The electrochemical behaviour of Ti-48Al-2Cr-2Nb produced by electron beam powder bed fusion process

Abstract

Titanium aluminides (TiAl) are distinguished by their exceptional strength-to-weight ratio, making them ideal for aerospace and medical applications. Notably, TiAl alloys offer a unique combination of high-temperature resistance and corrosion resilience, contributing to their growing prominence in advanced engineering and biomedical fields. Although initially developed for aerospace applications, TiAl alloys have demonstrated promising potential as implant materials over time. Hence, this research focuses on producing γ-TiAl alloy through electron beam powder bed fusion (EB-PBF) technology, utilising a powder with a composition of Ti-48Al-2Cr-2Nb. For comparative purposes, the corrosion characteristics of Ti6Al4V produced via EB-PBF were also evaluated under identical conditions. The findings indicate that the EB-PBF γ-TiAl exhibits exceptional resistance to corrosion. This is supported by the significantly high polarisation resistance and corrosion potential values, as well as the notably low corrosion current value. However, based on the analysis of the polarisation and impedance curves, it can be observed that the γ-TiAl sample displayed a less protective passive film formation. This occurrence can be attributed to the presence of aluminium ions within the passive layer, resulting in the formation of unstable oxides. As a consequence, it can be inferred that γ-TiAl exhibits inferior resistance to pitting corrosion when compared to Ti6Al4V alloy. The point defect model and Mott-Schottky test further revealed that the γ-TiAl alloy exhibited increased oxygen vacancies. Additionally, the presence of aluminium ions as impurities or dopants led to their substitution for titanium ions, creating cationic vacancies within the passive film. The accumulation of excessive cation vacancies ultimately led to the initiation of pitting corrosion.