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 ͑...

No abstract

In this article we report on the atomic displacement parameters, lattice expansions, heat capacity, and thermal conductivity of samples of Ti4AlN3 in the 298–1370 K temperature range. Rietveld refinement of high temperature neutron diffraction data shows that the nitrogen is substoichiometric and the formula is Ti4AlN2.9. In this structure, the atomic displacement parameters of the Al atoms are higher than those of either the Ti or N atoms. The Ti–N bonds adjacent to the Al planes are about 2.5% shorter than the Ti–N bonds in the inner layers. The thermal expansion coefficients along the a and c axes are, respectively, (9.6±0.1)×10−6 and (8.8±0.1)×10−6 K−1. The unit cell expansivity, (9.4±0.1)×10−6 K−1, is in agreement with the dilatometric bulk thermal expansivity (9.7±0.2)×10−6 K−1. The heat capacity, cp, is 150 J/mol K at ambient temperatures and extrapolates to ≈220 J/mol K at 1300 K. At all temperatures cp equals four times the molar heat capacity of TiN. The room temperature thermal conductivity is 12 W/m K and increases linearly to ≈20 W/m K at 1300 K.

The structure and chemistry of what initially was proposed to be Ti 3 Al 2 N 2 are incorrect. Using high-resolution transmission electron microscopy, together with chemical analysis, the stoichiometry of this compound is concluded to be Ti 4 AlN 3−␦ (where ␦ = 0.1). The structure is layered, wherein every four layers of almost-close-packed Ti atoms are separated by a layer of Al atoms. The N atoms occupy ∼97.5% of the octahedral sites between the Ti atoms. The unit cell is comprised of eight layers of Ti atoms and two layers of Al atoms; the unit cell is hexagonal with P6 3 /mmc symmetry (lattice parameters of a = 0.3 nm and c = 2.33 nm). This compound is machinable and closely related to other layered, ternary, machinable, hexagonal nitrides and carbides, namely M 2 AX and M 3 AX 2 (where M is an early transition metal, A is an A-group element, and X is carbon and/or nitrogen).