Additive manufacturing of nanostructured eutectic NiAl-(Cr,Mo) in-situ composites

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Lightweight NiAl-(Cr,Mo) in-situ composites are interesting candidate materials for high temperature applications in the realm of aerospace engineering and energy conversion. By conventional casting techniques with limited cooling rates relative coarse microstructures are produced with insufficient mechanical properties (e.g. relatively low strength, insufficient fracture toughness and creep strength). However, electron beam assisted additive manufacturing is very attractive to process nanoscale eutectic NiAl-(Cr,Mo) composites. To successfully process such crack free alloys and to tailor the mechanical properties, the alloy composition, the process parameters and the corresponding microstructure have to be optimized.Suitable alloys with exact eutectic compositions will be selected by thermodynamic calculations based on a new database coupled with multi-criterial optimization. Electron beam assisted additive manufacturing simultaneously enables extremely high local temperatures, cooling rates and temperature gradients as well as controllable in-situ heat treatments. This processing route is perfect for preparing nanostructured NiAl-(Cr,Mo) in-situ composites with adjusted microstructures. Additionally, the additively manufactured eutectic alloys show a solid state discontinuous precipitation reaction that has not been reported for NiAl-(Cr,Mo) so far. Sophisticated in-situ heat treatments during electron beam melting are planned to trigger this discontinuous precipitation reaction on purpose to get an ordered and well-aligned lamellar microstructure. For the first time, this discontinuous precipitation will be used in a beneficial way to generate homogeneous lamellar microstructures similar to lamellar Titanium Aluminides. Grain morphology, crystallographic orientation and lattice misfit between both phases will be varied depending on processing parameters and composition of the composite. Highly flexible scan strategies allow tailoring the microstructure towards isotropic as well as anisotropic fine microstructures fully composed of discontinuously precipitated phases which are expected to lead to a significant improvement in material properties. The optimized eutectic NiAl-(Cr,Mo) in-situ composites are expected to reach strength values 50% higher than those of conventionally cast materials and a room temperature fracture toughness of more than 20 MPa∙m1/2, comparable to that of intermetallic TiAl currently used in aerospace industries. It is key to understand the correlation between process strategies during electron beam melting, the associated temperature history and the corresponding microstructure. Therefore, the final aim of this project is to combine all this knowledge in processing maps to link the processing parameters and microstructures with its mechanical properties and present NiAl-(Cr,Mo) as a showcase material for eutectic in-situ composites fabricated by the advanced processing technique of selective electron beam melting.