The NiAl B2 intermetallic phase exhibits potential as a high-temperature material owing to its high melting point, low density, excellent thermal conductivity, and outstanding oxidation resistance. However, its high-temperature strength and fracture toughness at room temperature are poor, which disqualifies single-phase NiAl for structural applications as a high-temperature material. However, the addition of 28 at. % Cr and 6 at. % Mo to NiAl results in a eutectic alloy that forms a two-phase, rod- or lamellar eutectic microstructure with significantly improved mechanical properties with directional solidification. Additive manufacturing techniques such as electron beam powder bed fusion (PBF-EB) allow very high cooling rates, resulting in nanostructured composites with very small lamellae spacing between the B2 NiAl and the bcc-Cr solid solution [1]. This could potentially increase the fracture toughness at room temperature due to the high density of interfaces.

A new alloy composition (Ni_30.6Al₃₆Cr_31.4Mo₂) was developed to reduce the solidification interval by deviating from the stoichiometric NiAl composition. The alloy exhibits a fine lamellar microstructure in cast specimens. However, in PBF-EB manufactured specimens, the microstructure consists of nanostructured interconnected cells, as well as discontinuous coarsened areas with a rod-like morphology of the reinforcement phase.

In this study, dense and crack-free specimens of eutectic NiAl-(Cr,Mo) in-situ composites were processed by PBF-EB. The degree of discontinuous coarsening can be controlled by adjusting the process parameters. As a result, the morphology of the sample can range from equiaxed to columnar. This study focuses on the effect of discontinuous coarsening on the mechanical properties of PBF-EB fabricated samples. The effect of this microstructural change is investigated by fixed strain rate compression and creep experiments. Subsequently, the resulting defect structure is analyzed using conventional TEM. With a particular focus on anisotropic mechanical properties, the additively manufactured specimens are compared with arc-melted and directionally solidified samples.

[1] A. Förner et. al.; Ad Eng Mater, 2023, 25, 2300407.