Formation mechanism of aluminide diffusion coatings on Ti and Ti-6Al-4V alloy at the early stages of deposition by pack cementation

Abstract

The diffusion coatings were deposited on commercially pure Ti and Ti-6Al-4V alloy at up to 1000 ffiC for up to 10 h using the pack cementation method. The pack powders consisted of 4 wt% Al (Al reservoir) and 4 wt% NH4Cl (activator) which were balanced with Al2O3 (inert filler). The growth kinetics of coatings were gravimetrically measured by a high precision balance. The aluminised specimens were characterised by means of scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and X-ray diffraction (XRD). At the early stages of deposition, a TiO2 (rutile) scale, other than aluminide coating, was developed on both materials at <900 ffiC. As the experimental temperature arose above 900 ffiC, the rutile layer became unstable and reduced to the low oxidation state of Ti oxides. When the temperature increased to 1000 ffiC, the TiO2 scale dissociated almost completely and the aluminide coating began to develop. After a triple-layered coating was generated, the coating growth was governed by the outward migration of Ti species from the substrates and obeyed the parabolic law. The coating formed consisted of an outer layer of Al3Ti, a mid-layer of Al2Ti and an inner layer of AlTi. The outer layer of Al3Ti dominated the thickness of the aluminide coating.