T. El-Raghy

Oxidation Of Ti3SiC2 in Air

Polycrystalline samples of Ti{sub 3}SiC{sub 2} were oxidized in air in the 900 to 1,400 C temperature range. The oxidation was parabolic with parabolic rate constants, k{sub p}, that increased from 1 {times} 10{sup {minus}9} to 1 {times} 10{sup {minus}4} kg{sup 2}/m{sup 4}s as the temperature increased from 900 to 1,400 C, respectively, which yielded an activation energy of 370 {+-} 20 kJ/mol. The scale that forms was dense, adhesive, resistant to thermal cyclings and layered. The outer layer was pure TiO{sub 2} (rutile), and the inner layer consisted of mixture of SiO{sub 2} and TiO{sub 2}. The results are consistent with a model in which growth of the oxide layer occurs by the inward diffusion of oxygen and the simultaneous outward diffusion of titanium and carbon. The presence of small volume fractions ({approx} 2%) of TiC{sub x} in Ti{sub 3}SiC{sub 2} were found to have a deleterious effect on the oxidation kinetics.

Low temperature dependencies of the elastic properties of Ti4AlN3, Ti3Al1.1C1.8, and Ti3SiC2

In this article we report on the temperature dependencies of the longitudinal and shear sound velocities in Ti4AlN3, Ti3Al1.1C1.8, and Ti3SiC2. The velocities are measured using a phase sensitive pulse-echo ultrasonic technique in the 90–300 K temperature range. At room temperature, Young’s, ERT, and shear, μRT, moduli and Poisson’s ratio of Ti4AlN3 are 310±2, 127±2 GPa, and 0.22, respectively. The corresponding values for Ti3AlC2 are 297.5±2, 124±2 GPa, and 0.2. Both moduli increase slowly with decreasing temperature and plateau out at temperatures below ≈125 K. A least squares fit of the temperature dependencies of the shear and Young’s moduli of Ti4AlN3 yield: μ/μRT=1−1.5×10−4(T−298), and E/ERT=1−0.74×10−4(T−298), for T>125 K. The corresponding relationships for Ti3Al1.1C1.8 are: μ/μRT=1−1.2×10−4(T−298), and E/ERT=1−0.84×10−4(T−298) for T>125 K. The acoustic Debye temperatures calculated for Ti4AlN3 and Ti3AlC2, as well as Ti3SiC2, are all above 700 K, in agreement with values calculated from low tempe...

Fatigue-crack growth and fracture properties of coarse and fine-grained Ti3SiC2

An experimental study of fracture and cyclic fatigue-crack growth behavior was made in a reactively hot-pressed monolithic Ti{sub 3}SiC{sub 2} ceramic with both fine (3--10 {mu}m) and coarse-grained (50--200 {mu}m) microstructures. Whereas the fine-grained microstructure exhibited a steady-state (plateau) R-curve fracture toughness of K{sub c} {approximately} 9.5 MPa{radical}m, the coarse-grained Ti{sub 3}SiC{sub 2} exhibited a K{sub c} {approximately}16 MPa{radical}m. The latter value is though to be one of the highest fracture toughnesses ever observed in a monolithic, non-transforming ceramic, consistent with the profusion of crack-bridging processes active in the crack wake. Moreover, fatigue-crack growth thresholds, {Delta}K{sub TH}, were comparatively high for ceramic materials, as indicated by the coarse-grained microstructure which had a threshold {Delta}K{sub TH} of {approximately}9 MPa{radical}m. Such fatigue crack growth was associated with substantial evidence for wear degradation at active bridging sites behind the crack tip.

Diffusion kinetics of the carburization and silicidation of Ti3SiC2

The ternary carbide Ti3SiC2 possesses a unique set of properties that could render it a material of considerable technological impact. The motivation for this work was to enhance the hardness and oxidation resistance of Ti3SiC2 by altering its surface chemistry. Reaction of Ti3SiC2 with single crystal Si wafers in the 1200–1350 °C temperature range resulted in the formation of a dense surface layer composed of a two phase mixture of TiSi2 and SiC. This layer grows in two distinct morphologies; an outer layer with fine (1–5 μm) SiC particles and an inner coarser (10–15 μm) one. The overall growth rates of the layers were parabolic. Comparison with previously published results supports the conclusion that diffusion of Si through TiSi2 is rate limiting. In the 1400–1600 °C temperature range, reaction of Ti3SiC2 with graphite foils resulted in the formation of a 15 vol. % porous surface layer of TiCx (where x>0.8). It is shown that the carburization kinetics are rate limited by the diffusion of C through TiCx...

The topotactic transformation of Ti{sub 3}SiC{sub 2} into a partially ordered cubic Ti(C{sub 0.67}Si{sub 0.06}) phase by the diffusion of Si into molten cryolite

Immersion of Ti{sub 3}SiC{sub 2} samples in molten cryolite at 960 C resulted in the preferential diffusion of Si atoms out of the basal planes to form a partially ordered, cubic phase with approximate chemistry Ti(C{sub 0.67}, Si{sub 0.06}). The latter forms in domains, wherein the (111) planes are related by mirror planes; i.e., the loss of Si results in the de-twinning of the Ti{sub 3}C{sub 2} layers. Raman spectroscopy, X-ray diffraction, optical, scanning and transmission electron microscopy all indicate that the Si exists the structure topotactically, in such a way that the C atoms remain partially in their ordered position in the cubic phase.

