Rejuvenation Heat Treatment of the Second-Generation Single-Crystal Superalloy DD6

doi:10.1007/s40195-015-0323-8

Abstract

Abstract:

The second-generation single-crystal superalloy DD6 with [001] orientation was prepared by screw selecting method in the directionally solidified furnace. The long-term aging of the alloy after full heat treatment was performed at 1100 °C for 400 h. Then the rejuvenation heat treatment 1300 °C/4 h/AC + 1120 °C/4 h/AC + 870 °C/24 h/AC was carried out. The stress rupture properties were investigated at 760 °C/800 MPa, 850 °C/550 MPa, 980 °C/250 MPa and 1100 °C/140 MPa after different heat treatments. The microstructures of the alloy at different conditions were studied by SEM. The results show that γ′ phase of the alloy became very irregular and larger after long-term aging at 1100 °C for 400 h. A very small amount of needle-shaped TCP phase precipitated in the dendrite core. The coarsened γ′ phase and TCP phase dissolved entirely after rejuvenation heat treatment. The microstructure was restored and almost same with the original microstructure. The stress rupture life of the alloy decreased in different degrees at various test conditions after long-term aging. The stress rupture life of the alloy after rejuvenation heat treatment all restores to the original specimen more than 80% at different conditions. The microstructure degradation of the alloy during long-term aging includes coarsening of the γ′ phase, P-type raft and precipitation of TCP phase, which results in the degeneration of stress rupture property. The rejuvenation heat treatment succeeds in restoring the original microstructure and stress rupture properties of the alloy.

Key words: Single-crystal superalloy, DD6, Rejuvenation heat treatment, Stress rupture properties

Zhen-Xue Shi, Shi-Zhong Liu, Jia-Rong Li. Rejuvenation Heat Treatment of the Second-Generation Single-Crystal Superalloy DD6[J]. Acta Metallurgica Sinica (English Letters), 2015, 28(10): 1278-1285.

Figures/Tables 10

Table 1 Nominal chemical compositions of DD6 alloy (wt%)

Cr	Co	Mo	W	Ta	Re	Nb	Al	Hf	C	Ni
4.3	9	2	8	7.5	2	0.5	5.6	0.1	0.006	Bal.

Fig. 1 Microstructure of DD6 single-crystal superalloy after standard heat treatment

Fig. 2 Microstructures of the alloy after long-term aging at 1100 °C for 400 h: a γ′ phase; b TCP phase

Table 2 Chemical composition of the TCP phase in the alloy after long-term aging at 1100 °C for 400 h (wt%)

Al	Cr	Co	Mo	Re	Ta	W	Ni
2.3	4.1	7.6	2.8	13.2	6.2	11.1	Bal.

Fig. 3 Microstructures of the alloy after rejuvenation heat treatment: a solution heat treatment; b solution and aging heat treatment

Table 3 Stress rupture properties of DD6 alloy after different heat treatments under different test conditions

Test condition	Standard heat treatment		Long-term aging		Rejuvenation heat treatment
Test condition	t (h)	δ (%)	t (h)	δ (%)	t (h)	δ (%)
760 °C/800 MPa	208.4	28.3	52.5	36.6	174.5	32.1
850 °C/550 MPa	603.5	26.7	133.5	22.5	662.8	26.9
980 °C/250 MPa	254.5	36.6	145.1	46.7	250.9	36.4
1100 °C/140 MPa	155.2	8.1	70.9	42.6	124.3	21.2

Fig. 4 Microstructures of the stress-ruptured specimens after standard heat treatment: a 760 °C/800 MPa; b 850 °C/550 MPa; c 980 °C/250 MPa; d 1100 °C/140 MPa

Fig. 5 Microstructures of the stress-ruptured specimens after long-term aging at 1100 °C for 400 h: a-c 760 °C/800 MPa; d-f 850 °C/550 MPa; g-i 980 °C/250 MPa; j-l 1100 °C/140 MPa

Fig. 6 Microstructures of the stress-ruptured specimens after rejuvenation heat treatment: a 760 °C/800 MPa; b 850 °C/550 MPa; c 980 °C/250 MPa; d 1100 °C/140 MPa

Fig. 7 Calculated dependences of the precipitation phases of DD6 alloy on temperature

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