Pack cementation coatings on Ti 3Al–Nb alloys to modify the high-temperature oxidation properties

Abstract

Ti 3Al-based alloys have excellent high-temperature strength, but their poor oxidation resistance restricts the applications of these alloys at high temperature. In this study the microstructure, oxidation resistance, and mechanical properties of pack cementation Al, Si, and Cr coatings on Ti 3Al-based alloys, i.e. Ti–25Al–10Nb and Ti–25Al–13Nb, were investigated and discussed. The morphology of aluminized coating is a single layer of the TiAl 3 phase. The siliconized coating shows a multilayer structure with the composition of TiSi 2, TiSi, TiAl 2 and TiAl from the outer surface to the substrate. The chromized coating is a Cr-rich beta phase. The high-temperature oxidation resistance of Al and Si coatings is quite superior, and their temperature limit of oxidation resistance is up to 1100°C. In contrast to Al and Si coatings, the oxidation resistance of the chromized coating is poor, since it is unable to improve the oxidation resistance of the Ti–25Al–10Nb alloy. The high-temperature tensile test of Ti–25Al–13Nb with Al or Si coatings shows that the hard coating layer may reduce the ductility of the base alloy. The alloy with the Al or Si coating is suitable for applications which operate at high temperatures but low stress levels.