Nanostructured NiAl-(Cr,Mo) In-Situ Composites Processed by Selective Electron Beam Melting

The intermetallic B2 phase NiAl possesses promising properties as a high-temperature material with a very high melting point, low density, good thermal conductivity, and excellent oxidation resistance. However, the high-temperature strength and the room temperature fracture toughness show poor performances which disqualify the usage of single-phase NiAl for structural applications as a high-temperature material. However, the addition of 28 at. % Cr and 6 at. % Mo to NiAl leads to an eutectic alloy, which forms a two-phase rod-like or lamellar eutectic microstructure during directional solidification. These in-situ composites show superior mechanical properties over single-phase NiAl. New additive manufacturing methods, such as selective electron beam melting (SEBM) provides very high cooling rates which produces nanostructured composites with very small lamellar spacings between the B2-NiAl and the bcc Cr solid solution. The room temperature fracture toughness of NiAl-(Cr,Mo) composites benefits from a smaller lamellar spacing and the creep strength can be superior to the creep strength of TiAl-alloys.<br/>In this study, dense and crack-free specimens of eutectic NiAl-(Cr,Mo) in-situ composites have been processed via selective electron beam melting using an Arcam A2 machine. In dependence on the processing parameters, a lamellar network-like microstructure can be obtained. For alloys with a lower Mo content, also rod-like structures introduced by discontinuous precipitation can be found in the additively manufactured composites. These discontinuous precipitations are triggered by an in-situ heat treatment in the building chamber or an ex-situ heat treatment. The different types of microstructures are investigated by their mechanical properties, microstructure, and chemical composition. APT investigations are used to further analyze and understand the formation of the nanostructured phases. The high-temperature properties are analyzed by compression and creep experiments, whereby the post-creep defect structure is further investigated by TEM to understand the deformation mechanisms. Furthermore, the room temperature fracture toughness of the different morphologies is studied by testing FIB-milled micro bending cantilevers. The best creep results and high-temperature flow stress are shown by the network-like composites, while the lamellar composites show superior fracture toughness. Consequently, the correlation between microstructure and mechanical properties requires a trade-off between room temperature- and high-temperature properties.