Shi -Changxu Innovation Center for Advanced Materials, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
Cite this article:
ZHANG Shaohua, XIE Guang, DONG Jiasheng, LOU Langhong. Investigation on Eutectic Dissolution Behavior of Single Crystal Superalloy by Differential Scanning Calorimetry. Acta Metall Sin, 2021, 57(12): 1559-1566.
Ni-based single crystal (SX) superalloys are used for the production of blades in gas turbines and aircraft engines because of their superior mechanical performance at high temperatures. To improve the temperature capabilities of modern SX superalloys, specific refractory elements are added to the alloys. This leads to micro-segregation in alloys, requiring a complex heat treatment process to eliminate γ/γ′ eutectic. Therefore, the dissolution process of γ/γ′ eutectic must be understood. In this study, a second-generation Ni-based SX superalloy was used to investigate the effect of extended holding time at 1290oC and 1300oC on the γ′ phase dissolving temperature (Tγ′) and γ/γ′ eutectic phase-melting temperature (Tγ/γ′), respectively. The method involves measuring the differential heating curves of as-cast and as heat-treated samples using DSC. The results showed that Tγ′ and Tγ/γ′ increased at a holding time of 2 h. However, with an increase in the holding time, the temperature increase was not obvious. The volume fraction of γ/γ′ eutectic decreased with the extended holding at 1300oC, while the volume fraction of γ/γ′ eutectic increased after holding at 1290oC for 8 h. This abnormal phenomenon was confirmed by the metallographic experiments. The analyses showed that the increase in the eutectic volume fraction was due to the incomplete dissolution of coarse γ′ phase at the inter-dendritic region, which resulted in the diffusion of Ta element from dendrite core to the inter-dendritic region, promoting eutectic growth.
Fig.1 The temperature program for eutectic dissolution kinetics experiment
Fig.2 OM (a, c) and SEM (b, d) images of the as-cast (a, b) and heat-treatment (1310oC, 6 h) (c, d) DD414 single crystal superalloy
Position
Al
Cr
Mo
Co
Ta
W
Re
Ni
Nominal composition
5.5
5
2
10
9
6
3
Bal.
Dendritic core
5.12
5.02
1.66
11.40
4.94
7.69
3.40
Bal.
Fine γ/γ′ eutectic
6.52
3.87
1.61
9.26
11.08
3.53
1.35
Bal.
Coarse γ/γ′ eutectic
7.40
2.05
0.86
7.66
14.54
2.28
0.32
Bal.
Table 1 Chemical compositions of dendritic core, fine γ/γ′ eutectic, and coarse γ/γ′ eutectic of as-cast alloy tested by EPMA
Fig.3 Morphologies (a-c) and distributions (eutectic colored by image software) (d-f) of the eutectics in DD414 single crystal superalloy after solution heat treatment at 1290℃ for 2 h (a, d), 6 h (b, e), and 10 h (c, f)
Fig.4 Morphologies of the eutectics in DD414 single crystal superalloy after solution heat treatment at 1300℃ for 2 h (a), 4 h (b), 6 h (c), and 10 h (d)
Fig.5 DSC curves of DD414 single crystal superalloy heated at 1290oC (a) and 1300oC (b) for different time
Fig.6 The evolutions of γ′ phase dissolving temperature (Tγ′), the start and end melting temperatures of γ/γ′ eutectic phase (Tγ/γ′, TEnd, γ/γ′) with holding time when DD414 single crystal superalloy heated at 1290oC (a) and 1300oC (b)
Fig.7 The relationship between volume fraction of eutectic and holding time at 1290oC (a) and 1300oC (b) in DD414 single crystal superalloy
Fig.8 ln[-ln(1 - f(t))]-lnt curves of DD414 single crystal superalloy during isothermal holding process
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