Investigations on the solidification microstructures and room temperature compression properties of Nb-doped NiAl-32Cr-6Mo hypereutectic alloy

Solidification microstructures and compression properties of NiAl-32Cr-6Mo hypereutectic alloys containing different contents of Nb were investigated. The addition of Nb did not change the basic phase constituents, though a new phase of Laves Nb(Cr1-x-yNixAly)2 was observed. However, the primary Cr(Mo) phase no longer showed obvious dendritic morphology, the formation of lamellae was disturbed and the eutectic cellular microstructure degenerated. When the Nb content was 5%, eutectic lamellae coarsened and became spherical and poorly aligned. Fully eutectic microstructures were obtained in the directionally solidified hypereutectic alloys grown at 6 µm/s regardless of the addition amount of Nb. With the increasing content of Nb addition, the volume fraction of Laves phase located at the NiAl/Cr(Mo) boundary became more. The yield strength of directionally solidified NiAl-32Cr-6Mo-3Nb alloy reached to 2072 MPa, which is the highest value in the NiAl-based eutectic alloy at present. Well-aligned microstructure and more strengthening phase are conducive to the improvement of properties. All the alloys exhibited brittle quasi-cleavage fracture mode, cleavage in the NiAl phase and debonding of the NiAl/Cr(Mo) interface accompanied by Cr(Mo) lamellae and Cr2Nb phase pulled out were observed in the fracture surface.