Extraction Replication of Inert Particles in Additively Manufactured 800H Builds

Metal additive manufacturing (AM) technologies have now gained widespread acceptance as flexible approaches to produce high value-added components from many different metals and alloys. However, the microstructural development in such AM builds can be very different from conventionally processed components of the same alloy. These differences can arise from the characteristics of the alloy feedstock, the details of the AM technology used, and the specific process parameters required to build high-quality AM parts [1]. While transmission electron microscopy (TEM) has been an essential tool for the analysis of AM microstructures, certain features can be difficult to study using standard specimens. For example, many builds contain fine particles of metal borides, carbides, nitrides, and oxides. These “inert” particles can form as precipitates, dispersoids or inclusions. When the particles are substantially smaller than the TEM specimen thickness, analysis of such features can be challenging due to interference from the surrounding metallic matrix phases. In the ferrous metals community, inert particles were traditionally prepared for analysis by extraction replication [2], but this approach has seen little attention since the development of focused ion beam (FIB) instruments. More recently, extraction replicas have seen renewed attention for certain alloys with very complex and/or fine distributions of inert particles [3, 4]. We have found extraction replication to be particularly useful for AM builds, and here we illustrate the advantages using data obtained from laser powder bed fusion (LPBF) builds of Alloy 800H. Specimens were prepared from an LPBF build produced under conditions where there was no significant porosity, unmelts, cracking or other coarse inhomogeneities. Microstructural analyses were first performed on a metallographic section through the build using scanning electron microscopy (SEM) in an FEI Verios. It was found that the build exhibited a coarse-grained structure with cellular dendritic segregation within the grains and a distribution of fine inert particles at the cell or dendrite boundaries. More detailed analysis was performed by scanning transmission electron microscopy (STEM) in an FEI Talos F200X operating at an accelerating voltage of 200 kV and equipped with a Super-X silicon drift detector (SDD) energy-dispersive X-ray spectrometry (EDXS) system. Examples of STEM data from a FIB-cut cross-section prepared in an FEI Helios NanoLab 460F1 are shown in Fig. 1. The HAADF images in Figs. 1(a) and (b) reveal the overall structure and the defects/particles at the boundaries, respectively. The EDXS elemental maps in Figs. 1(c) and (d) show that there are Ti-rich particles and Cr segregation at the boundaries, but these features are not resolved clearly. Data from an extraction replication experiment on the same build are shown in Fig. 2. A metallographic section through the build was electropolished using perchloric acid in methanol to bring the inert particles into relief; Fig. 2(a) is a secondary electron SEM image of the resultant surface. The surface was then coated with carbon, and the replica was released by electropolishing using the same solution. Figs. 2(b) and (c) are BF TEM images of the replica on a Cu support grid; the higher magnification image reveals a much more complex population of inert particles than was evident in the FIB-cut section. The coarsest particles resemble those observed in Ti maps such as Fig. 1(c), but there are also finer and more irregularly-shaped particles present. The corresponding EDXS maps (Figs. 2(d) and (e) show that all of the equiaxed/spherical particles are Ti-rich, whereas the irregularly-shaped particles are Cr-rich. Further analysis of these particles by EDXS and electron diffraction showed that they are TiN and Cr23C6, respectively. The character and distribution of these particles give important insights into the development of the microstructure in such builds [5]. (a) HAADF image of a FIB-cut lift-out, and (b) detail of the region in the red box in (a); (c) and (d) EDXS elemental maps showing the distribution of Ti and Cr, respectively. (a) Secondary electron SEM image of an etched build surface with the particles in relief; (b) BF TEM image of a replica on a copper support grid; (c) BF TEM image of extracted inert particles on the carbon replica; (d) and (e) EDXS elemental maps showing the distribution of Ti and Cr, respectively.