Effects of boron on microstructure in cast zirconium alloys

Trace additions of boron to cast zirconium result in significant microstructural changes similar to those observed with additions of boron to titanium alloys. These changes include the promotion of dendritic growth and a refinement in both the prior beta and alpha grain size. The refinement of the prior beta grain size is explained using a model of grain refinement in association with values calculated from the binary Zr-B phase diagram. It is proposed that the refinement of the alpha phase occurs through a combination of increased nucleation and altered diffusion mechanisms during cooling through the beta transus.