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Acta Metall Sin  2017, Vol. 53 Issue (6): 695-702    DOI: 10.11900/0412.1961.2016.00508
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Precipitation Behavior of δ Phase of Deformation Induced GH3625 Superalloy Hot-Extruded Tube
Yutian DING(),Yubi GAO,Zhengyi DOU,Xin GAO,Dexue LIU,Zhi JIA
State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals, Lanzhou University of Technology, Lanzhou 730050, China
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Abstract  

GH3625 alloy is a wrought nickel-based superalloy mainly used in aeronautical, aerospace, chemical, nuclear, petrochemical, and marine applications industry due to its good mechanical properties, processability, weldability and resistance to high-temperature corrosion on prolonged exposure to aggressive environments. However, in medium and high temperature environment during long-term service, the γ'' is a metastable phase, easily transformed into stable δ phase, or δ phase directly formed in the γ matrix so that alloy performance was deteriorated, leading to the result of alloy failure. At the present work, mass fraction of δ phase in GH3625 superalloy hot-extruded tube cold deformed to different reductions and then aged at 800 ℃ for different times, were measured by XRD. The effect of cold deformation on the law and kinetics of δ phase precipitation was investigated by SEM, EDS and Image-Pro Plus metallographic analysis. The results show that δ phase first precipitates at the deformation twin and grain boundaries as well as deformation bands, and then precipitates in the grains. The amount of δ phase at the deformation bands increases with the increase of cold deformation. The morphologies of δ phase change gradually from needles to spheroids or rodlike with increasing cold deformation. With the extend of ageing time, the average size of δ phase increases which grows according to LSW theory. At 800 ℃, the relationship between the precipitation content of δ phase and ageing time follows Avrami equation. As cold deformation increases, the content of δ phase increases, the time index n decreases, whereas the δ phase precipitation rate increases. Cold deformation promotes the precipitation of δ phase. The solute drags of Nb in soild solution and pinning of δ phase inhibits the grain growth during ageing process of cold deformed GH3625 superalloy hot-extruded tube. The hardness of the alloy increases with the extension of the holding time at ε =35% but no obvious change at ε ≥50%.

Key words:  GH3625 superalloy      cold deformation      δphase      precipitation behavior      kinetics     
Received:  14 November 2016     
Fund: Supported by National Natural Science Foundation of China (No.51661019) and Science and Technology Project of Gansu Province (No.145RTSA004)

Cite this article: 

Yutian DING,Yubi GAO,Zhengyi DOU,Xin GAO,Dexue LIU,Zhi JIA. Precipitation Behavior of δ Phase of Deformation Induced GH3625 Superalloy Hot-Extruded Tube. Acta Metall Sin, 2017, 53(6): 695-702.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2016.00508     OR     https://www.ams.org.cn/EN/Y2017/V53/I6/695

Fig.1  Time-temperature-transformation diagram of the phases in GH3625 superalloy[14,15]
Fig.2  XRD spectra of GH3625 superalloy hot-extruded tube under different cold reductions (ε) and ageing times (t) (a) ε=35% (b) ε=50% (c) ε=65%
ε / % Mass fraction / %
25 h 50 h 75 h 100 h
35 1.58 1.77 1.95 2.15
50 1.73 1.89 2.12 2.24
65 1.88 2.07 2.28 2.34
Table 1  Mass fraction of δ phase in GH3625 superalloy tubes under different cold reductions and ageing times
Fig.3  SEM images (a~c) and EDS scaned along the line shown in Fig.3a (d) of δ phase in cold deformed GH3625 superalloy hot-extruded tube ageing at 800 ℃ for 75 h under ε =35 % (a), ε =50% (b) and ε =65% (c)
Fig.4  SEM images of δ phase in cold deformed GH3625 superalloy hot-extruded tube (ε=65%) ageing at 800℃ for 25 h (a), 50 h (b), 75 h (c) and 100 h (d)
Holding time / h l?/ μm w? / μm
25 1.462 0.204
50 1.854 0.260
75 2.205 0.326
100 2.536 0.377
Table 2  Average sizes of δ phase ageing for different holding times at 800 ℃ (ε=65%)
Fig.5  Relationship between average sizes of δ phase and the ageing holding times at 800 ℃
Fig.6  Relationship between δ phase content (Wδ) and ageing time of cold deformed GH3625 superalloy hot-extruded tube at 800 ℃
Fig.7  Relationship between lg[-ln(1-Wδ/Ws)] and lgt (Wsδ phase content at equilibrium state)
ε / % α / s-n n
35 1.144×10-2 0.364
50 1.693×10-2 0.342
65 2.463×10-2 0.322
Table 3  Parameters for precipitation kinetics of δ phase
Fig.8  Effect of cold deformation and ageing time on grain size of GH3625 superalloy hot-extruded tube
Fig.9  Effect of cold deformation and ageing time on hardness of GH3625 superalloy hot-extruded tube
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