Metallurgical Abstracts on Light Metals and Alloys vol.55

Analysis of the Oxidation and Nitridation of Ti-17 (Ti-5Al-2Sn-2Zr-4Mo-4Cr) Alloys with Added Si under Atmospheric Heating

Takayuki Narushima^1,2), Satoshi Suzuki¹⁾, Kyosuke Ueda¹⁾, Somesh Kr. Bhattacharya²⁾ and Ryoji Sahara²⁾
¹⁾Department of Materials Processing, Tohoku University
²⁾Research Center for Structural Materials, National Institute for Materials Scinence

[Published in ISIJ International, Vol. 62 (2022), pp. 1512–1521]

https://doi.org/10.2355/isijinternational.ISIJINT-2022-053
E-mail: narut[at]material.tohoku.ac.jp
Key Words: oxidation, nitridation, α-case, titanium nitride, parabolic rate constant, machine learning

We investigated the oxidation and nitridation of a near β-type Ti-17 (Ti-5Al-2Sn-2Zr-4Mo-4Cr) alloy with up to 1 mass% of added Si. The alloys were heated in air at 923–1223 K for up to 518.4 ks. The mass gain was determined, and the scales and oxygen-rich α-case layer that were formed were analyzed. At temperatures below 1123 K, the primary product was rutile-type TiO₂, along with traces of Al₂O₃. The presence of Si⁴⁺ decreased the oxygen vacancy concentration in TiO₂, whereas the presence of SiO₂ caused the relaxation of the internal stress in the scale. These factors were instrumental in lowering the parabolic rate constant and expanding the region conforming to the parabolic rate law. A TiN layer was detected beneath the oxide layer at 1223 K. At the interface between the TiN layer and the Ti alloy, a Sn-rich layer was observed. Both the presence of the Sn-rich layer and the incubation period following the formation of the Sn-rich layer play a crucial role in the formation of the TiN layer. Meanwhile, Si addition had no discernible effect on the formation of the α-case. The parabolic rate constants that were predicted via machine learning concurred well with the measured values. The current study is the first report that describes the scale and α-case formation in Ti-17 alloys, and it may serve as a guideline for determining the optimal application conditions, designing a practical manufacturing process, and improving the oxidation resistance of Ti-17 alloys with Si.