Tensile, creep behavior and microstructure evolution of an as-cast Ni-based K417G polycrystalline superalloy

doi:10.1016/j.jmst.2018.02.007

Abstract

Abstract:

The Ni-based K417G superalloy is extensively applied as aeroengine components for its low cost and good mid-temperature (600-900 °C) properties. Since used in as-cast state, the comprehensive understanding on its mechanical properties and microstructure evolution is necessary. In the present research, the tensile, creep behavior and microstructure evolution of the as-cast K417G superalloy under different conditions were investigated. The results exhibit that tensile cracks tend to initiate at MC carbide and γ/γ′ eutectic structure and then propagate along grain boundary. As the temperature for tensile tests increases from 21 °C to 700 °C, the yield strength and ultimate tensile strength of K417G superalloy decreases slightly, while the elongation to failure decreases greatly because of the intermediate temperature embrittlement. When the temperature rises to 900 °C, the yield strength and ultimate tensile strength would decrease significantly. The creep deformation mechanism varies under different testing conditions. At 760 °C/645 MPa, the creep cracks initiate at MC carbides and γ/γ′ eutectic structures, and propagate transgranularly. While at 900 °C/315 MPa and 950 °C/235 MPa, the creep cracks initiate at grain boundary and propagate intergranularly. As the creep condition changes from 760 °C/645 MPa to 900 °C/315 MPa and 950 °C/235 MPa, the γ′ phase starts to raft, which reduces the creep deformation resistance and increases the steady-state deformation rate.

Key words: Ni-based superalloy, Microstructure, Tensile, Creep

Beining Du, Ziyang Hu, Liyuan Sheng, Chuanyong Cui, Jinxia Yang, Yufeng Zheng, Xiaofeng Sun. Tensile, creep behavior and microstructure evolution of an as-cast Ni-based K417G polycrystalline superalloy[J]. J. Mater. Sci. Technol., 2018, 34(10): 1805-1816.

Figures/Tables 18

Table 1 Nominal composition of K417G superalloy (wt%).

	Cr	Co	Mo	V	Al	Ti	C	Zr	Ni
0.018	9.00	10.00	3.00	0.75	5.25	4.40	0.18	0.07	Balance

Fig. 1. Schematic diagram of standard tensile (a) and creep (b) specimen.

Fig. 2. SEM images of the microstructure of as-cast K417G superalloy: (a) grain distribution; (b) MC carbide and γ/γ′ eutectic structure; (c) grain boundary microstructure; (d) γ′ phase; (e) EDS spectrum of MC carbide.

Fig. 3. Tensile results of K417G superalloy.

Fig. 4. Elements distribution near grain boundary: (a) SEM image of the grain boundary of K417G superalloy (b) O distribution; (c) S distribution; (d) C distribution; (e) Cr distribution; (f) Ti distribution.

Fig. 5. Creep results of K417G superalloy: (a) creep curves; (b) creep life and the steady-state deformation rate.

Fig. 6. SEM images of the longitudinal section of the tensile fractured specimens: (a) 21 °C; (b) 700 °C; (c) 900 °C.

Fig. 7. EBSD plane sweep scanning images near the tensile fracture surface: (a)-(c) All Euler image; (d)-(f) Local Misorientation image; (a), (d) 21 °C; (b), (e) 700 °C; (c), (f) 900 °C. (The misorientation increases from blue to red as the color bar).

Fig. 8. SEM images of the longitudinal section of the creep fractured specimens: (a) 760 °C/645 MPa; (b) 900 °C/315 MPa; (c) 950 °C/235 MPa.

Fig. 9. EBSD plane sweep scanning images near the creep fracture surface: (a)-(c) All Euler image; (d)-(f) Local Misorientation image; (a), (d) 760 °C/645 MPa; (b), (e) 900 °C/315 MPa; (c), (f) 950 °C/235 MPa.

Fig. 10. SEM and TEM images of the microstructure of as-cast K417G superalloy after tensile tests (a), (d) 21 °C; (b), (e) 700 °C; (c), (f) 900 °C; (g)-(i) TEM dark-field image and diffraction patterns of the grain boundary M23C6 carbide at the grain boundary of K417G superalloy after tensile at 900 °C.

Fig. 11. SEM and TEM images of the microstructure of as-cast K417G superalloy after creep tests (a), (d) 760 °C/645 MPa; (b), (e) 900 °C/315 MPa; (c), (f) 950 °C/235 MPa; (g)-(i) TEM bright-field image and diffraction patterns of the grain boundary M3B2 boride and M23C6 carbide at the grain boundary of K417G superalloy after creep at 900 °C/315 MPa and 950 °C/235 MPa.

Fig. 12. Dislocation structures of K417G superalloy after tensile at different temperatures (a) 21 °C; (b) 700 °C; (c) 900 °C.

Fig. 13. SEM images of γ' phase and TEM images of dislocation structures of K417G superalloy after creep at different conditions (a), (d) 760 °C/645 MPa; (b) 900 °C/315 MPa; (c), (e) 950 °C/235 MPa.

Table 2 Microstructure evolution of K417G superalloy after tensile and creep tests.

Test condition		MC carbide	Eutectic	γ' phase	Grain boundary microstructure
Tensile	21 °C	broken	broken	no rafting	no new phase
	700 °C	broken	broken	no rafting	no new phase
	900 °C	broken	broken	no rafting	M₂₃C₆
Creep	760 °C/645 MPa	broken	broken	no rafting	no new phase
	900 °C/315 MPa	no broken decompose into M₂₃C₆	broken	rafting	M₂₃C₆ and M₃B₂
	950 °C/235 MPa	no broken decompose into M₂₃C₆	broken	serious rafting	M₂₃C₆ and M₃B₂

Table 3 Average size and distribution of MC carbide and γ/γ′ eutectic structure in K417G superalloy.

	Average size (μm)		Distribution
	At grain boundary	In grain interior	At grain boundary	In grain interior
MC carbide	3.2	2.7	76%	24%
Eutectic	15.3	10.4	77%	23%

Fig. 14. Schematic diagram of the microstructure evolution and fracture mechanism of K417G superalloy during tensile tests at different temperatures.

Fig. 15. Schematic diagram of the microstructure evolution and fracture mechanism of K417G superalloy during creep tests under different conditions.

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