High-temperature oxidation behavior and its oxide layer structure formed on Ti-Nb alloys

Abstract

It had been reported that Ti29Nb13Ta4.6Zr（TNTZ）alloy forms a dense oxide layer by hightemperature oxidation whereas CP Ti forms a multilayered oxide consisted of rutile monolayers and void layer. This morphological change is supposed to be mainly caused by Nb addition in Ti since the dense oxide layer of TNTZ consists of multiple oxide phases, at least with rutile TiO2 and TiNb2O7. In this study, hightemperature oxidation at 1273 K for 3.6 ks in the air of TixNb alloys（x＝1, 5, 7, 10, 13, 15, 18, 20, 23, 26, 28, 30 and 32 mol％）was investigated to discuss the effect of Nb addition to Ti on its highoxidation behavior, and on its oxide microstructure. From the results of the SEM observation, an oxide layer with a void layer was formed on TixNb substrate from 1 mol％ Nb up to 10 mol％ Nb. However, densification of the oxide layer was confirmed at Ti13Nb. Then, the dense oxide layer was formed up to 32 mol％ Nb. XRD results indicated that only rutiletype TiO2 was identified from 1 mol％ Nb up to 10 mol％ Nb, then both TiO2 and TiNb2O7 were formed from 13 mol％ Nb to 32 mol％ Nb. These results indicate that dense oxide layer formation attributes to phase separation from TiO2 to TiNb2O7. Until 10 mol％ Nb, the thickness of oxide layer was suppressed by Nb addition, whereas the layer thickness increased with increasing Nb content from 13 mol％ Nb. The maximum exfoliation resistance of the oxide layer was obtained at 20 mol％ Nb. The results of oxide growth rate at each TixNb alloys suggested that Nb diffusion in Ti may ratedetermining process of the dense oxide layer formation.