

Fig. 12. Average size (a) and number density (b) of skeleton-like MC carbides with different Y contents. The error bars represent the standard deviation.

Fig. 13. EPMA and EDS analyses of precipitates in the 330Y alloy: (a) morphology of Y-rich and MC symbiotic phase and corresponding EDS mapping results with regard to the yellow box area; (b) morphology of Y-rich and matrix symbiotic phase and corresponding EDS mapping results with regard to the yellow box area.

Fig. 14. The HAADF STEM images of multiphase zone and the corresponding EDS mapping.

Fig. 15. TEM analyses of the multiphase zone in the 330Y alloy: (a) morphology of multiphase zone; (b) SADP of the γ matrix [110] in Fig. 15; (c) SADP of the Ni17Y2 [10$\bar{1}$0] in Fig. 15; (d) SADP of the Ni17Y2 [42¯2¯9¯] in Fig. 15; and (e) SADP of the MC carbide [011] in Fig. 15.

Table 7 The room temperature tensile properties of K4169 alloys with different Y additions.

Alloy no.	0Y	50Y	70Y	330Y
Tensile strength (σ_p, MPa)	1090 ± 2	1007 ± 4	1019 ± 1	1046 ± 2
Yield strength (σ_p0.2, MPa)	1240 ± 3	1109 ± 6	1135 ± 6	1161 ± 2
Elongation (A, %)	11 ± 1	8 ± 1	9 ± 1	9 ± 1

Fig. 16. The room temperature tensile results of K4169 alloys, including the ultimate tensile strength (UTS, σp), the yield strength (YS, σp0.2), and the elongation (A). The corresponding stand errors are given in Table 7.

Fig. 17. The SEM images of fracture surfaces of samples with different Y addition: (a) 0Y; (b) 50Y; (c) 70Y; (d)330Y. The longitudinal sections of fracture surfaces of samples with different Y additions: (e) 0Y; (f) 50Y; (g) 70Y; (h) 330Y.

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