Microstructure, precipitates and mechanical properties of powder bed fused inconel 718 before and after heat treatment

doi:10.1016/j.jmst.2018.12.006

摘要/Abstract

Abstract:

IN718 alloy was fabricated by laser powder bed fusion (PBF) for examination of microstructure, precipitates and mechanical properties in the as-built state and after different heat treatments. The as-built alloy had a characteristic fine cellular-dendritic microstructure with Nb, Mo and Ti segregated along the interdendritic region and cellular boundary. The as-built alloys were then subjected to solution heat treatment (SHT) at 980 °C or 1065 °C for 1 h. SHT at 980 °C led to the formation of δ-phase in the interdendritic region or cellular boundary. The segregation was completely removed by the SHT at 1065 °C, but recrystallization was observed, and the carbides decorated along the grain boundaries. The as-built alloy and alloys with SHT at 980 °C and 1065 °C were two-step aged, which consisted of annealing at 720 °C for 8 h followed by annealing at 620 °C for 8 h. Transmission electron microscopy revealed the precipitation of γ' and γ” in all alloys after two-step aging, but the amount and uniformity of distribution varied. The Vickers hardness of the PBF IN718 alloy increased from 296 HV to 467 HV after direct aging. The hardness decreased to 267 HV and 235 HV after SHT at 980 °C and 1065 °C, respectively, but increased to 458 HV and 477 HV followed by aging. The evolution of Young’s modulus after heat treatment exhibited similar trend to that of hardness. The highest hardness was observed for IN718 after SHT at 1065 °C and two-step aging due to precipitation with greater amount and uniform distribution.

Key words: Laser powder bed fusion, IN718, Microstructure, Transmission electron microscopy

. [J]. 材料科学与技术, 2019, 35(6): 1153-1164.

Le Zhou, Abhishek Mehta, Brandon McWilliams, Kyu Cho, Yongho Sohn. Microstructure, precipitates and mechanical properties of powder bed fused inconel 718 before and after heat treatment[J]. J. Mater. Sci. Technol., 2019, 35(6): 1153-1164.

图/表 17

Table 1 Chemical composition in weight percent of the IN718 powders.

Ni	Cr	Co	Mo	Al	Fe	Ti	Nb	C	B	Cu	Mn	P	S	Si
50.0-55.0	17.0-21.0	1.0	2.80-3.30	0.20-0.80	Bal.	0.65-1.15	4.75-5.50	0.08	0.006	0.30	0.35	0.0015	0.0015	0.35

Table 2 Heat treatment parameters for different PBF IN718 alloy samples examined in this study.

Designation	Solution Heat Treatment	Aging
As-built	None	None
DA	None	720 °C for 8 h + 620 °C for 8 h
S980	980 °C for 1 h	None
SA980	980 °C for 1 h	720 °C for 8 h + 620 °C for 8 h
S1065	1065 °C for 1 h	None
SA1065	1065 °C for 1 h	720 °C for 8 h + 620 °C for 8 h

Fig. 1. X-ray diffraction patterns from the XY cross-sections of PBF IN718 alloy samples: (a) as-built, (b) DA, (c) S980, (d) SA980, (e) S1065 and (f) SA1065.

Fig. 2. Optical micrographs of the as-built IN718 alloy from the (a) XY and (b) XZ cross-sections.

Fig. 3. Backscatter electron micrographs from the as-built IN718 alloy sample in the (a, b) XY and (c, d) XZ cross-sections.

Fig. 4. Electron backscatter diffraction (EBSD) grain mapping illustrating the grain structure of the as-built IN718 alloy in the (a) XY and (b) XZ cross-sections, and (c) the corresponding distribution of grain size.

Fig. 5. TEM observations from the as-built IN718 alloy sample: (a) BF TEM micrograph of cellular structure; (b) SADP within the cell without any precipitates; (c) SADP from the cellular boundary with Laves phase; (d) HAADF STEM micrograph and (e) XEDS elemental mapping of Al, Ti, Cr, Fe, Ni, Nb, Mo, C and O from the white square marked in (d).

Fig. 6. Backscatter electron micrographs from the DA IN718 alloy sample in the (a, b) XY and (c, d) XZ cross-sections.

Fig. 7. TEM observations from the DA IN718 alloy sample: (a, b) HAADF STEM micrographs and (c) XEDS elemental mapping for Al, Ti, Cr, Fe, Ni, Nb, Mo, and C from the white square marked in (b).

Fig. 8. Backscatter electron micrographs from the SA980 IN718 alloy sample in the (a, b) XY and (c, d) XZ cross-sections.

Fig. 9. TEM observations from the SA980 IN718 alloy sample: (a) HAADF STEM micrograph; (b) BF TEM micrograph and (c) SADP of the δ-phase; (d) HAADF STEM micrograph; (e) XEDS elemental mapping of Al, Ti, Cr, Fe, Ni, Nb, Mo, C and O from the white square marked in (d); and (f) SADP of the γ” needle.

Fig. 10. Backscatter electron micrographs from the SA1065 IN718 alloy sample in the (a, b) XY and (c, d) XZ cross-sections.

Fig. 11. TEM observations from the SA1065 IN718 alloy sample: (a) HAADF STEM micrograph; (b) SADP of the carbide particle; (c) HAADF STEM micrograph and (d) XEDS elemental mapping of Al, Ti, Cr, Fe, Ni, Nb, Mo, B/C and O from the white square marked in (d).

Fig. 12. (a) A SADP from the γ <001> orientation and its schematic representation; (b) DF TEM micrograph of γ' and γ” precipitates; High resolution TEM micrograph of γ” from the (c) [100] and [010] orientation and the (d) [001] orientation.

Fig. 13. DF TEM micrographs showing the γ’’ distribution and the corresponding SADP in the (a, b) DA, (c, d) SA980 and (e, f) SA1065 IN718 alloy samples.

Table 3 Young’s modulus and nano-scale hardness measured from instrumented nanoindentation, and Vickers hardness for PBF IN718 alloy samples examined in this study. The standard deviations were listed in the parentheses.

	E (GPa)	H (GPa)	Vickers
As-built	208 (7.7)	4.9 (0.13)	296 (2.9)
DA	226 (8.6)	6.8 (0.19)	467 (4.3)
S980	193 (7.7)	4.6 (0.14)	267 (2.1)
SA980	209 (6.0)	6.6 (0.18)	458 (4.2)
S1065	181 (9.0)	4.4 (0.16)	235 (4.2)
SA1065	226 (6.3)	7.0 (0.14)	477 (6.6)

Fig. 14. Vickers hardness change in the PBF IN718 alloy samples examined in this study.

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