Effect of Heat Treatment Parameters on Microstructure and Hot Workability of As-Cast Fine Grain Ingot of GH4720Li Alloy
WANG Tao,WAN Zhipeng(),LI Zhao,LI Peihuan,LI Xinxu,WEI Kang,ZHANG Yong
Science and Technology on Advanced High Temperature Structural Materials Laboratory, AECC Beijing Institute of Aeronautical Materials, Beijing 100095, China
Cite this article:
WANG Tao,WAN Zhipeng,LI Zhao,LI Peihuan,LI Xinxu,WEI Kang,ZHANG Yong. Effect of Heat Treatment Parameters on Microstructure and Hot Workability of As-Cast Fine Grain Ingot of GH4720Li Alloy. Acta Metall Sin, 2020, 56(2): 182-192.
GH4720Li was Ni-Cr-Co base precipitation strengthened superalloy and widely used for high performance applications such as disks and blades of either aircraft engines or land-based gas turbines attributing to its excellent properties including resistance to creep and fatigue, together with corrosion, fracture and microstructural stability for the intended applications. Compared with the double-melting process (vacuum induction melting (VIM)+electroslag remelting (ESR) or VIM+vacuum arc remelting (VAR), a triple-melting process (VIM+ESR+VAR) can eliminate the segregation coefficient of the alloying elements and reduce the content of impurity elements, while the ingot fabricated by the triple-melting process also exhibited lots of shortcomings, such as the coarse grains, dendritic structure, microstructure defects and high forging temperature. The as-cast fine grain ingot prepared by grain refining casting process can eliminate the microscopic shrinkage, reduce the differences among three crystalline regions and improve the hot workability as a result. However, it was hardly to avoid the microstructure defects by simply improving the casting process attributing to its large number of alloying elements. Therefore, the homogenization treatment was always performed on the superalloy ingot. In this work, the optimized homogenization parameter was identified according to the investigation on the microstructure evolution under various homogenization treatment conditions and hot workability of as-cast fine grain ingot after homogenization treatment. The relationships of one-stage as well as two-stage homogenization treatment parameters and segregation coefficient as well as volume fraction of eutectic phase were investigated indepth. The hot workability of the homogenized samples under various conditions was also analyzed with the help of hot compression tests. Experimental results revealed that the increased homogenization treatment temperature and extended holding time were able to decrease the volume fraction of eutectic phase and segregation coefficient of the alloying element significantly. Hot compression tests by the Gleeble 3800 dynamic thermal-mechanical testing machine indicated that the samples suffered two-stage homogenization treatment followed by the slowly cooling rate (1140 ℃, 12 h+1170 ℃, 10 h, 0.2 ℃/min furnace cooling to 1010 ℃, and then air cooling) exhibited better hot workability (the maximum reduction rate of 50% deformed at 1120 ℃, 1 s-1). Discontinuous dynamic recrystallization was identified as the mainly nucleation mechanism of the alloy, and the recrystallized grains preferred to nucleate at the boundaries of the original grains according to the microstructure observation of hot compressed samples. In additions, the M(C, N) type precipitates were able to promote the occurrence of dynamic recrystallization behavior. Homogenization treatment experiments and microstructure observation suggested that the optimized treatment parameters of the as-cast fine grain ingot was 1140 ℃, 12 h+1170 ℃, 10 h, 0.2 ℃/min furnace cooling to 1010 ℃, and then by air cooling.
1160 ℃, 14 h, 0.2 ℃·min-1 FC to 1010 ℃, and then AC
Table 1 One-stage homogenization heat treatment (OSHT) schedules for the GH4720Li alloy
No.
