Influence of Substituting W for Nb or Hf on Solidification Behavior of a Typical Co-Ni-Al-W Based Superalloy

doi:10.1007/s40195-024-01749-2

Abstract

Abstract:

Understanding the effects of various elements on solidification behavior is crucial for designing the composition of γ’-strengthened Co-based superalloys and is fundamental for controlling the as-cast structure and formulating subsequent heat treatment processes. This research investigated the effects of replacing 1 at.% W with 1 at.% Nb or Hf elements on the solidification behavior of Co-Ni-Al-W-based superalloys. The findings revealed that substituting W with Nb and Hf resulted in a notable decrease in both the solidus temperature (T_S) and liquidus temperature (T_L). Specifically, the substitution of W with Nb lowered T_S from 1353 °C to 1332 °C and T_L from 1383 °C to 1368 °C, while replacing W with Hf decreased T_S from 1353 °C to 1330 °C and T_L from 1383 °C to 1366 °C. Moreover, both Nb and Hf element are positive segregation element, while Nb decreases and Hf increases W segregation, respectively. During the final solidification stage, the substitution of W with Nb resulted in the formation of eutectic (γ + γ’), Co₃Ta, and a small amount of μ-Co₇Nb₆ phase, while replacing W with Hf resulted in the formation of the Laves phase and β-CoAl phase. The solidification paths of the three alloys were confirmed based on the result of differential scanning calorimetry, isothermal solidification experiment and Thermo-calc simulation. These results offer a theoretical basis for the composition design and optimization of heat treatment processes for Co-Ni-Al-W-based superalloys.

Key words: Co-Ni-Al-W-based superalloys, Solidification microstructure, Segregation, Solidification path

Huifang Zhang, Jun Xie, Qi Li, Hao Wu, Jinjiang Yu, Hongyu Chai, Fengjiang Zhang, Jinguo Li, Yizhou Zhou, Xiaofeng Sun. Influence of Substituting W for Nb or Hf on Solidification Behavior of a Typical Co-Ni-Al-W Based Superalloy[J]. Acta Metallurgica Sinica (English Letters), 2024, 37(11): 1907-1920.

Figures/Tables 15

Table 1 Measured compositions and density of three experimental alloys

Alloys	ρ (g·cm⁻³)	Percentage	Co	Al	W	Ni	Cr	Ta	Ti	C	B	Zr	Nb	Hf
Base	9.18	at.%	58.79	9.38	5.21	16.46	6.09	1.81	1.28	0.79	0.17	0.01	-	-
Base	9.18	wt%	54.48	3.98	15.06	15.19	4.98	5.15	0.96	0.15	0.03	0.02	-	-
Nb	9.02	at.%	58.77	9.40	4.14	16.54	6.08	1.81	1.30	0.78	0.13	0.01	1.04	-
Nb	9.02	wt%	55.10	4.05	12.14	15.50	5.05	5.24	0.99	0.15	0.02	0.02	1.54	-
Hf	9.10	at.%	59.05	9.30	4.14	16.55	6.13	1.88	1.34	0.80	0.17	0.01	-	1.09
Hf	9.10	wt%	54.23	3.94	11.96	15.26	5.01	5.34	1.01	0.15	0.03	0.02	-	3.05

Fig. 1 Schematic drawing that illustrates the isothermal solidification process

Fig. 2 Backscattered scanning electron microscope micrographs of experimental alloys microstructures: a-c microstructures at low magnification, d-f interdendritic precipitation phases

Table 2 Compositions of interdendritic phases measured by SEM-EDS and TEM-EDS (at.%)

Presumed phase	Measure method	Co	Al	W	Ni	Cr	Ta	Ti	Zr	Nb	Hf
M₃B₂	SEM-EDS	46.7	0.2	15.3	5.9	2.7	24.7	4.4	0.1	-	-
MC (Base alloy)	SEM-EDS	5.5	-	-	2.1	0.4	60.8	29.5	2.0	-	-
MC (Nb alloy)	SEM-EDS	6.6	-	3.6	2.1	0.6	41.1	20.2	0.1	25.8	-
MC (Hf alloy)	SEM-EDS	7.3	-	4.4	1.5	0.7	45.0	18.5	-	-	22.7
Co₃Ta	TEM-EDS	58.6	1.8	2.0	7.6	4.0	12.1	2.3	2.4	9.1	-
μ-Co₇Nb₆	SEM-EDS	47.5	0.3	9.7	6.1	3.8	11.0	2.7	0.5	18.4	-
Eutectic γ + γʹ	SEM-EDS	56.4	9.3	3.4	17.4	4.7	3.3	2.2	-	2.1	-
β	TEM-EDS	31.1	19.5	-	15.0	2.0	19.9	2.5	0.6	-	9.2
Laves	TEM-EDS	54.1	3.6	1.6	16.7	1.8	4.8	2.0	-	-	15.4

