Multi-scale Study of the Formation and Evolution of M6C Carbides in High-Tungsten Superalloys

doi:10.1007/s40195-024-01754-5

Abstract

Abstract:

The formation and evolution of M₆C carbides in high-W superalloy following solution treatment was investigated at different temperatures. Initially, during solid solution treatment, MC and M₆C carbides was precipitated in the alloy. As the temperature increased, the morphology of M₆C carbides transitioned from granular to needle-like. During the solution treatment at 1255 °C, the MC carbides degraded and transformed into M₆C carbides, forming a symbiotic relationship between them. Nonetheless, no clear orientation relationship was observed between the two types of carbides. After further increasing the temperature to 1270 °C, the precipitation of needle-like M₆C carbides in the dendrite arm was confirmed. This was supported by electron probe X-ray micro-analyzer and selected area electron diffraction patterns. Subsequently, a detailed examination of the three-dimensional morphology and orientation relationship of the needle-like phase with the matrix was carried out using focused-ion-beam and transmission electron microscopy techniques. The results indicated that the flat interface of the needle phase exhibited a specific orientation relationship with the matrix. However, in the three-dimensional plane, the interfaces between the needle-like phase and the matrix were not straight. Furthermore, no clear orientation relationship between the non-straight interfaces and the matrix was observed. As the solution temperature increased, the tensile properties at room temperature progressively decreased, while the stress rupture properties peaked at 1260 °C, suggesting that the alloy demonstrated its optimal comprehensive performance at this temperature. A subsequent analysis was conducted on the longitudinal section of the fracture using electron backscattered diffraction. The results showed a noticeable concentration of stress at the interface between MC and M₆C carbides, which ultimately led to crack initiation at this interface. In addition, as the solid solution temperature increased, the quantity of symbiotic phases also increased. This phenomenon led to the initiation of cracks at multiple locations, which then propagated and interconnected. As a consequence, the tensile properties and stress rupture life of the alloy progressively deteriorated.

Key words: Superalloys, Tungsten, Solution treatment, Carbides, Mechanical properties

Xiang Fei, Naicheng Sheng, Shijie Sun, Shigang Fan, Jinjiang Yu, Guichen Hou, Jinguo Li, Yizhou Zhou, Xiaofeng Sun. Multi-scale Study of the Formation and Evolution of M₆C Carbides in High-Tungsten Superalloys[J]. Acta Metallurgica Sinica (English Letters), 2024, 37(12): 1995-2007.

Figures/Tables 15

Table 1 Main chemical composition of K416B alloy (wt%)

Al	Ti	Hf	Co	Cr	Nb	W	C	Ni
5.9	0.8	0.9	7.8	5.1	1.8	15.8	0.11	Bal.

Fig. 1 a DSC heating and b cooling results of as-cast K416B alloy

Table 2 Phase transition temperature of K416B alloy by DSC curve

	L−γ (°C)	MC (°C)	L−γ/γ′ (°C)	γ−γ′ (°C)
Heating-up	1376	1334	1291	1231
Cooling	1361	1317	1274	1211

Fig. 2 As-cast and solid solution microstructure of K416B alloy: a as-cast; b 1255 °C; c 1260 °C; d 1270 °C; e 1280 °C; f 1290 °C

Table 3 Composition of various carbides revealed by EDS

		C	Cr	Co	Ni	W	Nb	Hf	Ti
MC of as-cast	wt%	12.07	1.31	0.79	6.45	35.55	27.95	7.62	8.26
MC of as-cast	at.%	53.93	1.35	0.72	5.90	10.40	16.14	2.28	9.27
MC of symbiosis	wt%	10.11	0.68	0.89	7.49	20.49	33.70	20.89	5.74
MC of symbiosis	at.%	49.32	0.76	0.88	7.36	6.49	21.28	6.91	7.00
M₆C of symbiosis	wt%	2.34	2.08	0.54	4.36	90.68
M₆C of symbiosis	at.%	24.00	4.93	1.14	9.14	60.79
M₆C of needle-like	wt%	3.77	2.07	2.41	17.49	74.27
M₆C of needle-like	at.%	28.63	3.62	3.72	27.15	36.88

Fig. 3 Element distribution in symbiotic phase by EPMA: a SEM; b Nb; c Al; d C; e Hf; f Ni; g W; h Ti; i Cr

Fig. 4 a SEM image and b, c, e, h TEM images of the symbiotic phase; d the magnification image of area D in b; f, g SAED pattern at area B and C in b

Fig. 5 Element distribution in the needle-like M6C phase by EPMA: a SEM; b Nb; c Al; d C; e Hf; f Ni; g W; h Ti; i Cr

Fig. 6 a, b SEM images and c-f TEM images of the needle-like M6C phase; g, h SAED pattern at area C and D in c; i SAED pattern at two-phase interface

Fig. 7 a SEM and b-e TEM images of the needle-like M6C phase; f the magnification of area D in b; g, h SAED pattern at area B and C in b; i SAED pattern at two-phase interface

Table 4 Tensile properties at room temperature and stress rupture life at 975 ℃/235 MPa of different solution temperatures

Temperature (°C)	Tensile properties			Stress rupture life (h, 975 ℃/235 MPa)
Temperature (°C)	σ_m (MPa)	σ_0.2 (MPa)	A (%)	Stress rupture life (h, 975 ℃/235 MPa)
1255	1070 ± 10	837.5 ± 15	5.05 ± 1.45	45.84 ± 2.04
1260	972.5 ± 17	797.5 ± 8	5.05 ± 1.45	61.22 ± 8.76
1270	840 ± 0	760 ± 5	1.75 ± 0.45	50.19 ± 7.33
1280	770 ± 55			1.23 ± 0.3

Fig. 8 Phase distribution diagram and KAM diagram of longitudinal section of tensile fracture after solution treatment at different temperatures: a, e, i 1255 °C; b, f, j 1260 °C; c, g, k 1270 °C; d, h, l 1280 °C

Table 5 Effect of element content on the type of carbides

Empirical formulas	Types of carbides
Cr. at.%/(Cr + Mo + 0.7 W) at.% < 0.72	M₆C carbide
Cr. at.%/(Cr + Mo + 0.7 W) at.% > 0.82	M₂₃C₆ carbide
0.72 < Cr. at.%/(Cr + Mo + 0.7 W) at.% < 0.82	Types of carbides change with heat treatment

Table 6 Diffusion constant and diffusion activation energy of the alloy elements in Ni [29,30,31,32]

	D₀ (m² s⁻¹)	Q_{m Ni} (kJ mol⁻¹)
Ni	1.9 × 10^-4	284
Co	7.5 × 10^-5	285.1
Cr	3 × 10^-6	170.7
W	8.0 × 10^-6	264
Nb	8.8 × 10^-5	251
Al	7.52 × 10^-4	284
Ti	8.6 × 10^-5	257

Fig. 9 Growth mechanism diagram of needle-like M6C carbides: a distribution of γ′ phase; b nucleation of M6C carbides; c a flat interface between M6C and matrix; d non-flat interface between M6C and matrix

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Multi-scale Study of the Formation and Evolution of M₆C Carbides in High-Tungsten Superalloys

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