Tailoring microstructure via heat treatment to improve the corrosion resistance of directed energy deposited nickel-aluminum bronze alloy

Abstract

The microstructure and corrosion resistance of nickel-aluminum bronze (NAB) alloy prepared by directed energy deposition (DED) and various heat treatments after DED were investigated. The results show that the microstructure of as-built NAB alloy consists of the α-phase matrix and various κ-phases (κⅡ∼κⅣ). The coarse κⅡ-κⅢ phases and needle-like κⅤ phase are observed in the annealed samples. In comparison, only the α-phase matrix, high-temperature residual β′-phase and a small amount of κⅡ-phase are found in quenched samples. When the quenched sample is aged, the unstable martensite β′-phase transforms into the globular discontinuous κⅢ phase. The annealing treatment induces a strengthened (111) texture, while quenched and aged samples exhibit minimal anisotropy. The corrosion morphology observation releases the obvious cracks presented in the passive film of the annealed sample, and each phase is severely corroded, while the microstructures of other samples are well preserved. Mitigating micro-galvanic effects and preventing corrosion crack formation in quenched samples has been attributed to the reduction of κ-phases. The corrosion resistance of the as-built NAB alloy is greatly improved via a quenching process at 900 °C.