Effect of niobium additions on the fracture of Ni-19Si-based alloys

The mechanical properties of Ni-19Si polycrystals alloyed with niobium and boron were investigated by microhardness and tensile tests. The microstructure change caused by the addition of niobium was also examined by X-ray diffraction, high temperature differential scanning calorimetry, scanning electron microscopy and transmission electron microscopy. The results reveal that the cast Ni-19Si-xNb alloys consist of two phases, α (f.c.c.) and β(L1₂). The addition of niobium, which substitutes for silicon in these alloys, will decrease their melting points, but increase their hardness and yield strengths. This strengthening effect arises owing to the lattice expansion of the β phase matrix by the addition of niobium and the hardening by the hard niobium-rich precipitates in the β matrix. In addition, it appears that with 5 at.% Nb, the β phase formed directly from the melt, instead of a liquid-solid or solid-solid reaction. The fracture behavior of the Ni-19Si-3.5Nb alloy with boron doping also exhibits significant ductility improvement.