Microstructures, Compressive Properties, and Microhardness of NiAl-Cr(Mo) Eutectic Alloys With Various Ni Contents

The microstructures and mechanical properties of 66(NixAl)-28Cr-6Mo (x=1.0, 1.5, 2.0, 2.5, 3.0, and 3.5) alloys were investigated using scanning electron microscopy, energy dispersive spectroscopy, transmission electron microscope, microhardness, and compression tests. The microstructure of NiAl-28Cr-6Mo (Ni-1.0) eutectic alloy consists of NiAl and Cr(Mo) phases. With increasing the Ni content to 2.0, the microstructure changes from eutectic (Ni-1.0) to eutectic+primary NiAl dendrite (Ni-1.5 and Ni-2.0), and the morphologies of part of precipitates in primary NiAl dendrite evolve from granular to needle-like. When the Ni content increases further, besides eutectic and primary NiAl dendrite, the gray phase forms and is identified as an ordered FCC (L1(2)) (Ni,Cr)(3)(Al,Mo) phase. Moreover, the more needle-like precipitates emerge in the primary NiAl dendrite of Ni-2.5, Ni-3.0, and Ni-3.5 alloys, and the precipitate is identified as a bcc Cr(Mo) phase. The deep etching reveals that the three-dimensional morphology of Cr(Mo) precipitate is not needle-like but lath-like. Among the investigated alloys, both Ni-2.0 and Ni-2.5 alloys possess the higher fracture strength and microhardness. The relevant strengthening mechanisms are discussed.