Polycrystalline bulk samples of Ti 3 Al 1.1 C 1.8 have been fabricated by reactively hot isostatically pressing a mixture of titanium, graphite, and Al 4 C 3 powders at a pressure of 70 MPa and temperature of 1400°C for 16 h. The hot isostatically pressed samples are predominantly single phase (containing ϳ4 vol% Al 2 O 3 ), fully dense, and have a grain size of ϳ25 m. This carbide is similar to Ti 3 SiC 2 , with which it is isostructural, and has an unusual combination of properties. It is relatively soft (Vickers hardness of ϳ3.5 GPa) and elastically stiff (Young's modulus of 297 GPa and shear modulus of 124 GPa); yet, it is lightweight (density of 4.2 g/cm 3 ) and easily machinable. The room-temperature electrical resistivity is 0.35 ؎ 0.03 ⍀⅐m and decreases linearly as the temperature decreases. The temperature coefficient of resistivity is 0.0031 K ؊1 . The coefficient of thermal expansion, in the temperature range of 25°-1200°C, is 9.0 (؎ 0.2) ؋ 10 ؊6 K ؊1 . The roomtemperature compressive and flexural strengths are 560 ؎ 20 and 375 ؎ 15 MPa, respectively. In contrast to flexure, where the failure is brittle, the failure in compression is noncatastrophic and is accompanied by some plasticity. The origin of that plasticity is believed to be the formation of a "shear" band that is oriented at an angle of ϳ45°to the applied load. Ti 3 Al 1.1 C 1.8 also is a highly damage-tolerant material; a 10-kg-load Vickers indentation made in a bar 1.5 mm thick reduces the postindentation flexural strength by ϳ7%. This material also is quite resistant to thermal shock. At temperatures of >1000°C, the deformation in compression is accompanied by significant plasticity and very respectable ultimate compressive stresses (200 MPa at 1200°C).

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 ͑...

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