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

Abstract

Abstract 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 2.5×10 21 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 SiO 2 and Al 2 O 3 in the scale during the initial periods of oxidation. This initially formed SiO 2 layer seems to have resulted in the enrichment of Al 2 O 3 beneath it. The influence of acceleration voltage is small and the three-step implantation is not so effective.