Effects of boron and phosphorous on nano-amorphous Ni-B and Ni-P alloys studied by x-ray spectroscopy

Both XAFS and XRD techniques have been used to investigate the effects of boron and phosphorous in nano-amorphous Ni-B and Ni-P alloys annealed at various temperatures. The XAFS and XRD results indicate that the initial Ni-B and Ni-P samples are amorphous alloys with large disorder. The static disorder factor (σ_S) of the nearest Ni-Ni subshell is 0.33 and 0.28 Å for the initial Ni-B and Ni-P alloys, respectively. Under an annealing temperature of 573 K, the nano-amorphous Ni-B alloy is crystallized into mixed phases of a crystalline Ni₃B and a metastable intermediate with smaller grain size and metallic Ni-like structure whose average grain size is about 30 nm, and the nano-amorphous Ni-P alloy forms two phases of crystalline Ni and Ni₃P with an average grain size of about 120 nm. It means that boron can retard the growth of Ni-B alloy into particles with larger size, in comparison with phosphorous. When the annealing temperature is increased to 773 K, most of the nano-crystalline Ni₃B is decomposed into crystalline Ni with larger grain size, but crystalline Ni₃P is rather stable. The XAFS results further show that the local structure around Ni atoms in the Ni-B sample annealed at 773 K is almost the same as that of Ni foil. However, the static disorder σ_S (0.125 Å) of the Ni-P sample annealed at 773 K is still much larger, which is attributed to the existence of some amount of crystalline Ni₃P. These results suggest that there is a great difference in the crystallization mechanism between nano-amorphous Ni-B and Ni-P alloys.