Grain boundary and microstructure engineering of Inconel 690 cladding on stainless-steel 316L using electron-beam powder bed fusion additive manufacturing

doi:10.1016/j.jmst.2018.09.059

摘要/Abstract

Abstract:

This research explores the prospect of fabricating a face-centered cubic (fcc) Ni-base alloy cladding (Inconel 690) on an fcc Fe-base alloy (316 L stainless-steel) having improved mechanical properties and reduced sensitivity to corrosion through grain boundary and microstructure engineering concepts enabled by additive manufacturing (AM) utilizing electron-beam powder bed fusion (EPBF). The unique solidification and associated constitutional supercooling phenomena characteristic of EPBF promotes [100] textured and extended columnar grains having lower energy grain boundaries as opposed to random, high-angle grain boundaries, but no coherent {111} twin boundaries characteristic of conventional thermo-mechanically processed fcc metals and alloys, including Inconel 690 and 316 L stainless-steel. In addition to [100] textured grains, columnar grains were produced by EPBF fabrication of Inconel 690 claddings on 316 L stainless-steel substrates. Also, irregular 2-3 μm diameter, low energy subgrains were formed along with dislocation densities varying from 10⁸ to 10⁹ cm^-2, and a homogeneous distribution of Cr₂₃C₆ precipitates. Precipitates were formed within the grains (with ～3 μm interparticle spacing), but not in the subgrain or columnar grain boundaries. These inclusive, hierarchical microstructures produced a tensile yield strength of 0.527 GPa, elongation of 21%, and Vickers microindentation hardness of 2.33 GPa for the Inconel 690 cladding in contrast to a tensile yield strength of 0.327 GPa, elongation of 53%, and Vickers microindentation hardness of 1.78 GPa, respectively for the wrought 316 L stainless-steel substrate. Aging of both the Inconel 690 cladding and the 316 L stainless-steel substrate at 685 °C for 50 h precipitated Cr₂₃C₆ carbides in the Inconel 690 columnar grain boundaries, but not in the low-angle (and low energy) subgrain boundaries. In contrast, Cr₂₃C₆ carbides precipitated in the 316 L stainless-steel grain boundaries, but not in the low energy coherent {111} twin boundaries. Consequently, the Inconel 690 subgrain boundaries essentially serve as surrogates for coherent twin boundaries with regard to avoiding carbide precipitation and corrosion sensitization.

Key words: Additive manufacturing, Electron-beam powder bed fusion (EPBF), Inconel 690 cladding, 316L stainless steel, Grain boundary engineering, Materials characterization, Mechanical properties

. [J]. 材料科学与技术, 2019, 35(2): 351-367.

I.A. Segura, L.E. Murr, C.A. Terrazas, D. Bermudez, J. Mireles, V.S.V. Injeti, K. Li, B. Yu, R.D.K. Misra, R.B. Wicker. Grain boundary and microstructure engineering of Inconel 690 cladding on stainless-steel 316L using electron-beam powder bed fusion additive manufacturing[J]. J. Mater. Sci. Technol., 2019, 35(2): 351-367.

图/表 23

Table 1 Nominal alloy chemical compositions (wt%).

Alloy	Ni	Fe	Cr	C	Mo	Mn	Co	Si
In. 690	bal.	7-11	27-31	0.05	0.01	0.5	0.2	0.5
SS 316 L	<10-14	bal.	16-19	<0.03	2-3	2	-	1

Fig. 1. As-received Inconel 690 powder (a) and powder internal microstructure (b).

Fig. 2. Schematic of Arcam EBM system (From Segura et al. [22]).

Fig. 3. Temperature vs time plot for the aging process.

Fig. 4. Composite 3D OM image of wrought 316 L stainless-steel substrate.

Fig. 5. Composite 3D OM image of EPBF fabricated Inconel 690 cladding. The build direction is vertical.

Fig. 6. Composite 3D OM image of EPBF cladded Inconel 690 on 316 L (see Fig. 4 for substrate microstructure).

Fig. 7. OM image for aged (50 h), vertical plane EPBF fabricated Inconel 690 showing an overview of columnar grains with various precipitate densities in the elongated grain boundaries in the build direction, B. Dotted circles show random, high energy grain boundary segments with high precipitate density.

Fig. 8. Magnified views of columnar grains in Fig. 7 showing grain boundary and matrix precipitation (a). (b) Magnified region comparing low carbide precipitation along a low angle grain boundary at left and high precipitate density along irregular and high-energy grain boundary portions at right. Homogeneous precipitation within the surrounding grain matrices is noted in (b).

