Influence of silicon ion implantation and post-implantation annealing on the oxidation behaviour of TiAl under thermal cycle conditions

TiAl coupon specimens measuring 15×10×1 in mm were implanted with Si ions at varying acceleration voltages ranging from 80 to 260 keV with a constant dose of 2.5×1021 ions·m−2. In addition, three-step implantation was performed to obtain a wider and flat distribution of Si along a depth direction. Their oxidation resistance was assessed by a cyclic oxidation test with temperature varying between room temperature and 1200 K in a flow of purified oxygen under atmospheric pressure. The holding time at temperature was 72 ks (20 h). Conventional metallographic examinations were performed for implanted specimens and oxidised specimens using X-ray diffractometry, glancing angle X-ray diffractometry, AES, SEM and EDS. The implantation under all the conditions examined does not improve the oxidation resistance. The scales on all the specimens partially spall after a few oxidation cycles. The post-implantation annealing at 800 K for 600 s in an Ar atmosphere enhances the oxidation with the formation of scales spalling earlier. However, the vacuum annealing at 1100 K for 3.6 ks decreases the oxidation rates and improves the scale adherence. The best oxidation resistance is obtained by vacuum annealing at 1100 K for 36 ks. The scales are very adherent and oxidation becomes very slow. This excellent oxidation resistance is attributable to the formation of a layer consisting mainly of SiO2 and Al2O3 in the scale during the initial periods of oxidation. This initially formed SiO2 layer seems to have resulted in the enrichment of Al2O3 beneath it. The influence of acceleration voltage is small and the three-step implantation is not so effective.