Effect of cooling rate on microstructure, microsegregation and mechanical properties of cast Ni-based superalloy K417G

doi:10.1016/j.jmst.2017.03.023

Abstract

Abstract:

The effects of different solidification rates after pouring on the microstructures, microsegregation and mechanical properties of cast superalloy K417G were investigated. Scheil-model was applied to calculate the temperature range of solidification. The casting mould with different casting runners was designed to obtain three different cooling rates. The microstructures were observed and the microsegregation was investigated. Also, high temperature tensile test was performed at 900 °C and stress rupture test was performed at 950 °C with the stress of 235 MPa. The results showed that the secondary dendrite arm spacing, microsegregation, the size and volume fraction of γ′ phase and the size of γ/γ′ eutectic increased with decreasing cooling rate, but the volume fraction of γ/γ′ eutectic decreased. In the cooling rate range of 1.42 °C s^-1-0.84 °C s^-1, the cast micro-porosities and carbides varied little, while the volume fraction and size of γ′ phase and γ/γ′ eutectic played a decisive role on mechanical properties. The specimen with the slowest cooling rate of 0.84 °C s^-1 showed the best comprehensive mechanical properties.

Key words: Ni-based cast superalloy, Cooling rate, Microstructure, Microsegregation, Mechanical property

Li Gong, Bo Chen, Long Zhang, Yingche Ma, Kui Liu. Effect of cooling rate on microstructure, microsegregation and mechanical properties of cast Ni-based superalloy K417G[J]. J. Mater. Sci. Technol., 2018, 34(5): 811-820.

Figures/Tables 19

Table 1 Chemical compositions of K417G superalloy (wt%).

Ni	C	Cr	Co	Mo	Al	Ti	V	Zr	B
Bal.	0.166	9.37	9.65	3.08	5.21	4.56	0.71	0.086	0.023

Fig. 1. Shell structure of casting mould designed for different cooling rates after pouring: (a) stereogram; (b) profile of casting mould.

Fig. 2. Simulation results of K417G superalloy calculated by Scheil model.

Fig. 3. Temperature vs. time curves during casting experiment for K417G superalloy.

Table 2 Element segregation analysis of different cooling rates (v) between dendrite core and interdendritic regions.

Sample	v (°C s^-1)		V	Co	Al	Cr	Mo	Ti
A	1.42	C_d (%)	0.78	10.75	5.28	9.39	2.46	3.89
		C_i (%)	0.80	10.03	5.36	9.54	2.52	4.46
		k	0.98	1.07	0.98	0.98	0.98	0.87
B	1.06	C_d (%)	0.83	10.90	5.31	9.60	2.51	3.45
		C_i (%)	0.81	10.19	5.30	9.96	2.63	4.36
		k	1.02	1.07	1.00	0.96	0.95	0.79
C	0.84	C_d (%)	0.80	11.09	5.26	9.87	2.34	3.41
		C_i (%)	0.78	10.49	5.16	10.47	2.60	4.51
		k	1.02	1.06	1.02	0.94	0.90	0.76

Fig. 4. Microstructure at gauge position of high temperature tensile specimens with different cooling rates: (a, b) specimen A, 1.42 °C s-1; (c, d) specimen B, 1.06 °C s-1, (e, f) specimen C, 0.84 °C s-1.

Fig. 5. OM images showing secondary dendrite arm spacing (λ) for specimen A (a), specimen B (b) and specimen C (c) with cooling rates of 1.42 °C s-1, 1.06 °C s-1 and 0.84 °C s-1, respectively.

Table 3 Microstructure features at gauge position of specimens under different cooling rates.

