Influences of Cooling Rate After Solution Treatment on Microstructural Evolution and Mechanical Properties of Superalloy Rene 80

doi:10.1007/s40195-016-0500-4

Abstract

Abstract:

Influences of the cooling rate after solution treatment on microstructural evolution and mechanical properties of Rene 80 nickel-based superalloy were investigated by optical microscopy, scanning electron microscopy, transmission electron microscopy and mechanical test. Results showed that the high cooling rate decreased the size of secondary γ′ particles in the supersaturated matrix, but led to a high coarsening rate of γ′ particles during subsequent aging treatment. Despite various coarsening rates, the size and morphology of γ′ particles in the final microstructures of all samples were close due to the long enough holding time to an equilibrium state. During the aging of 870 °C/2 h, primary MC started to decompose with the carbide reaction: MC + γ → M₆C or M₂₃C₆ + γ′. And a number of observations showed that the coarsening of γ′ particles on grain boundaries resulted in the depletion of γ′ during aging treatment. The test results indicated that high cooling rate resulted in the presence of quench crack, and the air cooling method following solution treatment was an optimum heat treatment method for Rene 80.

Key words: Ni-based superalloy, Cooling rate, γ′, Precipitates Ripening theory, Heat treatment, Microstructural evolution

Shuang Gao,Jie-Shan Hou,Kai-Xin Dong,Lan-Zhang Zhou. Influences of Cooling Rate After Solution Treatment on Microstructural Evolution and Mechanical Properties of Superalloy Rene 80[J]. Acta Metallurgica Sinica (English Letters), 2017, 30(3): 261-271.

Figures/Tables 14

Table 1 Chemical composition of the alloy (wt%)

C	Cr	Co	W	Mo	Al	Ti	B	Zr	Ni
0.15	13.98	9.62	4.17	4.02	2.87	4.8	0.013	0.061	Bal.

Table 2 Heat treatment schedule

Sample code	Solution	Aging
WC	1200 °C/2 h/WC	1095 °C/4 h/AC + 1080 °C/4 h/AC + 870 °C/12 h/AC
AC	1200 °C/2 h/AC
FC	1200 °C/2 h/FC

Fig. 1 Microstructures of the as-cast experimental alloy and EDS analysis: a OM image showing the dendritic microstructure; b SEM micrograph showing MC carbides morphology in as-cast microstructure; c morphology of γ′ precipitates; d SEM micrograph showing γ-γ′ eutectic in detail; e EDS spectrum of Ti-rich MC carbide

Fig. 2 Effects of the cooling methods after solution treatment on the microstructures: a-cmicrostructures of WC, AC and FC samples, respectively, d-f corresponding γ′ image of WC, AC and FC samples, respectively

Fig. 3 SEM micrographs of γ′ precipitates in three samples at different aging heat treatment stages: a-c at 1095 °C/4 h/AC; d-f at 1080 °C/4 h/AC; g-i at 870 °C/12 h/AC. Insetsin upper right show hyperfine tertiary γ′ particles in the matrix as marked by thearrows

Table 3 Statistical size and area fraction of secondary γ′ precipitates at different aging heat treatment stages

Heat treatment stage	WC		AC		FC
Heat treatment stage	d (nm)	Area fraction (%)	d (nm)	Area fraction (%)	d (nm)	Area fraction (%)
1	295 ± 24	15.7	290 ± 34	16.5	316 ± 54	14.7
2	345 ± 46	19.8	355 ± 51	17.8	372 ± 35	15.9
3	391 ± 36	24.7	402 ± 21	29.2	415 ± 24	26.5

Fig. 4 SEM microstructures of the carbides and grain boundaries (marked by dotted lines) of AC sample in different heat treatment stages of a 1095 °C/4 h/AC, b 1080 °C/4 h/AC, cBS-SEM microstructure after aging of 870 °C/12 h/AC

Table 4 EDS analysis of different carbides

	MC		M₆C		M₂₃C₆
	wt%	at.%	wt%	at.%	wt%	at.%
Al	0.7	1.7	1.9	6.2	0.9	2.1
Mo	26.0	19.3	44.5	39.9	9.9	6.3
Ti	42.7	63.5	4.2	7.5	2.5	3.2
Cr	1.6	2.2	14.0	23.4	51.9	60.4
Co	0.3	0.3	1.0	1.4	2.8	2.8
Ni	2.2	2.7	5.7	8.3	21.1	21.7
W	26.5	10.2	28.8	13.5	10.9	3.6

Fig. 5 TEM microstructures and SAED patterns of the carbides on grain boundaries of AC sample at 870 °C/12 h/AC aging: aM6C topography, M6C SAED pattern (inset in upper right corner) and MC SAED pattern (inset in lower left corner); bM23C6 topography andM23C6 SAED pattern (inset)

Fig. 6 SEM images: aγ′ coarsening on grain boundary and γ′ depleting on both sides of grain boundary after 870 °C/12 h/AC, b corresponding higher magnification image

Fig. 7 Growth rate (K) of γ′ particles of three samples during the aging treatment

Fig. 8 Schematic model of solute atom diffusing with different supersaturation degrees: a high supersaturation degree, b low supersaturation degree

Table 5 Results of mechanical tests for three kinds of samples

Sample code	Tensile test at 25 °C				Tensile test at 870 °C				Hardness (HRC)
Sample code	YS (MPa)	UTS (MPa)	El (%)	RA (%)	YS (MPa)	UTS (MPa)	El (%)	RA (%)	Hardness (HRC)
WC	-	520.5	-	-	-	337.5	-	-	41.39
AC	938	1108	6.1	8.2	585	795	18.7	29	41.49
FC	937.5	1092	6.3	8	545	692.5	21	32.25	41.51

Fig. 9 Intergranular cracks as marked by the arrows in SEM micrographs of fractured surface:a longitudinal section, b WC sample at 25 °C

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