Fig. 3 TEM images and selected area diffraction patterns (SADP) of alloy Nb: a FIB cutting position, b FIB cutting completed sample, c TEM images of two phases, d TEM morphology and SADP pattern of Co7Nb6, e SADP of the TEM morphology and SADP pattern of Co3Ta, f TEM-EDS element distributions mapping

Fig. 4 TEM images and selected area diffraction patterns (SADP) of alloy Hf: a FIB cutting position, b FIB cutting completed sample, c TEM images of two phases, d TEM morphology and SADP pattern of β-CoAl phase, e SADP of the TEM morphology and SADP pattern of Laves phase, f TEM-EDS element distributions mapping

Fig. 5 DSC heating and cooling curves of alloy Base, alloy Nb and alloy Hf: a heating curves, b cooling curves

Table 3 Phase transformation temperatures measured by DSC (°C)

Alloys	Methods	T_L	T_S	MC	Eutectic (γ + γʹ)	M₃B₂	Co₃Ta	Laves	β	Solidification temperature range
Base	Heating	-	1353	-	1256	-	-	-	-	T_L-T_S = 30
	Cooling	1383	-	1351	-	1141	-	-	-
Nb	Heating	-	1332	-	1201	-	-	-	-	T_L-T_S = 36
	Cooling	1368	-	1337	1153	-	1110	-	-
Hf	Heating	-	1330	-	1125	-	-	1173	1138	T_L-T_S = 36
	Cooling	1366	-	1343	-	-	-	1147	-

Fig. 6 Solidification microstructures of alloys quenched at different temperatures by SEM(BSE): a-e alloy Base at 1400 °C, 1380 °C, 1360 °C, 1340 °C, 1300 °C, f-j alloy Nb at 1380 °C, 1360 °C, 1340 °C, 1320 °C, 1300 °C, k-o alloy Hf at 1380 °C, 1360 °C, 1340 °C, 1320 °C, 1300 °C

Fig. 7 Relationship between the isothermal temperature and the volume fraction of liquid during the solidification process: a alloy Base, b alloy Nb, c alloy Hf

Fig. 8 Solute partition coefficient of different elements at different isothermal temperatures: a alloy Base, b alloy Nb, c alloy Hf

Fig. 9 Solidification microstructures of alloys quenched at different temperatures: a-c alloy Base at 1250 °C, 1140 °C, 1130 °C, d-f alloy Nb at 1150 °C, 1140 °C, 1110 °C, g-i alloy Hf at 1150 °C, 1140 °C, 1130 °C

Fig. 10 Simulation results of three experimental alloys calculated by Thermo-calc: a, c, e equilibrium phases fraction corresponding to different temperatures, b, d, f solidification sequence calculated by Scheil model, a-b alloy Base, c-d alloy Nb, e-f alloy Hf

Table 4 Phase transformation temperature measured by DSC, isothermal solidification experiments, TC equilibrium calculation and Scheil model calculation (°C)

Methods	Alloy	T_L	MC	T_S	Laves	M₃B₂	μ	β	Eutectic (γ + γʹ)	Co₃Ta
DSC	Base	1383	1351	1353	-	1141	-	-	1256	-
	Nb	1368	1337	1332	-	-	1110	-	1177	-
	Hf	1366	1343	1330	1160	-	-	1138	1125	-
Isothermal solidification	Base	1380-1400	~ 1340	~ 1130	-	~ 1140	-	-	~ 1250	-
	Nb	1360-1380	~ 1340	~ 1110	-	-	~ 1110	-	~ 1150	~ 1140
	Hf	1360-1380	~ 1360	~ 1130	~ 1140	-	-	~ 1140	~ 1130	-
TC equilibrium	Base	1380	1340	1216	-	1220	-	1040	-	-
	Nb	1370	1360	1222	-	1223	-	1040	-	-
	Hf	1378	1354	1240	1448	1241	-	1040	-	-
Scheil model	Base	1387	1347	1090	1207	1128	1148	-	-	-
	Nb	1375	1368	1080	1220	1105	1105	-	-	-
	Hf	1476	1355	1080	1476	1142	1120	-	-	-

Fig. 11 Structure map for TCP phase prediction, with average valence band filling $\overline{N }$ and relative volume difference $\Delta V/\overline{V }$ (Eqs. 2, 3): a structure map analysis of intermetallic precipitates, b variation of residual liquid during solidification

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