Selective laser melting (SLM) is an additive manufacturing technique where successive laser beam passes are used to melt metal powder which forms a solid layer on solidification with high densification, little material waste, and large design freedom [1]. The application of SLM to repair high temperature components that often need reconditioning requires an understanding of the microstructural and compositional developments of the chosen material throughout the SLM process. Alloy 718 is a Ni-Cr-Fe-Nb-Ti-Al alloy used in applications where high strength is needed while maintaining corrosion and creep resistance, making this alloy a prime candidate for SLM structural and compositional characterization. Precipitation hardening is one of the primary strengthening mechanisms of Alloy 718, where intermetallic phases of L1₂-ordered Ni₃₍Al, Ti) (γ’) or D2₀₀-ordered Ni₃(Nb, Ti) (γ’’) may form coherent precipitate particles in the face-centered cubic matrix (γ) [2]. Additional phases that may be present in the microstructure of Alloy 718 include D0_a-ordered Ni₃Nb (δ), MC, M₆C, M₂₃C₆, and (Ni, Cr, Fe)₂(Nb, Mo, Ti) Laves [3, 4]. The complex microstructures in this alloy system are further complicated by the multiple heating and cooling cycles present in the SLM process, thus requiring characterization on the nanoscale in order to understand the microstructural development during processing.                           

Analytical electron microscopy allows the identification of the particular microstructural components on the micro and nano scales.  Alloy 718 is of particular interest in that the γ’ precipitates can nucleate on the (001) surface of γ’’ precipitates in the as-SLM condition. The structure and chemical composition of these precipitates was investigated through X-ray energy dispersive spectrometry (XEDS) using an aberration-corrected FEI Titan G2 ChemiSTEM equipped with the Super X EDX X-ray detector configuration. Figure 1 shows a γ’’ precipitate with γ’ precipitates on the two elongated sides of the γ’’ in both scanning transmission electron microscopy (STEM) bright-field (BF) and high-angle annular dark-field (HAADF) imaging modes. The understanding the formation of γ’/γ’’ requires both structural information about the interface and chemical analysis across the interface of the two precipitates. Figure 2 displays 4 XEDS spectrum images of Ni, Nb, Ti, and Al showing the location of these elements throughout the precipitates present in the γ matrix. Quantitative XEDS analysis was performed on an as printed Alloy 718 specimen where the γ matrix composition was found to be 50.5 wt % Ni, 1.4 wt % Nb, 0.3 wt % Al, and 0.09 wt % Ti with γ’ and γ’’ having compositions of 66.6 wt % Ni, 7.13 wt % Nb,  3.18 wt % Ti, and 2.4 wt % Al and 65.0 wt % Ni, 25.4 wt % Nb, 0.37 wt % Al and 3.6 wt % Ti, respectively.             

References:

[1] Gu, D.D., Meiners, W., Wissenbach, K., Poprawe, R., Int. Mater. Rev., 2012, 57, 133-164.

[2] Azadian, S., Wei, L.Y., and Warren, R., Mater. Charact., 2004, 53, 7.

[3] Burke, M.G. and Miller, M.K., J. de Physique, 1989, 50, C8 395-400.

[4] Rama, J.G.D., Reddya, A.V., Raob, K.P., Reddyc, G.M., Sundar, J.K.S., J. Mater. Process. Tech., 2005, 167, 73.

Figures:

Figure 1 - STEM images of γ’’ particle in γ matrix with γ’ present on the sides of the (001) γ’’ surface shown in (a) BF-STEM and (b) HAADF-STEM.

Figure 2 – XED specrum images of (a) Ni, (b) Nb, (c), Ti, and (d) Al showing the enrichment of Ni and Ti in both γ’ and γ’’ compared to the matrix γ as well as increased Al in the γ’ and increased Nb in the γ’’.

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EMC Abstracts - https://emc-proceedings.com/abstract/improved-quantitative-compositional-analysis-of-%ce%b3-and-%ce%b3-in-additively-manufactured-alloy-718-using-stem-x-ray-energy-dispersive-spectrometry/