Two stage heat treatment
TSHT 1
1130 ℃, 8 h＋1160 ℃, 4 h, WC
TSHT 2
1130 ℃, 16 h＋1160 ℃, 8 h, WC
TSHT 3
1160 ℃, 8 h+1180 ℃, 2 h, WC
TSHT 4
1140 ℃, 12 h+1170 ℃, 10 h, AC
TSHT 5
1140 ℃, 12 h+1170 ℃, 10 h, 0.2 ℃·min-1 FC to 1010 ℃, and then AC
Table 2 Two-stage homogenization heat treatment (TSHT) schedules for the GH4720Li alloy
Fig.1 Macrostructures of as-cast fine grain ingot of GH4720Li alloy(a) overall profile of ingot (b) horizontal direction (c) vertical direction
Fig.2 OM (a) and SEM (b) images of as-cast fine grain ingot of GH4720Li alloy at R/2 region (Insets show the morphology of γ' precipitates in the interdendritic and dendrite regions, R—radius of the as-cast fine grain ingot)
Element
Mass fraction / %
Segregation coefficient (K)
Interdendritic
Dendrite core
Al
2.47
2.18
1.133
Ti
12.13
4.67
2.597
Cr
8.40
16.84
-2.005
Co
11.59
14.95
-1.290
Ni
63.13
56.43
1.119
Mo
1.83
3.06
-1.672
W
0.46
1.86
-4.043
Table 3 Element segregation analyses of initial as-cast fine grain ingot of GH4720Li alloy at R/2 between dendrite core and interdendritic regions
Fig.3 Microstructures of as-cast fine grain ingot of GH4720Li alloy under heat treatment temperatures of 1130 ℃ (a), 1140 ℃ (b), 1150 ℃ (c), 1160 ℃ (d), 1170 ℃ (e), 1180 ℃ (f), 1200 ℃ (g) and 1220 ℃ (h) for 1 h (Insets show the morphologies of incipient melting phase)
Fig.4 Volume fraction of eutectic phase of GH4720Li alloy under different heat treatment temperatures for 1 h
Fig.5 Microstructures of as-cast fine grain ingot of GH4720Li alloy under one-stage homogenization heat treatment parameters (Insets show the morphologies of eutectic γ+γ' phase and γ' precipitate)(a) OSHT 1 (b) OSHT 2 (c) OSHT 3 (d) OSHT 4 (e) OSHT 5 (f) OSHT 6 (g) OSHT 7
Fig.6 Volume fraction of eutectic phase during various one stage homogenization treatment parameters of as-cast fine grain ingot of GH4720Li alloy
Fig.7 Elemental segregation coefficient of as-cast fine grain ingot of GH4720Li alloy under different homogenization heat treatment parameters
Fig.8 Microstructures of as-cast fine grain ingot of GH4720Li alloy under different two-stage homogenization heat treatment parameters (Insets show the morphologies of eutectic γ+γ' phase and γ' precipitate)(a) TSHT 1 (b) TSHT 2 (c) TSHT 3 (d) TSHT 4 (e) TSHT 5
Fig.9 The volume fraction of eutectic phase of as-cast fine grain ingot of GH4720Li alloy under different two-stage homogenization treatment parameters
No.
1120 ℃
1140 ℃
1160 ℃
0.01 s-1
0.1 s-1
1 s-1
0.01 s-1
0.1 s-1
1 s-1
0.01 s-1
0.1 s-1
1 s-1
ACFGI
30
30
20
30
30
20
30
30
20
OSHT 6
30
30
30
50
40
40
50
40
40
TSHT 4
40
40
40
50
50
40
50
50
40
TSHT 5
40
40
50
50
50
60
50
50
60
Table 4 Critical reduction for crack initiation of homogenized as-cast fine grain ingot of GH4720Li alloy samples during hot compression tests (%)
Fig.10 True stress-strain curves of as-cast fine grain ingot of GH4720Li alloy under homogenization treatment parameter of TSHT 5 at 1140 ℃ (a) and 0.01 s-1 (b)
Fig.11 Microstructures of as-cast fine grain ingot of GH4720Li alloy under homogenization treatment parameter of TSHT 5 (Inset shows the morphology of locally magnified microstructure, T—temperature, —deformation rate, ε—strain, DRX—dynamic recrystallization, CDRX—continuous dynamic recrystallization, DDRX—discontinuous dynamic recrystallization)(a) T=1160 ℃, =1 s-1, ε=0.3 (b) T=1160 ℃, =1 s-1, ε=0.8(c) T=1160 ℃, =0.01 s-1, ε=0.8 (d) T=1140 ℃, =0.01 s-1, ε=0.8
[1]
