1 University of Science and Technology of China, Hefei 230022 2 Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016
Cite this article:
SUN Wen, QIN Xuezhi, GUO Yongan, GUO Jianting, LOU Langhong, ZHOU Lanzhang. EFFECTS OF Nb/Ti RATIOS ON THE MICROSTRUCTURAL EVOLUTIONS OF CAST Ni-BASED SUPERALLOYS DURING LONG-TERM THERMAL EXPOSURE. Acta Metall Sin, 2014, 50(6): 744-752.
Effects of Nb/Ti ratios on the microstructural evolutions of cast Ni-based suerpalloys during long-term thermal exposure are investigated by OM, SEM and TEM. The results show that Nb/Ti ratios have no influence on the evolution of γ? morphology and size during long-term thermal exposure. However, with decrease of Nb/Ti ratios in alloys, the volume fraction of γ? phase increases. Both parameters Nb/Ti and (Nb+Ti)/(W+Mo) of primary MC have a good linear relationship with Nb/Ti ratios in alloys. With decrease of Nb/Ti ratios in alloys, both parameters for primary MC linearly decrease and sequentially thermal stability of primary MC is weakened. However, the results also show that Nb/Ti and (Nb+Ti)/(W+Mo) ratios of primary MC are not the principle factors determining the thermal stability of primary MC. The degeneration degree of primary MC can be calculated by the volume fraction of primary MC before and after degeneration, while the thermal stability of primary MC can be quantitatively characterized by degeneration degree of primary MC. Furthermore, with decreased Nb/Ti ratios in alloys, the grain boundaries coarsen more severely during long-term thermal exposure. Meanwhile, precipitation tendency of M23C6 carbide on grain boundaries increases and that of M6C carbide on grain boundaries decreases. However, the precipitation and evolution of μ phase during long-term thermal exposure is not affected by Nb/Ti ratios obviously.
Fig.1 Typical microstructures of sample A0 after standard heat treatment (1110 ℃, 4.5 h, A.C.+750 ℃, 10.5 h, A.C.)
(a) OM image (b) SEM image of γ' phase (c) SEM image of grain boundary
(d) TEM image of grain boundary (Insets show SAED of M6C and M23C6)
Fig.2 SEM images of γ? phase in samples A0 (a, d, g), A2 (b, e, h) and A4 (c, f, i) after aging at 850 ℃ for 1.0×103 h (a~c), 6.0×103 h (d~f) and 10.0×103 h (g~i)
Fig.3 Size (a) and volume fraction (b) changes of γ? phase with exposure time
Fig.4 Dependence of Nb/Ti and (Ti+Nb)/(W+Mo) ratios of MC on Nb/Ti ratios in alloys
Sample
Nb
Ti
Mo
W
Cr
Ni
Nb/Ti
Nb+Ti
(Nb+Ti)/(W+Mo)
A0
41.0
29.3
9.0
16.0
1.6
3.1
1.4
70.3
2.8
A2
23.9
37.6
11.1
23.5
1.3
2.6
0.6
61.5
1.8
A4
4.7
43.0
13.6
31.1
2.6
5.0
0.1
47.7
1.1
Table 2 Compositions of primary MC after heat treatment
Fig.5 SEM images of MC degeneration in samples A0 (a~c), A2 (d~f) and A4 (g~i) alloy after aging at 850 ℃ for 0.5×103 h (a, d, g), 6.0×103 h (b, e, h) and 10.0×103 h (c, f, i)
Fig.6 Degeneration degree (D) of primary MC changes with aging time (t)
Fig.7 SEM images of grain boundaries in samples A0 (a, d), A2 (b, e) and A4 (c, f) alloys after aging at 850 ℃ for 0.5 ×103 h (a~c) and 10.0×103 h (d~f), respectively
Fig.8 SEM images of μ phase in sample A4 after aging at 850 ℃ for 0.5×103 h (a), 3.0×103 h (b), 6.0×103 h (c) and 10.0×103 h (d)
Fig.9 Change of volume fraction of μ phase with exposure time
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FANG Hongsheng; BO Xiangzheng; WANG Jiajun; XU Ning(Deperment of Materials Science & Engineering; Tsinghua University; Beijing 100084)Correspondent : BO Xiangzheag Fas:(010)62771160; Tel: (010)62782361;E-mail: fhsdms tsinghua edu .cn. THREE DIMENSIONAL MORPHOLOGY AND MICROSTRUCTURAL EVOLUTION OF BAINITE[J]. 金属学报, 1998, 34(6): 579-585.
