First-Principles Study on the Effect of Cu on Nb Segregation in Inconel 718 Alloy
LI Yamin(), ZHANG Yaoyao, ZHAO Wang, ZHOU Shengrui, LIU Hongjun
State Key Laboratory of Advanced Processing and Reuse of Nonferrous Metals, Lanzhou University of Technology, Lanzhou 730050, China
Cite this article:
LI Yamin, ZHANG Yaoyao, ZHAO Wang, ZHOU Shengrui, LIU Hongjun. First-Principles Study on the Effect of Cu on Nb Segregation in Inconel 718 Alloy. Acta Metall Sin, 2022, 58(2): 241-249.
Inconel 718 alloy is an Fe-Ni based superalloy precipitation-strengthened by γ″ phase (Ni3Nb) and γ′ phase (Ni3(AlTi)). It has been widely used in aviation, energy, chemicals, and other fields because of its outstanding mechanical properties, resistance to high-temperature oxidation, and corrosion resistance. Because the mechanical properties of Inconel 718 alloy are primarily determined by the γ″ precipitates, Nb becomes one of the most important alloying elements. Due to the high content, large atomic radius, and small partition coefficient of Nb, Nb segregation occurs easily during the solidification process of casting, welding, and laser cladding. The segregation drastically degrades mechanical properties and increases the difficulty of subsequent heat treatment. The composition of Inconel 718 alloy comprises many elements, and some trace elements are inevitably introduced from the raw materials. The interaction of the elements has a certain effect on Nb segregation. In this study, the effect of the interaction of elements caused by doping of Cu on Nb segregation in Inconel 718 alloy was studied by first-principles calculation and experiment. The Ni-Fe-Cr-Nb supercell model was constructed with and without Cu doping. The enthalpy of formation, cohesive energy, state density, electron density difference, and population analysis was calculated. The calculation results show that the doping of Cu reduces the stability of the system. Doping will change the interaction between elements and affects the strength and density distribution ratio of the charged density between atomic bonds in the system. The addition of Cu increases the bond strength between the Fe atoms and Cr atoms and the repulsive force between Fe atoms and Nb atoms in the matrix. The experimental results show that the addition of 0.1%Cu (mass fraction) decreases the segregation of Fe and Cr, but promotes the segregation of Nb. Experimental results and first-principles calculations show that the increase in the repulsive force between the Nb atom and Fe atom, which is caused by the interaction between the alloying elements after doping with Cu, is the essential reason for Cu to promote Nb segregation.
Fund: Provincial and Ministry Co-Construction of the Open Fund Project of the State Key Laboratory of Advanced Processing and Reuse of Nonferrous Metals(SKLAB02019014)
About author: LI Yamin, associate professor, Tel: 13993195230, E-mail: leeyamin@163.com
Table 1 Nominal chemical composition of experimental alloy
Fig.1 Supercell model for calculation
Formula
a / nm
b / nm
c / nm
α / (°)
β / (°)
γ / (°)
V / nm3
Ni20Fe6Cr6
0.6988
0.7086
0.7041
90.0034
89.9982
90.0058
0.3487
Ni19Fe6Cr6Nb
0.7063
0.7114
0.7081
90.0018
89.9990
90.0058
0.3558
Ni18Fe6Cr6NbCu
0.7077
0.7124
0.7067
89.0081
90.0010
90.0010
0.3563
Table 2 Equilibrium lattice constants of each supercell
Formula
Formation enthalpy / (eV·atom-1)
Cohesive energy / (eV·atom-1)
Total energy / eV
Ni19Fe6Cr6Nb
-81.6314
-10.3420
-47349.2553
Ni18Fe6Cr6NbCu
-81.6141
-10.2053
-47470.1808
Table 3 Calculated formation heats and binding energies of the system
Fig.2 Diagrams of density of states (DOS) of the system before and after doping (PDOS—partial density of states, EF—Fermi level)
Fig.3 Differential charge density diagrams of the system (010) before and after doping
System
Atom
s
p
d
Total
Charge
Ni19Fe6Cr6Nb
Ni
0.58
0.82
8.65
10.05
-0.04
Fe
0.50
0.64
6.73
7.87
0.13
Cr
2.66
6.53
4.97
14.16
-0.18
Nb
2.67
5.31
3.93
11.91
1.09
Ni18Fe6Cr6NbCu
Ni
0.58
0.81
8.65
10.04
-0.04
Fe
0.49
0.62
6.73
7.85
0.17
Cr
2.65
6.53
4.98
14.16
-0.15
Cu
0.79
0.98
9.62
11.39
-0.39
Nb
2.69
5.33
3.93
11.95
1.05
Table 4 Atomic populations of systems before and after doping
Atom
Ni19Fe6Cr6Nb
Ni18Fe6Cr6NbCu
Population
Length / nm
Population
Length / nm
Fe-Ni
0.19
0.251086
0.20
0.251930
Cr-Fe
-0.02
0.247538
0.03
0.248490
Cr-Ni
0.04
0.250789
0.05
0.250421
Ni-Ni
0.19
0.250051
0.19
0.250101
Fe-Nb
-0.17
0.258629
-0.18
0.257657
Ni-Nb
-0.13
0.253356
-0.13
0.253710
Ni-Cu
-
-
0.13
0.250674
Cr-Cu
-
-
-0.16
0.249645
Fe-Cu
-
-
0.11
0.244092
Table 5 Overlapping populations of systems before and after doping
Fig.4 XRD spectra of as-cast Inconel 718 alloy before and after Cu doping
Fig.5 OM images of microstructures of as-cast Inconel 718 alloy before and after Cu doping
Fig.6 SEM images of microstructures of as-cast Inconel 718 alloy before and after Cu doping
Alloy
Area
Ni
Nb
Al
Ti
Cr
Fe
Mo
Cu
No.0
Matrix
51.74
3.76
0.39
0.72
20.68
19.93
2.78
-
Segregation zone
51.90
7.59
0.26
1.31
18.56
17.35
3.03
-
Laves phase
46.46
15.53
-
1.23
16.94
15.66
4.18
-
No.1
Matrix
48.79
1.94
0.42
0.29
21.25
24.22
3.02
0.08
Segregation zone
45.52
4.69
0.46
0.29
20.92
22.09
3.84
0.18
Laves phase
37.15
23.04
0.15
0.31
14.29
14.78
10.0
0.29
Table 6 EDS results of different regions of as-cast Inconel 718 alloy
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