Atom probe tomography study of κ-phases in additively manufactured nickel aluminum bronze in as-built and heat-treated conditions

Abstract

The distribution, morphology, and chemical composition of various κ-phases present in the wire-arc additive manufactured (WAAM) nickel aluminum bronze (NAB) of nominal composition Cu-9Al-4Ni-4Fe-1Mn (wt%) was investigated in the as-built and three different heat-treated conditions (350 °C for 2 h, 550 °C for 4 h, and 675 °C for 6 h). The precipitation of intermetallic κ-phases plays a crucial role in determining the mechanical and corrosion properties. The microstructural changes were analyzed using scanning electron microscopy (SEM), transmission electron microscopy (TEM) with energy dispersive spectroscopy (EDS), and complemented with atom probe tomography (APT) at the nano-scale. The as-built microstructure consists of copper-rich α, κII (globular Fe3Al) and κIII (lamellar NiAl) phases in the interdendritic regions, and nano-scale Fe-rich κIV particles (5–10 nm) in the Cu-matrix. Heat-treatment at 350 °C for 2 h (HT-1) has not produced any significant microstructural changes. When heat-treated at 550 °C for 4 h (HT-2), a new-phase needle-like κv (NiAl based) was formed, which differs from other κ-phases in morphology. After HT-2, globular κII was coarsened, lamellar κIII was partially spheroidized, and κIV precipitation in the matrix was reduced. Under conditions of 675 °C for 6 h (HT-3), globular κII and needle-like κv were coarsened, lamellar κIII was completely spheroidized, and the amount of κIV was significantly reduced.