Effect of Ti and Al addition on solidification structure of Ni-Fe-Mo-Cu alloys

The effect of the addition of Ti and Al on the solidification structure of Ni-4%Mo-5%Cu-13%Fe (mass%) alloys was investigated. It was found that the addition of an optimal amount of both Ti and Al affects on the formation of equiaxed fine grains. Oxide films formed on the molten alloy surface (scum) were examined using a transmission electron microscope equipped with energy dispersive spectrometers. The scum was analyzed and found to contain two separated phases of TiO with the monoclinic structure and Al ₂O ₃ with the corundum structure. The TiO might be the NaCl structure at the melting point of the matrix phase. According to the lattice disregistry between the fcc Ni and oxides TiO with the NaCl structure has lower disregistry than Al ₂O ₃ with the corundum structure. These facts suggest that TiO precipitated with Al ₂O ₃ in Ni melt can form heterogeneous nucleation sites for the fcc Ni phase. The distribution conditions of TiO may be different between the Ti-added alloys and those with the addition of both Al and Ti. It is considered that the addition of both Al and Ti causes fine dispersion of TiO and promotes nucleation of the primary fcc Ni phase.