Oxidation of Ti[sub n+1]AlX[sub n] (n=1-3 and X=C, N): II. Experimental Results

Abstract

In this, Part II of a two-part study, the oxidation kinetics in air of the ternary compounds Ti 2 AlC, Ti 2 AlC 0.5 N 0.5 , Ti 4 AlN 2.9, and Ti 3 AlC 2 are reported. For the first two compounds, in the 1000-1100°C temperature range and for short times ͑Ϸ20 h͒ the oxidation kinetics are parabolic. The parabolic rate constants are k x (m 2 /s) ϭ 2.68 ϫ 10 5 exp Ϫ 491.5 (kJ/mol)/RT for Ti 2 AlC, and 2.55 ϫ 10 5 exp Ϫ 458.7 (kJ/mol)/RT for Ti 2 AlC 0.5 N 0.5 . At 900°C, the kinetics are quasi-linear, and up to 100 h the outermost layers that form are almost pure rutile, dense, and protective. For the second pair, at short times ͑Ͻ10 h͒ the oxidation kinetics are parabolic at all temperatures examined ͑800-1100°C͒, but become linear at longer times. The k x values are 3.2 ϫ 10 5 exp Ϫ 429 ͑kJ/mol͒/RT, for Ti 4 AlN 2.9 and 1.15 ϫ 10 5 exp Ϫ 443 ͑kJ/mol͒/RT for Ti 3 AlC 2 . In all cases, the scales that form are comprised mainly of a rutile-based solid solution, (Ti 1Ϫy Al y ͒O 2Ϫy/2 where y Ͻ 0.05, and some Al 2 O 3 . The oxidation occurs by the inward diffusion of oxygen and the outward diffusion of Al and Ti. The C and N atoms are presumed to also diffuse outward through the oxide layer. At the low oxygen partial pressure side, the Al 3ϩ ions dissolve in and diffuse through the (Ti 1Ϫy Al y ͒O 2Ϫy/2 layer and react with oxygen to form Al 2 O 3 at the high oxygen pressure side. This demixing results in the formation of pores that concentrate along planes, especially at longer times and higher temperatures. These layers of porosity impede the diffusion of Al, but not those of Ti and oxygen, which results in the formation of highly striated scales where three layers, an Al 2 O 3 -rich, a TiO 2 -rich, and a porous layer repeat multiple ͑Ͼ10͒ times. The presence of oxygen also reduces the decomposition ͑into TiX x and Al͒ temperatures of Ti 4 AlN 2.9 and Ti 3 AlC 2 from a T Ͼ 1400°C, to one less than 1100°C.In this, Part II of a two-part study, 1 we report on the oxidation in air in the 800-1100°C temperature range, of the ternary compounds Ti 2 AlC, Ti 2 AlC 0.5 N 0.5 , Ti 4 AlN 2.9 , and Ti 3 AlC 2 . Since this is the first report on the oxidation of these compounds, there are no previous results with which to compare; it is thus instructive to review the oxidation behavior of some related solids such as Ti, and some Ti-aluminides such as TiAl, Ti 3 Al, and ''Ti 2 Al,'' which is a twophase mixture of the first two. The oxidation of Ti 3 SiC 2 2 was briefly reviewed in Part I. 1 The oxidation of pure Ti in the 600-1000°C temperature range is parabolic. [3][4][5][6][7] In this temperature range, individual rutile TiO 2 layers form that range in thickness from 1 to 8 m depending inversely on temperature. 3-7 These stratified layers tend to spall off periodically. Simultaneously with the formation of a TiO 2 scale, substantial amounts of oxygen dissolve in the Ti substrate. The same is true for the Ti-aluminides; most, but especially the ones for which the Ti:Al ratio is around 2:1, dissolve substantial amounts of oxygen ͑...

Microstructural evolution during high-temperature oxidation of Ti2AlC ceramics

“…Ti 2 AlC oxidation kinetics have been measured by various researchers [29][30][31]. Parabolic oxidation kinetics have been observed, which implies growth of the oxide scale is diffusion-controlled [26,[29][30][31].…”

“…Parabolic oxidation kinetics have been observed, which implies growth of the oxide scale is diffusion-controlled [26,[29][30][31]. In Ti 2 AlC, the oxide scale growth is caused by inward diffusion of O 2-and outward diffusion of Al 3+ and Ti 4+ [29][30][31].…”

Microstructure and Residual Stress of Alumina Scale Formed on Ti2AlC at High Temperature in Air

“…Oxidation resistance is one of the critical properties of MAX phases for high temperature applications [16][17][18]. Wang et al [8,19] reported that the oxidation resistance of Al 2 O 3 -forming Ti 3 AlC 2 was superior to that of SiO 2 -forming Ti 3 SiC 2 .…”

Cyclic-Oxidation Behavior of Ti3Al0.7Si0.3C2 Compounds Between 900 and 1100 °C in Air

“…The isothermal oxidation behavior of Ti 3 SiC 2 [2][3][4][5][6][7][8] and Ti 3 AlC 2 [9][10][11][12][13] has been extensively studied. The isothermal oxidation of Ti 3 SiC 2 at temperatures between 900 and 1200°C in air resulted in the formation of an outer TiO 2 layer, an intermediate SiO 2 -rich layer and an inner (TiO 2 ?…”