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

Pack aluminization - a chemical vapor deposition process widely used to form protective coatings on Ni-based superalloy components - was used to form shells of Ni2Al3, NiAl and/or γ′-Ni3Al 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 Ni2Al3) 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.