Microstructure development during pack aluminization of nickel and nickel–chromium wires

Abstract Pack aluminization – a chemical vapor deposition process widely used to form protective coatings on Ni-based superalloy components – was used to form shells of Ni 2 Al 3 , NiAl and/or γ′-Ni 3 Al on the surface of γ-Ni wires with diameters of 127 μm. The growth kinetics of these Al-rich intermetallic shells are studied as a function of aluminization time and pack activity at 1000 °C. Similar kinetics but additional phases (Cr/Ni two-phase shell, Cr silicide particles and Al-rich particles distributed in Ni 2 Al 3 ) are found in the shells of pack-aluminized Ni–20 wt.% Cr wires with similar diameters. Fully homogenous Ni–Al and Ni–Cr–Al wires are achieved by interdiffusion at 1200 °C between the deposited Al-rich intermetallic shells and the Ni-rich core of both types of wires. Upon subsequent aging at 900 °C, wires with γ/γ′ structure and high hardness indicative of precipitation strengthening are obtained.