NiAl-Cr and NiAl-Cr(Mo) eutectic in-situ composites are promising high temperature materials due to a high melting point, excellent oxidation properties and their low density. To enhance the poor room temperature fracture toughness of these composites high cooling rates are beneficial to obtain fine cellular – lamellar structure. The aim of this research is to characterize the microstructure and mechanical properties of a NiAl-28Cr(6Mo) eutectic in-situ composite processed by electron beam melting. This process provides very high solidification rates that lead to a very fine cellular-lamellar microstructure on the nanometer scale. This approach of enhancing hardness, fracture toughness and creep resistance of these functional interfaces is analyzed using REM and FIB as well as APT technique.
The fracture toughness was determined using FIB milled micro cantilevers tested with in-situ micromanipulator or ex-situ nanoindenter experiments. Cantilevers have been oriented in different lamellae and cellular directions. Fracture surfaces where analyzed to get information of toughness increasing mechanisms like crack bridging, renucleation or deflection at NiAl - Cr(Mo) interfaces. The preferred crack propagation in the cellular structure is shown using a FIB-tomography. The extremely high microhardness of the microstructure, determined by nanoindentation, was successfully attributed to the increase of interfaces according to the Hall-Petch mechanism. The atom probe measurement revealed a break off from the strict lamellar structure. This random arrangement of lamellae and rod-like structures showed weak crack resisting properties and still causes brittle fracture behavior. New alloy compositions will be created in the future to stabilize the lamellar structure even at highest cooling rates.