Structure observations by high-resolution electron microscopy of Ni-B melt-spun alloys (B<30 at%)

Abstract

The microstructure of a series of Ni-B alloys (15 at%<B<30 at%) rapidly quenched by melt spinning has been investigated by means of conventional transmission and high-resolution electron microscopy (HREM), and analysed by electron energy loss spectroscopy (EELS) adapted on a scanning transmission electron microscope. A metastable phase is observed in the composition range 21 to 28 at% B where the orthorhombic Ni₃B phase (a=0.44 nm,b=0.52 nm,c=0.66 nm) is expected. High-resolution electron microscopy images of Ni-25 at% B rapidly quenched indicate that a two-phase structure is observed. A metastable phase is observed with a periodicity of 1.0 nm. HREM images and EELS analyses led us to propose the structure of this metastable phase. This phase (Ni₅B₂) is correlated with the monoclinic Hägg carbide Fe₅C₂. Depending on the velocity of the substrate and on the boron concentration, a variety of microstructures has been observed from a faulted to a semi-amorphous structure. The structures are related to the solidification behaviour and the heat flow in the undercooled melt. Particular attention was paid to the eutectic Ni-Ni₃B composition.