Revealing 3D Strain and Carbide Architectures in Additively Manufactured Ni Superalloys

Abstract

Fast directional solidification during Laser Additive Manufacturing (LAM) produces a complex microstructure in nickel-based superalloys, comprising columnar grains with cellular sub-grain structures and carbides. Using non-destructive Scanning 3D X-ray Diffraction (S3DXRD), we reveal spatially complex orientation and intergranular strain relationships that couple strongly to processing-induced cellular sub-grain networks and a primary cubic metal carbide (MC) phase. We have examined 3D orientation and elastic strain tensor fields across 82 $γ$ grains together with the spatial distribution of over 37,000 MC carbides in an ABD-900AM alloy sample manufactured by the Directed Energy Deposition (DED) LAM process. Carbides are spatially associated with the cellular sub-grain network with a weak but present orientation relationship with their parent $γ$ grains. The MC carbides, known to be Ti, Ta and Nb rich, form in regions of high solute segregation, resulting in a significant volumetric lattice parameter patterning in the associated $γ$ phase regions. These chemically distinct solute-rich regions possess a higher associated elastic modulus compared to intercellular regions and determine the local residual stress patterning. These results provide the first non-destructive 3D study of the relationship between rapid solidification-induced segregation, deformation heterogeneity and carbide architectures in an additively manufactured Ni-based superalloy. The insights provide crucial detail to rationalise LAM process parameter optimisation and the coupled spatially governed structural performance.

Figures (4)

• Figure 1: Correlative SEM and S3DXRD of Ni-based superalloy microstructure comprising a $\gamma$ matrix and primary MC carbides. (a, d) Backscatter electron micrographs of a sectioned sample with build direction parallel to the labelled $z$-axis, at low (a) and high (d) magnifications. Annotated features include an example MC carbide circled in red in (a) and sub-grain cellular structures highlighted in white in (d). (b) Sample mounted on Nanoscope diffractometer (ID11, ESRF) showing stage names and the laboratory coordinate system. (c) Reconstructed S3DXRD volume, $\gamma$ phase, in IPF-$Z$ colouring. (f) Single $\gamma$ grain with two dendrites (orange and magenta) isolated from (c), two perspectives. (e) Top $Z$-layer of $\gamma$ grain from (f), diffracted intensity colouring (left) with positions of MC overlaid in red (right).
• Figure 2: Unit cell length fields of the $\gamma$ matrix reveal cellular sub-grain structures, via hydrostatic strain magnitude. evident when viewed as (a) a single $Z$ layer, $\gamma$ phase, with the grain boundaries (GB) and a grain of interest (GOI) indicated. In (b), a magnified single grain is shown, and (c), the GOI viewed along the transverse to the vertical dashed line in (a). The inhomogeneity of the solutes is evident in (d) with an EDS map of the Ti concentration (wt.%) as an example element, showing the concentration field matching the length scale of the hydrostatic strain modulation.
• Figure 3: Spatial and orientation distributions of MC carbides. (a) Single $Z$ layer with hydrostatic strain colouring for the $\gamma$ phase with the grain boundaries (GB) highlighted. The MC carbide positions are overlaid in red (when near to a GB) and black (otherwise). (b) Pole figures of misorientation density functions for the MC carbides with respect to their parent $\gamma$ grains, coloured by multiples of random distribution (MRD). (c) Fraction of MC carbides (all layers) located within 1 of grain boundaries (MC) versus the expected distribution for random points (Random). The $Z$-score indicates the number of standard deviations between the random and observed distributions.
• Figure 4: Proposed residual stress formation mechanism. Hydrostatic stress field (a), combined with intragranular stress field (b), yields proposed von-Mises stress field (c) which is compared with observation (d), a grain-of-interest from a full 2D slice (e) of the sample volume.