Boron Effect on Solidification in the Two Phase (NiAl+Ni3Al) Alloy

It was found that addition of boron improved room temperature fracture strength by formation of Ni 3 Al(γ') thin layer around the NiAl(β) phase after heat-treatment in the NiAl+Ni 3 Al two phase alloy. Directional solidification technique with quenching of the solidifying interface has been applied to understand the mechanism of formation of the γ' layer around the β phase. The quenched solid/liquid interface showed that solidification ended up with γ phase at the β interdendritic region in the boron added alloy, while the γ' phase formed at the interdendritic region in the binary alloy. It is thought that the γ' phase formed around the β dendritic phase grows between γ and β phases by a diffusion couple manner in the boron added alloy. It was also interesting to find that boron addition changed the phase field and solidification behavior.