Characterization of κ-precipitates in wire-arc additive manufactured nickel aluminum bronze: A combined transmission Kikuchi diffraction and atom probe tomography study

The morphology, size, volume fraction, and distribution of κ-precipitates play a vital role in determining the mechanical and corrosion properties of nickel aluminum bronze (NAB). A square bar of NAB ~16 cm in height (side: 2.5 cm) was built using a wire-arc additive manufacturing (WAAM) technique. This is the first work where the effect of height of deposition on the precipitation of various κ-intermetallic phases is investigated using combined Transmission Kikuchi Diffraction (TKD) and Atom Probe Tomography (APT) techniques. The fast-cooling rates prevalent during the WAAM process produced more refined grains and finer κ-precipitates in the microstructure compared to the cast-NAB. The microstructure in the bottom, middle, and top regions consist of copper-rich α matrix phase, κII (globular Fe3Al) and κIII (lamellar NiAl) phases in the interdendritic regions, and nano-scale Fe-rich clusters constituting κIV precipitates dispersed homogeneously in the α-matrix. The average size of the κIV precipitates is slightly higher (~14.6 nm) in the middle sample, whereas it remained nearly the same (~ 8.5 nm) in the bottom and top samples. The APT cluster analysis indicated no significant difference in the number density, volume fraction, and morphology of κIV precipitates. Overall, a uniform microstructure and precipitate formation was observed along the build direction, demonstrating the WAAM process's feasibility to produce larger NAB parts.