Gopinath K, Gogia A K, Kamat S V, et al. Dynamic strain ageing in Ni-base superalloy 720Li [J]. Acta Mater., 2009, 57: 1243
[2]
Pang H T, Reed P A S. Microstructure effects on high temperature fatigue crack initiation and short crack growth in turbine disc nickel-base superalloy Udimet 720Li [J]. Mater. Sci. Eng., 2007, A448: 67
[3]
Wang T, Wan Z P, Sun Y, et al. Dynamic softening behavior and microstructure evolution of nickel base superalloy [J]. Acta Metall. Sin., 2018, 54: 83
Gopinath K, Gogia A K, Kamat S V, et al. Low cycle fatigue behaviour of a low interstitial Ni-base superalloy [J]. Acta Mater., 2009, 57: 3450
[5]
Na Y S, Park N K, Reed R C. Sigma morphology and precipitation mechanism in Udimet 720Li [J]. Scr. Mater., 2000, 43: 585
[6]
Jackson M P, Reed R C. Heat treatment of UDIMET 720Li: The effect of microstructure on properties [J]. Mater. Sci. Eng., 1999, A259: 85
[7]
Yu Q Y, Yao Z H, Dong J X. Deformation and recrystallization behavior of a coarse-grain, nickel-base superalloy Udimet720Li ingot material [J]. Mater. Charact., 2015, 107: 398
[8]
Wan Z P, Wang T, Sun Y, et al. Dynamic softening mechanisms of GH4720Li alloy during hot deformation [J]. Acta Metall. Sin., 2019, 55: 213
Semiatin S L, Kramb R C, Turner R E, et al. Analysis of the homogenization of a nickel-base superalloy [J]. Scr. Mater., 2004, 51: 491
[10]
Miao Z J, Shan A D, Wu Y B, et al. Quantitative analysis of homogenization treatment of INCONEL718 superalloy [J]. Trans. Nonferrous Met. Soc. China, 2011, 21: 1009
[11]
Sohrabi M J, Mirzadeh H, Rafiei M. Solidification behavior and Laves phase dissolution during homogenization heat treatment of Inconel 718 superalloy [J]. Vacuum, 2018, 154: 235
[12]
Hosseini S A, Madar K Z, Abbasi S M. Effect of homogenization heat treatments on the cast structure and tensile properties of nickel-base superalloy ATI 718Plus in the presence of boron and zirconium additions [J]. Mater. Sci. Eng., 2017, A689: 103
[13]
Yang J X, Sun Y, Jin T, et al. Microstructure and mechanical properties of a Ni-based superalloy with refined grains [J]. Acta Metall. Sin., 2014, 50: 839
Zhang Y, Li P H, Jia C L, et al. Research progress of melting purification techniques and equipment for cast & wrought superalloy [J]. Mater. Rev., 2018, 32: 1496
Gao Z T, Guo W, Zhang C W, et al. Development of fine-grained structure in Ni-Cr-W based superalloy and its effect on the mechanical properties [J]. Mater. Sci. Eng., 2017, A682: 156
[16]
Ma Y, Sun J H, Xie X S, et al. An investigation on fine-grain formation and structural character in cast IN718 superalloy [J]. J. Mater. Process. Technol., 2003, 137: 35
[17]
Men H, Jiang B, Fan Z. Mechanisms of grain refinement by intensive shearing of AZ91 alloy melt [J]. Acta Mater., 2010, 58: 6526
[18]
Chang L T, Jin H, Sun W R. Solidification behavior of Ni-base superalloy Udimet 720Li [J]. J. Alloys Compd., 2015, 653: 266
[19]
Gong L, Chen B, Yang Y Q, et al. Effect of N content on microsegregation, microstructure and mechanical property of cast Ni-base superalloy K417G [J]. Mater. Sci. Eng., 2017, A701: 111
[20]
Qin L, Pei Y L, Li S S, et al. Effect of thermal stability of γ' phase on the recrystallization behaviors of Ni-based single crystal superalloys [J]. Mater. Des., 2017, 130: 69
[21]
Zhang H, Liu Y, Chen X, et al. Microstructural homogenization and high-temperature cyclic oxidation behavior of a Ni-based superalloy with high-Cr content [J]. J. Alloys Compd., 2017, 727: 410
[22]
