Homogenization of Porous, Intermetallic Ni₃Al by Additive Manufacturing and In Situ Alloying

Nickel is a common and well-researched catalyst material with a wide variety of industrial applications. However, pure Ni also comes with limitations such as poisoning and sintering at the elevated temperatures required for some catalytic processes. Intermetallic Ni₃Al, by contrast, has been found to exhibit catalytic properties comparable to Ni, while being less prone to both poisoning and sintering. Therefore, we have started to investigate potential approaches to fabricate a porous Ni₃Al catalyst.<br/>To obtain a large surface area as required for catalysis, samples were produced through additive manufacturing. In particular, laser powder bed fusion (LPBF) was used, whereby the printing parameters were optimized for maximum bulk porosity. Furthermore, in order to keep the fabrication route economically competitive, so-called in-situ alloying was used to print the samples. In-situ alloying refers to the procedure of mixing two elemental powders mechanically and creating the alloy during the printing process.<br/>In the case of a nominal chemical composition of 76 at.% Ni and 24 at.% Al, the printing parameters optimized for a porous structure were found to cause an inhomogeneous microstructure with pronounced local variations in the chemical composition. Although the powder particles were fully melted during the printing process, the melt solidified before complete mixing and equilibrium could be reached. Using X-ray diffraction, the phases Ni(Al), Ni₃Al, NiAl, Al₃Ni₅, and Al₃Ni were detected in these as-printed samples. To homogenize the microstructure and increase the phase fraction of Ni₃Al, heat treatments have been conducted. Due to the relatively low melting points of the Al-rich phases, a stepwise heat treatment was designed. At 600 °C, the dissolution of Al-rich phases was investigated, before increasing the temperature to 1000 °C in a second step. To obtain further information about the phase fractions and to trace the phase evolution during different heat treatments, in-situ heating experiments were conducted in a dilatometer setup at the Hereon-run beamline P07B at the Deutsches Elektronen-Synchrotron in Hamburg, Germany. The data collected by means of high-energy X-ray diffraction offer valuable insights into the effect of the various heat treatments on the as-printed samples and allow to draw conclusions as to their final catalytic properties.