The simultaneous improvements of strength and ductility in additive manufactured Ni-based superalloy via controlling cellular subgrain microstructure

doi:10.1016/j.jmst.2020.07.011

Abstract

Abstract:

Fine cellular subgrain structure was formed in the Selective Laser Melting (SLM) manufactured IN718 alloy via optimizing the processing parameters. During the subsequent homogenization heat treatment process, the Laves phase dispersed at the subgrain boundaries can be eliminated while the cellular subgrain structure is reserved in the printed samples after holding at 1080 ℃ for 50 min. With the prolongation of the holding time, the subgrain boundaries undergo low angle rotation via the motion of dislocation, which leads to the annihilation of the cellular subgrain structure. Moreover, during the subsequent double aging heat treatment process, the reserved cellular subgrain structure in the homogenized samples promotes the precipitation of γ” second phase nanoparticles, and these precipitated γ” phase nanoparticles prefer to distribute at subgrain boundaries. It was found that these unique subgrain boundaries with γ” phase precipitates can hinder but not fully terminate the motion of dislocation during the plastic deformation process, which contributes to increasing the strength as well as holding the stable plastic flow. Hence, the strength and ductility of final prepared IN718 alloy with cellular subgrain microstructure were improved simultaneously compared to the prepared alloy without cellular subgrain structure, which even exceed the mechanical properties standards (AMS 5662) of wrought IN718 alloy. These results in our work suggest that controlling the subgrain structure is a promising effective strategy to improve the mechanical properties of SLM manufactured nickel-based superalloy.

Key words: Selective laser melting, Superalloy, Subgrain structure, Strengthening and toughening

Yanan Zhao, Zongqing Ma, Liming Yu, Ji Dong, Yongchang Liu. The simultaneous improvements of strength and ductility in additive manufactured Ni-based superalloy via controlling cellular subgrain microstructure[J]. J. Mater. Sci. Technol., 2021, 68: 184-190.

Figures/Tables 8

Table 1 The chemical composition of gas-atomized IN718 powder.

Element	Ni	Cr	Nb	Mo	Ti	Al	Co	Cu	Si	C	Fe
Content(wt.%)	54.76	18.24	5.15	3.0	0.95	0.58	0.16	0.04	0.16	0.031	Bal

Table 2 The detailed information of heat treatment for the SLM manufactured IN718 alloy.

Sample No.	Heat treatment parameters
H0	As-printed
H1	1080 ℃ × 50 min, AC
H2	1080 ℃ × 80 min, AC
H3	1080 ℃ × 110 min, AC
H4	1080 ℃ × 50 min, AC; 720 ℃ × 8 h, cooled at 55 ℃/h to 620 ℃, 620 ℃ × 8 h, AC
H5	1080 ℃ × 110 min, AC; 720 ℃ × 8 h, cooled at 55 ℃/h to 620 ℃, 620 ℃ × 8 h, AC

Fig. 1. TEM images of cellular subgrain microstructure in as-printed IN718 alloy with (a) low magnification image, (b) high magnification image and (c) The EDX spectrum analysis diagram of the segregation phase at subgrain boundaries.

Fig. 2. The TEM images of subgrain structures for the (a) H0 as-printed sample, (b) H1 sample heated at 1080 ℃ for 50 min, (c) H2 sample heated at 1080 ℃ for 80 min and (d) H3 sample heated at 1080 ℃ for 110 min. The inset figure in Fig. 2(d) is the diffraction patterns of the NbC at initial grain boundary.

Fig. 3. Schematic of evolution model of subgrain boundaries during homogenization treatment process.

Fig. 4. The SEM microstructure of SLM manufactured samples after homogenization and aging treatment with (a) H4 sample and (b) H5 sample. And the TEM images of samples after homogenization and aging treatment with (c) and (e) for H4 sample, and (d) and (f) for H5 sample.

Fig. 5. Room temperature tensile properties of H4 and H5 samples.

Fig. 6. The SEM microstructure of fractures for (a) H4 and (b) H5 samples.

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