Pan X L, Wang B, Sun W R, et al. Effect of homogenization treatment on the hot deformation of GH742 alloy [J]. Acta Metall. Sin., 2012, 48: 1403
Dong J X, Li L H, Li H Y, et al. Effect of extent of homogenization on the hot deformation recrystallization of superalloy ingot in cogging process [J]. Acta Metall. Sin., 2015, 51: 1207
Hegde S R, Kearsey R M, Beddoes J C. Designing homogenization-solution heat treatments for single crystal superalloys [J]. Mater. Sci. Eng., 2010, A527: 5528
[25]
Du J H, Qu J L, Deng Q, et al. As-cast microstructure and homogenization process of alloy GH720Li [J]. J. Iron Steel Res., 2005, 17(3): 60
Liu F F, Chen J Y, Dong J X, et al. The hot deformation behaviors of coarse, fine and mixed grain for Udimet 720Li superalloy [J]. Mater. Sci. Eng., 2016, A651: 102
[27]
Cao L, Zhou Y Z, Jin T, et al. Effects of Re on surface eutectic formation for Ni-base single crystal superalloys during directional solidification [J]. J. Mater. Sci. Technol., 2017, 33: 1308
[28]
Mishin Y, Herzig C. Diffusion in the Ti-Al system [J]. Acta Mater., 2000, 48: 589
[29]
Liu M C, Sheng G M, He H J, et al. Microstructural evolution and mechanical properties of TLP bonded joints of Mar-M247 superalloys with Ni-Cr-Co-W-Ta-B interlayer [J]. J. Mater. Process. Technol., 2017, 246: 245
[30]
Neumeier S, Rehman H U, Neuner J, et al. Diffusion of solutes in fcc cobalt investigated by diffusion couples and first principles kinetic Monte Carlo [J]. Acta Mater., 2016, 106: 304
[31]
Ruan J J, Ueshima N, Oikawa K. Phase transformations and grain growth behaviors in superalloy 718 [J]. J. Alloys Compd., 2018, 737: 83
[32]
Yang C, Mo D G, Lu H Z, et al. Reaction diffusion rate coefficient derivation by isothermal heat treatment in spark plasma sintering system [J]. Scr. Mater., 2017, 134: 91
[33]
Egbewande A T, Chukwukaeme C, Ojo O A. Joining of superalloy Inconel 600 by diffusion induced isothermal solidification of a liquated insert metal [J]. Mater. Charact., 2008, 59: 1051
[34]
Viswanathan G B, Sarosi P M, Henry M F, et al. Investigation of creep deformation mechanisms at intermediate temperatures in René 88 DT [J]. Acta Mater., 2005, 53: 3041
[35]
Sinharoy S, Virro-Nic P, Milligan W W. Deformation and strength behavior of two nickel-base turbine disk alloys at 650 ℃ [J]. Metall. Mater. Trans., 2001, 32A: 2021
[36]
Henry M F, Yoo Y S, Yoon D Y, et al. The dendritic growth of γ' precipitates and grain [J]. Metall. Trans., 1993, 24A: 1733
[37]
Fahrmann M, Suzuki A. Effect of cooling rate on gleeble hot ductility of Udimet alloy 720 billet [A]. Superalloys 2008 [C]. Warrendale: TMS, 2008: 311
[38]
Liu G W, Han Y, Shi Z Q, et al. Hot deformation and optimization of process parameters of an as-cast 6Mo superaustenitic stainless steel: A study with processing map [J]. Mater. Des., 2014, 53: 662
[39]
Kumar S S S, Raghu T, Bhattacharjee P P, et al. Work hardening characteristics and microstructural evolution during hot deformation of a nickel superalloy at moderate strain rates [J]. J. Alloys Compd., 2017, 709: 394
[40]
Kumar S S S, Raghu T, Bhattacharjee P P, et al. Strain rate dependent microstructural evolution during hot deformation of a hot isostatically processed nickel base superalloy [J]. J. Alloys Compd., 2016, 681: 28
[41]
Han Y, Liu G W, Zou D N, et al. Deformation behavior and microstructural evolution of as-cast 904L austenitic stainless steel during hot compression [J]. Mater. Sci. Eng., 2013, A565: 342