Structure of Ti4AlN3—a layered Mn+1AXn nitride

Recent high resolution transmission electron microscopy and electron probe X-ray microanalysis show that the compound originally thought to be Ti3Al2N2 is Ti4AlN3. In this paper we report on the crystal structure determination by Rietveld refinement on neutron and X-ray powder diffraction data. Ti4AlN3 crystallizes with a hexagonal unit cell, space group P63/mmc, and with lattice parameters a = 2.9880(2) and c = 23.372(2) A. The stacking sequence is such that every four layers of Ti atoms is separated by a layer of Al atoms. The N atoms occupy octahedral sites between the Ti atoms making up a network of corner shared octahedra. This compound is closely related to other layered, ternary, machinable, hexagonal nitrides and carbides, namely, M2AX and M3AX2, where M is an early transition metal, A is a A-group element and X is either C and/or N.

THE RAMAN SPECTRUM OF TI3SIC2

In this paper, the Raman spectrum of Ti3SiC2 is reported and compared with that of TiC0.67. All the TiC0.67 first order Raman disorder-induced modes are active, but shifted, in Ti3SiC2. Two additional peaks at 150 and 372 cm−1 are observed in Ti3SiC2. The former is ascribed to a shear mode between the Si and Ti planes; the origin of the latter is unknown. No second order Raman bands are detected. Micro-Raman spectroscopy also reveal the presence of ≈50 A graphite crystallites in samples hot pressed in graphite dies—these crystallites are not detected in samples processed by hot isostatic pressing in molten glass containers.

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

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 (20 h) the oxidation kinetics are parabolic. The parabolic rate constants are k x (m 2 /s) = 2.68 × 10 5 exp -491.5 (kJ/mol)/RT for Ti 2 AlC. and 2.55 × 10 5 exp - 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 ( 10) times. The presence of oxygen also reduces the decomposition (into TiX x and Al) temperatures of Ti 4 AlN 2.9 and Ti 3 AlC 2 from a T> 1400°C, to one less than 1100°C.

Long Time Oxidation Study of Ti3SiC2 , Ti3SiC2 / SiC , and Ti3SiC2 / TiC Composites in Air

We report herein on the oxidation kinetics and morphology of the oxide phases that form after long term (up to 1500 h) oxidation in ambient air of fine and coarse-grained samples of Ti 3 SiC 2 , Ti 3 SiC 2 , with 30 vol % TiC and Ti 3 SiC with 30 vol %SiC in the 875-1200°C temperature range. In all cases, the oxidation resulted in a duplex scale, an outer rutile and an inner rutile/ silica layer. At 875°C, and up to at least 100 h, the oxidation kinetics of the Ti 3 SiC 2 /30 vol % TiC samples are parabolic: at 925°C, the oxidation kinetics of the Ti 3 SiC 2 /30 vol % SiC are subparabolic, up to at least 500 h. The oxidation kinetics of all other runs are initially parabolic, but at times >30 h they become linear. The reason for the transition is not entirely clear, but could he due to the buildup of stresses in the external oxide scales. Comparison with previously published results indicate that the rate limiting step, when the oxidation kinetics are parabolic, is most probably the inward diffusion of oxygen and the simultaneous outward diffusion of titanium. The results also strongly suggest that the activation energies for diffusion of oxygen and titanium in rutile are almost identical over at least the 1000-1200°C temperature range.

Synthesis and characterization of Hf2PbC, Zr2PbC and M2SnC (M=Ti, Hf, Nb or Zr)

Abstract Predominantly single phase (92–94 vol.%), fully dense samples of Hf2SnC, Zr2SnC, Nb2SnC, Ti2SnC, Hf2PbC and Zr2PbC were fabricated by reactively HIPing the stoichiometric mixture of the corresponding elemental powders in the 1200–1325°C temperature range for 4–48 h. The latter two, fabricated here for the first time, required a further anneal of 48–96 h to increase the volume fraction of ternary phases. Hf2PbC and Zr2PbC are unstable in ambient atmospheres at room temperature. As a family these compounds are good electrical conductors; the lowest and highest values of the electrical conductivities were, respectively, 2.2×106 (Ω.m)−1 for Hf2SnC and 13.4×106 (Ω.m)−1 for Hf2PbC. The Vickers hardness values range from 3 to 4 GPa. All compounds are readily machinable. The Youngs moduli of Zr2SnC, Nb2SnC and Hf2SnC are, respectively, 178, 216 and 237 GPa. The thermal coefficients of expansion, TCEs, of the ternaries scale with those of the corresponding binaries, and are relatively low for such readily machinable solids. The lowest TCE belonged to Nb2SnC [(7.8±0.2)×10−6 K−1], and the highest to Ti2SnC [(10±0.2)×10−6 K−1]. The TCEs of Hf and Zr containing ternaries cluster around (8.2±0.2)×10−6 K−1. All the synthesized ternary carbides were found to dissociate into the transition metal carbide and the A-group element in the 1250–1390 °C temperature range.