Oxidation and Diffusion in Ti-Al-(Mo, Nb) Intermetallics

Abstract

The oxidation resistance of Ti-Al intermetallics is superior to many of their counterparts at high temperatures. High-temperature stability of these intermetallics appears to improve with a ternary alloying addition such as Mo, Nb, etc. A detailed analysis of oxidation of the Ti 3 Al intermetallics, Ti 3 Al-2.9 at. pct Mo and Ti 3 Al-1.1 at. pct Nb, in pure oxygen using crystallographic and microscopic techniques is presented here. The alloy containing 1.1 at. pct Nb did not show an improvement in oxidation resistance over the base alloy but that containing 2.9 at. pct Mo showed marked resistance to oxidation. Activation energies of oxidation for both the alloys were comparable with those reported in the literature for similar compositions. TiO 2 and Al 2 O 3 were the major phases present in the oxide scales of the oxidized alloys. The crystal orientations from the X-ray diffraction (XRD) patterns and the morphologies from scanning electron microscopy (SEM) were in good agreement and were helpful in further analysis of the oxidation process. The effective diffusion of oxygen in the oxide layers was calculated using the mole fraction of individual oxide and the diffusivity of oxygen in pure oxide. The activation energy for the effective diffusion of oxygen through the oxide layers was determined to bẽ 24 kJ/mol. The activation energy for the oxidation process was higher than that of the diffusion of oxygen. Hence, it can be concluded that the oxidation process in both the alloys studied is not diffusion controlled.

ВЛИЯНИЕ ПРИМЕСЕЙ НА ДИФФУЗИЮ КИСЛОРОДА В Al2O3

“…В то же время формирование плотного оксидного слоя α-Al 2 O 3 на поверхности сплавов с низким содержанием титана обеспечивает им высокую коррозионную стойкость [1,4]. Согласно одной из точек зрения [5] скорость окисления Ti-Al сплавов с меньшим содержанием алюминия и морфология шкалы оксидов контролируется диффузией кислорода с поверхности в сплав, тогда как в работе [6]…”

RASChETY KOJeFFICIENTA DIFFUZII KISLORODA V SPLAVAH Ti-Al: PERVOPRINCIPNYJ PODHOD

“…Формирование TiO2 связано с окислением титана на поверхности оксида, тогда как окисление алюминия происходит на внутренней границе раздела оксид-сплав [3,4]. Поэтому, согласно одной из точек зрения, кинетика окисления определяется диффузией кислорода [4], тогда как в [5] Рассчитанные энергии абсорбции кислорода в ряде октаэдрических и тетраэдрических позициях представлены на рис. 1г,д.…”