Effect of Nb Addition on Oxide Formation on Ti–xNb Alloys

Abstract

It had been reported that Ti29Nb13Ta4.6Zr (TNTZ) alloy forms a dense oxide layer by high-temperature 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 TiO 2 and TiNb 2 O 7 . 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 high-oxidation 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 rutile-type TiO 2 was identified from 1 mol%Nb up to 10 mol%Nb, then both TiO 2 and TiNb 2 O 7 were formed from 13 mol%Nb to 32 mol%Nb. These results indicate that dense oxide layer formation attributes to phase separation from TiO 2 to TiNb 2 O 7 . 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 rate-determining process of the dense oxide layer formation. [

Exfoliation Resistance, Microstructure, and Oxide Formation Mechanisms of the White Oxide Layer on CP Ti and Ti–Nb–Ta–Zr Alloys

“…Such a remarkable change in the exfoliation resistance due to the difference in the oxide structure was confirmed in our previous study [ 22 ], which investigated the effect of the Nb content on the oxide structure and exfoliation resistance of Ti-Nb alloys. Therefore, the apparent difference in the exfoliation stress of the oxide layer formed on the Ti-Nb-Ta-Zr alloy or CP Ti can be qualitatively attributed to the reasons mentioned in previous paragraphs.…”

“…From the results, it is hypothesized that the slower diffusion rates of alloying elements, such as Nb, Ta, and Zr, affected the oxide formation mechanism, and that, as a consequence, the layer thickness and morphology changed. According to our previous study on the oxidation behavior of Ti-Nb alloys [ 22 ], the layer thickness growth rate of the oxide followed a parabolic function, and the activation energy E of the layer thickness growth obtained from it increased with increasing Nb content. The E value approached the E value of the diffusion of Ti and Nb in Ti-Nb from that of O in Ti.…”

“…The addition of Nb to Ti reportedly increases oxidation resistance [ 22 , 28 , 29 , 30 ]. The following two reasons have been proposed for the suppression of oxidation by the addition of Nb to Ti: (1) reduction in O vacancies in the oxide by the principle of valence control; (2) Nb addition enhances the formation of the titanium nitride layer near the oxide/metal interface, which acts as a diffusion barrier and suppresses O diffusion.…”

“…The following two reasons have been proposed for the suppression of oxidation by the addition of Nb to Ti: (1) reduction in O vacancies in the oxide by the principle of valence control; (2) Nb addition enhances the formation of the titanium nitride layer near the oxide/metal interface, which acts as a diffusion barrier and suppresses O diffusion. Ogawa and Miura-Fujiwara [ 22 ] concluded that the reduction in O vacancies in the oxide by (1) is more dominant than that by (2) in the densification of the TiO 2 layer microstructure by Nb addition. Furthermore, in the case of Ti-Nb eutectic oxide precipitation, the densification is due to the suppression of outward diffusion of Ti by Nb at the reaction interface and the simultaneous redistribution of Nb into the TiO 2 and TiNb 2 O 7 phases during oxide precipitation.…”

Effect of Nb addition on high-temperature oxidation behavior, oxide layer structure, and its exfoliation resistance of Ti-Nb Alloys

“…9 shows HAADF images of the cross-section of oxidized Ti-13 mol%Nb and Ti-20 mol%Nb alloy. Exfoliation stress of the oxide layer on Ti-13 mol%Nb and Ti-20 mol%Nb was about 4 MPa and 70 MPa, respectively [13]. In both alloys, highly continuous interfaces without zonal layer and fine voids were observed, while in Ti-13 mol%Nb contained the dispersion of voids of several tens nm size in the oxide.…”