Sample	v (°C s^-1)	λ (μm)	Size of γ′ phase (nm)		Volume fraction of γ′ phase (%)		Size of γ/γ′ eutectic (μm)	Volume fraction of γ/γ′ eutectic (%)	Volume fraction of cast porosity (%)
			dendrite	interdendrite	dendrite	interdendrite
A	1.42	44.3 ± 0.3	521 ± 16	550 ± 11	63.3 ± 0.4	64.0 ± 0.8	31.7 ± 2.2	6.0 ± 1.3	0.24 ± 0.1
B	1.06	48.7 ± 0.3	569 ± 11	601 ± 15	63.9 ± 0.8	64.7 ± 0.6	32.2 ± 2.3	5.6 ± 1.3	0.25 ± 0.1
C	0.84	54.3 ± 0.8	612 ± 19	640 ± 9	64.7 ± 0.5	65.5 ± 0.6	41.3 ± 4.9	4.7 ± 1.2	0.25 ± 0.1

Fig. 6. Morphologies of γ′ phases in dendrite (a, c, e) and interdendrite (b, d, f) for specimen A (a, b), specimen B (c, d) and specimen C (e, f) with cooling rates of 1.42 °C s-1, 1.06 °C s-1 and 0.84 °C s-1, respectively.

Fig. 7. Morphologies and distribution of γ/γ′ eutectic specimen A (a, b), specimen B (c, d) and specimen C (e, f) at low (a, c, e) and high (b, d, f) magnification with cooling rates of 1.42 °C s-1, 1.06 °C s-1 and 0.84 °C s-1, respectively.

Fig. 8. Morphologies and distribution of the carbides for specimen A (a), specimen B (b) and specimen C (c) with cooling rates of 1.42 °C s-1, 1.06 °C s-1 and 0.84 °C s-1, respectively.

Fig. 9. SEM images of carbides (a) and EDS analysis of different areas (b, c).

Fig. 10. TEM image and SAED pattern of MC carbide in specimen C with cooling rate of 0.84 °C s-1.

Fig. 11. SEM image showing morphology of carbides in specimen C with cooling rate of 0.84 °C s-1 (a) and corresponding EDS analysis (b).

Fig. 12. TEM image and SAED pattern of M23C6 carbide in specimen C with cooling rate of 0.84 °C s-1.

Table 4 Microstructure features at screw position of specimens under different cooling rates.

Sample	v (°C s^-1)	λ (μm)	Size of γ′ phase (nm)		Volume fraction of γ′ phase (%)		Size of γ/γ′ eutectic (μm)	Volume fraction of γ/γ′ eutectic (%)	Volume fraction of cast porosity (%)
			dendrite	interdendrite	dendrite	interdendrite
A	1.42	47.1 ± 0.5	565 ± 12	589 ± 17	64.3 ± 0.4	64.9 ± 0.6	33.2 ± 2.4	4.9 ± 1.2	0.21 ± 0.1
B	1.06	50.9 ± 0.7	594 ± 15	622 ± 10	64.9 ± 0.7	65.7 ± 0.7	34.7 ± 2.8	4.8 ± 0.9	0.24 ± 0.2
C	0.84	54.7 ± 0.3	627 ± 10	653 ± 13	65.9 ± 0.6	66.2 ± 0.9	44.4 ± 3.7	4.7 ± 0.5	0.24 ± 0.1

Table 5 High temperature tensile properties and stress rupture properties of K417G superalloy with various cooling rates.

Sample	v (°C s^-1)	Ultimate tensile strength (MPa)	Elongation (%)	Reduction of area (%)	Rupture life (h)
A	1.42	761.0	7.4	8.8	67.5
B	1.06	735.5	8.2	8.9	68.0
C	0.84	714.0	11.3	15.8	70.5

Fig. 13. SEM images of fracture surfaces after tensile test at 900 °C for specimen A (a, b), specimen B (c, d) and specimen C (e, f) at low (a, c, e) and high (b, d, f) magnification with cooling rates of 1.42 °C s-1, 1.06 °C s-1 and 0.84 °C s-1, respectively.

Fig. 14. OM images of fracture surfaces after tensile tests at 900 °C for specimen A (a), specimen B (b) and specimen C (c) with cooling rates of 1.42 °C s-1, 1.06 °C s-1 and 0.84 °C s-1, respectively.

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