Identification of second phase precipitates in a nickel-based superalloy fabricated by Laser Powder Bed Fusion (L-PBF)
- Abstract number
- European Microscopy Congress 2020
- Corresponding Email
- [email protected]
- PSA.2 - Metals & Alloys
- Dr. Arthur Despres (2), Dr. Guilhem Martin (2), Pr. Muriel Véron (2), Dr. Edgar F. Rauch (2), Dr Jean-Jacques Blandin (2), Dr. Charlotte Mayer (1)
1. Aubert & Duval, Research and Development Department
2. Univ. Grenoble Alpes, CNRS, Grenoble INP, SIMaP
Additive manufacturing, ASTAR, Nickel superalloy, TEM
- Abstract text
Nickel-based superalloys strengthened by large fraction of γ’-precipitates are prone to hot cracking during welding and fabrication by additive manufacturing. Hot cracking susceptibility is known to depend on the presence of residual elements (B, Zr) in the last fraction of solidifying liquid [1,2,3,4]. In this context, microstructural characterizations are required to understand the distribution of these elements in the as-built microstructure.
In this study, we use TEM to investigate the as-built microstructure of a commercial grade of nickel-based superalloy fabricated by L-PBF. The fast cooling rates occurring during L-PBF, typically 106 K/s induce fine precipitation of secondary phases that justify the utilization of this instrument. Observations are conducted on thin foils of materials that contain the precipitates embedded in the matrix phase. To detect local solute enrichment, EDX maps are acquired. On the same regions of interest, ASTAR maps are carried out to identify the crystallographic phases. ASTAR is an automated technique of diffraction pattern indexing which allows maps of phases and crystallographic orientations to be generated. Here, the recently developed multi-index strategy is implemented in ASTAR to separate the diffraction signal of the matrix from that of the embedded precipitates .
The results allow the precipitates of intermetallic phase Ni7Zr2 grown heterogeneously on (Ti,Nb)C carbides to be identified (Fig. 1). Annular dark-field observations confirm this superposition of precipitates (Fig. 2). Precipitates are located mainly in zones of high solute segregation, i.e. near grain boundaries and in interdendritic zones. The Ni7Zr2 phase is reported as having a low melting point. It has been previously observed in weld lines of similar alloys , but not yet in parts made by additive manufacturing. These results thus inform on the spatial distribution of Zr during solidification and its potential role on hot cracking.
In summary, the combine use of EDX and ASTAR in the TEM allows the precipitate phases and their position relative to solute segregations in nickel based superalloys fabricated by L-PBF to be identified. These results provide insights on the thermal path of the part during solidification and allow adjustments of the nominal composition to be suggested in order to suppress hot cracking.
Figure 1: (Left) Combined phase and correlation index maps in conventional ASTAR. (Center) Map of phases after substraction of the matrix phase diffraction peaks. (Right) example of indexation of the Ni7Zr2 phase. Red circles indicate the template pattern calculated by ASTAR.
Figure 2: Annular dark-field image of nearly the same area as in Figure 1. Composition profile through the large precipitate is shown on the right.
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