Microstructural evolution of a Ni-Co based superalloy during hot compression at γ′ sub-/super-solvus temperatures

doi:10.1016/j.jmst.2020.10.042

Abstract

Abstract:

The effects of strain rate on the microstructural evolution and deformation mechanism of a Ni-Co based superalloy were investigated by isothermal compression tests performed at γ′ sub-solvus (1090 °C) and γ′ super-solvus temperatures (1150 °C) with a wide strain rate range from 0.001 to 10 s^-1 under a true strain of 0.693. Electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI) and transmission electron microscope (TEM) techniques were used to characterize the microstructures. The results revealed that the dynamic recrystallization (DRX) volume fraction increased and stored energy of the γ matrix grains decreased with increasing the strain rate during γ′ sub-solvus temperature deformation, while the opposite phenomena were observed during γ′ super-solvus temperature deformation. The comprehensive effect of initial grain size, primary γ′ phase, twins and adiabatic temperature rise led to these results. The primary γ′ particles undergone the deformation behavior within itself and obviously accelerated the DRX of the matrix. The microstructural evolution proved that discontinuous dynamic recrystallization (DDRX) was the dominant mechanism during the hot deformation carried out at both γ′ sub-solvus and γ′ super-solvus temperatures. Primary γ′ particles obviously accelerated the nucleation step and retarded the growth step of DDRX during γ′ sub-solvus temperature deformation. Besides, the acceleration effect of primary γ′ particles on DDRX increased with the increase of strain rate. Continuous dynamic recrystallization (CDRX) was confirmed to be an assistant mechanism during γ′ super-solvus temperature deformation and was promoted with the increase of strain rate.

Key words: Ni-Co based superalloy, Strain rate, Primary γ′ precipitates, Dynamic recrystallization, Microstructural evolution

Peng Liu, Rui Zhang, Yong Yuan, Chuanyong Cui, Faguang Liang, Xi Liu, Yuefeng Gu, Yizhou Zhou, Xiaofeng Sun. Microstructural evolution of a Ni-Co based superalloy during hot compression at γ′ sub-/super-solvus temperatures[J]. J. Mater. Sci. Technol., 2021, 77: 66-81.

Figures/Tables 16

Table 1. Nominal chemical composition (wt%) of the studied Ni-Co based alloy.

Ni	Co	Cr	Ti + Al	Mo	W	C + B+Zr
Bal	20∼26	13∼15	7.72∼8.40	2.4∼2.8	1.1∼1.3	0.04∼0.11

Fig. 1. Schematic diagram of hot compression test.

Fig. 2. Initial microstructure of the alloy soaked at 1090 °C for 10 min prior to hot compression: (a) IPF plus color-coded grain boundary map (the inset image is color legend), (b) GOS plus color-coded grain boundary map, (c) ECCI image, (d) TEM image (the inset image is the higher magnification figure of tertiary γ′ precipitates).

Fig. 3. Initial microstructure of the alloy soaked at 1150 °C for 10 min prior to hot compression: (a) IPF plus color-coded grain boundary map (the inset image is color legend), (b) GOS plus color-coded grain boundary map, (c) ECCI image, (d) TEM image.

Fig. 4. True stress-strain curves of the alloy deformed up to a 0.693 true strain with various strain rates at: (a) 1090 °C, (b) 1150 °C, and (c) temperature increment varying with strain at the strain rate of 1 s-1 and 10 s-1.

Fig. 5. GOS distributions of the alloy deformed up to a 0.693 true strain with various strain rates at: (a) 1090 °C, (b) 1150 °C.

Fig. 6. Microstructural evolution of the alloy deformed up to a 0.693 true strain with various strain rates at 1090 °C: IPF plus color-coded grain boundary maps: (a) 10 s-1, (b) 0.1 s-1, (c) 0.001 s-1, GOS plus color-coded grain boundary maps: (d) 10 s-1, (e) 0.1 s-1, (f) 0.001 s-1, Misorientations measured along the lines marked as: (g) A1, (h) B1, (i) C1, (j) A2, (k) B2, (l) C2.

Fig. 7. Microstructural evolution of the alloy deformed up to a 0.693 true strain with various strain rates at 1090 °C: (a) volume fraction of DRX grains, (b) initial grain size and DRX grain sizes, (c) LAGBs, MAGBs and HAGBs fraction of initial microstructure and deformed microstructure, (d) ΣCSL grain boundaries fraction of initial microstructure and deformed microstructure.

Fig. 8. Microstructural evolution of the alloy deformed up to a 0.693 true strain with various strain rates at 1150 °C: IPF plus color-coded grain boundary maps: (a) 10 s-1, (b) 0.1 s-1, (c) 0.001 s-1, GOS plus color-coded grain boundary maps: (d) 10 s-1, (e) 0.1 s-1, (f) 0.001 s-1, Misorientations measured along the lines marked as: (g) A1, (h) B1, (i) C1, (j) A2, (k) B2, (l) C2.

Fig. 9. Microstructural evolution of the alloy deformed up to a 0.693 true strain with various strain rates at 1150 °C: (a) volume fraction of DRX grains, (b) initial grain size and DRX grain sizes, (c) LAGBs, MAGBs and HAGBs fraction of initial microstructure and deformed microstructure, (d) ΣCSL grain boundaries fraction of initial microstructure and deformed microstructure.

Fig. 10. KAM maps of the alloy deformed up to a 0.693 true strain with various strain rates of 10 s-1, 0.1 s-1, and 0.001 s-1 at (a-c) 1090 °C, (d-f) 1150 °C, respectively.

Fig. 11. ECCI images of the alloy deformed up to a 0.693 true strain with various strain rates at 1090 °C: (a) 10 s-1, (b) 1 s-1, (c) 0.1 s-1, (d) 0.01 s-1, (e) 0.001 s-1, and (f) volume fraction of primary γ′ precipitates in initial microstructure and deformed microstructure (the arrows indicate the presence of dislocations or stacking faults in primary γ′ particles and the inset image is the enlarged figure of the white-square region).

Fig. 12. TEM images of the alloy deformed up to a 0.693 true strain with various strain rates at 1090 °C: (a), (d) 10 s-1, (b), (e) 0.1 s-1, (c), (f) 0.001 s-1 ((a-c) show high magnification of primary γ′ particles and the arrows in (e-f) indicate the presence of dislocations in γ grain).

Fig. 13. ECCI images (a-c) and TEM images (d-f) of the alloy deformed up to a 0.693 true strain with various strain rates at 1090 °C: (a), (d) 10 s-1, (b), (e) 0.1 s-1, (c), (f), 0.001 s-1.

Fig. 14. TEM images of the alloy deformed up to a 0.693 true strain with various strain rates at 1150 °C: (a), 10 s-1, (b), 0.1 s-1, (c), (d), 0.001 s-1 (the inset image in (c) is the enlarged figure of the white-square region).

Fig. 15. Summary schematic diagram of the microstructure evolution deformed at γ′ sub-solvus temperature: (a) initial microstructure, (b-d) microstructure deformed under high to low strain rate; and at γ′ super-solvus temperature: (e) initial microstructure, (f-h) microstructure deformed under high to low strain rate.

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