Fig. 9. OM images showing columnar grain structures in the horizontal plane (perpendicular to the build direction) exhibiting an essentially equiaxed grain structure after aging the EPBF fabricated Inconel 690 cladding. (a) Grain boundary and homogeneous matrix precipitation. (b) Magnified view of regions of dense grain boundary precipitation and Cr depletion (arrows).

Fig. 10. Magnified OM image area similar to Fig. 9(b) showing apparent low-energy grain boundary segments devoid of precipitates in contrast to high density precipitation in other, proximate grain boundary segments.

Fig. 11. Comparison of OM images for aged, EPBF-fabricated Inconel 690 cladding (in the vertical plane) (a), and aged, wrought 316 L stainless-steel substrate (b). Note the absence of coherent {111} annealing twins in (a) and the complete absence of precipitation along the {111} coherent twin boundaries (tb) in contrast to precipitation in noncoherent twin boundaries (TB).

Fig. 12. XRD spectra (a and b) for EPBF- fabricated Inconel 690 in the vertical and horizontal plane, respectively. Peaks are indexed for fcc-γ; a = 0.36 nm.

Fig. 13. XRD spectra (a and b) for aged, EPBF- fabricated Inconel 690 in the vertical and horizontal plane, respectively.

Fig. 14. XRD spectra (a and b) for wrought 316 L stainless-steel and aged, wrought 316 L stainless-steel, respectively.

Fig. 15. TEM (bright-field) images and SAED patterns for EPBF-fabricated Inconel 690 cladding (vertical plane section). (a) Grain boundary (GB) segment separating grains indicated A and B. (b) and (c) show corresponding SAED patterns for A and B in (a). The operating reflection, g, is [020]. (d) Grain boundary (GB) with right grain exhibiting dislocation traces and chromium carbide precipitates. Boundary portion is coincident with [020] γ direction parallel to the build direction, and columnar grain boundaries shown typically in Fig. 5. (e) Enlarged precipitate image at B in (d). (f) SAED pattern for the right grain in (d). The traces of dislocations in (d) are shown by trace direction, “t”, representing the [02 2ˉ] direction for (111) planes.

Fig. 16. TEM images for EPBF-fabricated Inconel 690 cladding (continued). (a) Dislocation traces (t) along [022] in Fig. 15 (d) and (e). (b) Chromium carbide (Cr23C6) precipitates in an area tilted for no dislocation contrast. (c) and (d) Grain boundary (GB) segments. (c) Heavy dislocation tangles in upper grain. (d) Carbide precipitates (P) in grain boundary (GB) segment.

Fig. 17. TEM (bright-field) images for EPBF-fabricated and aged Inconel 690 cladding viewed in the horizontal plane, corresponding to Fig. 9(a). (a) Chromium carbides in a grain boundary (GB). (b) Distribution of matrix carbides with essentially no precipitation in low-angle grain boundaries (arrows). (c) Carbides in (110) grain (SAED pattern inset). Note general absence of subgrain boundary carbides as in (b) above. (d) Homogeneous carbide distribution within a columnar grain transverse plane. (e) Typical area having high dislocation density. Note boundary images. Low dislocation density grain section.

Fig. 18. TEM (bright-field) image showing strongly diffracting Cr23C6 carbide crystal (left). SAED pattern for this crystal, with many carbide diffraction spots, is shown at right. The fcc (γ) 690 alloy matrix operating reflection is shown at [020]. Cr23C6 carbide reflections are indexed (1) and (2): [3ˉ 31] and [0120], respectively. The fcc alloy matrix orientation is [100]. The Cr23C6 crystal orientation is [103].

Fig. 19. Stress-strain plots for EPBF -fabricated Inconel 690, EPBF Inconel 690 cladding/316 L stainless-steel transition, and wrought 316 L stainless-steel.

Table 2 Average Mechanical Properties for EPBF Fabricated Inconel 690, Wrought 316 L and Cladded EPBF Inconel 690 on Wrought 316 L.

Specimen	UTS (MPa)	YS (MPa)	Elongation (%)	HV (GPa)
EPBF Inconel 690	670 ± 44.5	527 ± 19	21 ± 2.0	2.33 ± 0.12
Wrought 316 L	620 ± 4.5	327 ± 10	53 ± 0.8	1.78 ± 0.04
EPBF Inconel 690 - Wrought 316 L	603 ± 34.0	377 ± 39	23 ± 8.0	N/A

Fig. 20. Vickers hardness profile starting at the EPBF cladded Inconel 690 (left) and ending with the wrought 316 stainless-steel. Dashed lines represent the averages.

Fig. 21. SEM fracture surface images of wrought 316 L stainless-steel (a), EPBF Inconel 690 (b) and EPBF cladded Inconel 690/wrought 316 L stainless-steel transition (c).

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