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Acta Metall Sin  2019, Vol. 55 Issue (9): 1211-1220    DOI: 10.11900/0412.1961.2019.00121
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Stress Relaxation Mechanism for Typical Nickel-Based Superalloys Under Service Condition
JIANG He(),DONG Jianxin,ZHANG Maicang,YAO Zhihao,YANG Jing
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
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Since nickel-based superalloys are more and more used as fasteners, it is necessary to investigate the stress relaxation behavior and mechanism of nickel-based superalloy. In present work, the stress relaxation mechanism for four typical nickel-based superalloys (GH4169, GH4169D, GH4738, GH350) for fasteners under service condition was investigated. The stress relaxation tests were carried out according to GB/T 10120-2013 in the temperature range of 600~780 ℃ and initial stress range of 260~510 MPa, and the stress relaxation curves were recorded. The microstructure was studied by FESEM and TEM. The results show that the stress decreases fast in the initial stage of stress relaxation test and then trends to be steady. The stress relaxation stability decreases with increasing temperature. There is no apparent change in the microstructure after stress relaxation test. TEM observation shows that the major mechanism of stress relaxation is the movement of dislocations, and the stress relaxation properties of different alloys depend on the inhibition of dislocation movement. The species, size, shape and distribution of phases determine the ability to hinder dislocation movement and the stress relaxation property of different alloys. GH4169 alloy gets the stress relaxation property mainly by γ' phase, γ'' phase and δ phase hindering the movement of dislocations. In GH4169D alloy, both γ' phase and η phase participate in the stress relaxation process. γ' phase in GH4738 alloy can effectively impede the movement of dislocations and provide good stress relaxation property. The combined effect of γ' phase and η phase guarantees the stress relaxation stability of GH350 alloy.

Key words:  superalloy      stress relaxation      microstructure evolution      mechanism     
Received:  18 April 2019     
ZTFLH:  TG146.1  
Fund: Supported by National Natural Science Foundation of China(51771016)
Corresponding Authors:  He JIANG     E-mail:

Cite this article: 

JIANG He,DONG Jianxin,ZHANG Maicang,YAO Zhihao,YANG Jing. Stress Relaxation Mechanism for Typical Nickel-Based Superalloys Under Service Condition. Acta Metall Sin, 2019, 55(9): 1211-1220.

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Fig.1  Specimen for stress relaxation test (a) and typical stress relaxation curve (b) (σ—stress, σ0—initial stress, σ'0—threshold stress to divide first and second stage, σr—stress relaxation limit, t—time. In stage I, the stress decreases rapidly. The stress decreases more and more slowly to a stable value in stage II)
Table 1  Nominal chemical compositions of alloys used in present work (mass fraction / %)
Fig.2  Stress relaxation curves for different alloys (a) GH4169 and GH4169D (b) GH4738 and GH350
Fig.3  Typical microstructures and local magnifications (insets) of alloys before and after stress relaxation tests (b) GH4169 alloy after stress relaxation test at 650 ℃, 260 MPa (d) GH4738 alloy after stress relaxation test at 700 ℃, 510 MPa
Fig.4  TEM bright field images of GH4738 alloy after stress relaxation test(a, b) interaction between γ' particles and dislocations after stress relaxation test at 600 ℃, 510 MPa (a) and 700 ℃, 510 MPa (b)(c, d) stacking fault in γ' particles (c) and subgrain in matrix (d) after stress relaxation test at 700 ℃, 510 MPa
Fig.5  TEM bright field images of γ'' phase in GH4169 alloy after stress relaxation tests at 600 ℃, 380 MPa (a) and 650 ℃, 380 MPa (b)
Fig.6  TEM bright field images of GH4169D alloy after stress relaxation test at 700 ℃, 510 MPa(a) interaction between γ' particles and dislocation (b) dislocation pile-up around η phase
Fig.7  TEM bright field images of GH350 alloy after stress relaxation tests at 600 ℃, 510 MPa (a, b) and 700 ℃, 510 MPa (c, d)(a) strain contrast field around γ' particles (b) dislocation pile-up around grain boundary η phase(c) interface dislocation of intragranular η phase (d) deformation band in matrix
Fig.8  TEM bright field images of GH4169 alloy after stress relaxation test at 650 ℃ with different initial stresses(a) 260 MPa (b) 380 MPa (c, d) 510 MPa
Fig.9  Stress relaxation curve (a) and typical TEM bright field images showing a large amount of twins (b), grain boundary migration (c) and defects in δ phase (d) after stress relaxation test at 650 ℃, 1020 MPa
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