Polycrystalline bulk samples of Ti3SiC2 were fabricated by reactively hot‐pressing Ti, graphite, and SiC powders at 40 MPa and 1600°C for 4 h. This compound has remarkable properties. Its compressive strength, measured at room temperature, was 600 MPa, and dropped to 260 MPa at 1300°C in air. Although the room‐temperature failure was brittle, the high‐temperature load‐displacement curve shows significant plastic behavior. The oxidation is parabolic and at 1000° and 1400°C the parabolic rate constants were, respectively, 2 × 10−8 and 2 × 10−5 kg2‐m−4.s−1. The activation energy for oxidation is thus =300 kJ/mol. The room‐temperature electrical conductivity is 4.5 × 106Ω−1.m−1, roughly twice that of pure Ti. The thermal expansion coefficient in the temperature range 25° to 1000°C, the room‐temperature thermal conductivity, and the heat capacity are respectively, 10 × 10−6°C−1, 43 W/(m.K), and 588 J/(kgK). With a hardness of 4 GPa and a Young's modulus of 320 GPa, it is relatively soft, but reasonably stiff. Furthermore, Ti3SiC2 does not appear to be susceptible to thermal shock; quenching from 1400°C into water does not affect the postquench bend strength. As significantly, this compound is as readily machinable as graphite. Scanning electron microscopy of polished and fractured surfaces leaves little doubt as to its layered nature.

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In this article, we report on the fabrication and characterization of Ti 2 AlC, Ti 2 AlN, and Ti 2 AlC 0.5 N 0.5 . Reactive hot isostatic pressing (hipping) at Ϸ40 MPa of the appropriate mixtures of Ti, Al 4 C 3 graphite, and/or AlN powders for 15 hours at 1300 ЊC yields predominantly single-phase samples of Ti 2 AlC 0.5 N 0.5 ; 30 hours at 1300 ЊC yields predominantly single-phase samples of Ti 2 AlC. Despite our best efforts, samples of Ti 2 AlN (hot isostatic pressed (hipped) at 1400 ЊC for 48 hours) contain anywhere between 10 and 15 vol pct of ancillary phases. At Ϸ25 m, the average grain sizes of Ti 2 AlC 0.5 N 0.5 and Ti 2 AlC are comparable and are significantly smaller than those of Ti 2 AlN, at Ϸ100 m. All samples are fully dense and readily machinable. The room-temperature deformation under compression of the end-members is noncatastrophic or graceful. At room temperature, solid-solution strengthening is observed; Ti 2 AlC 0.5 N 0.5 is stronger in compression, harder, and more brittle than the end-members. Conversely, at temperatures greater than 1200 ЊC, a solid-solution softening effect is occurring. The thermal-expansion coefficients (CTEs) of Ti 2 AlC, Ti 2 AlN, and Ti 2 AlC 0.5 N 0.5 are, respectively, 8.2 ϫ 10 Ϫ6 , 8.8 ϫ 10 Ϫ6 , and 10.5 ϫ 10 Ϫ6 ЊC Ϫ1 , in the temperature range from 25 ЊC to 1300 ЊC. The former two values are in good agreement with the CTEs determined from hightemperature X-ray diffraction (XRD). The electrical conductivity of the solid solution (3.1 ϫ 10 6 (⍀ m) Ϫ1 ) is in between those of Ti 2 AlC and Ti 2 AlN, which are 2.7 ϫ 10 6 and 4.0 ϫ 10 6 ⍀ Ϫ1 m Ϫ1 , respectively.

Microstructural observations of damage around indentations in Ti 3 SiC 2 are presented. The Vickers hardness decreased with increasing load and asymptotically approached 4 GPa at the highest loads. No indentation cracks were observed even at loads as high as 300 N. Preliminary strength versus indentation plots indicate that, at least for the large-grained material ( 100 m) studied here, Ti 3 SiC 2 is a damagetolerant material able to contain the extent of microdamage to a small area around the indent. The following multiple energy-absorbing mechanisms have been identified from scanning electron micrographs of areas in the vicinity of the indentation: diffuse microcracking, delamination, crack deflection, grain push-out, grain pull-out, and the buckling of individual grains.

Transmission electron microscopy (TEM) of aligned, macrograined samples of Ti 3 SiC 2 , deformed at room temperature, shows that the deformed microstructure is characterized by a high density of perfect basal-plane dislocations with a Burgers vector of 1/3͗112 0͘. The dislocations are overwhelmingly arranged either in arrays, wherein the dislocations exist on identical slip planes, or in dislocations walls, wherein the same dislocations form a low-angle grain boundary normal to the basal planes. The arrays propagate across entire grains and are responsible for deformation by shear. The walls form as a result of the formation of kink bands. A dislocation-based model, that builds on earlier ideas proposed for kink-band formation in hexagonal metallic single crystals, is presented, which explains most of the microstructural features. The basic elements of the model are shear deformation by dislocation arrays, cavitation, creation of dislocation walls and kink boundaries, buckling, and delamination. The delaminations are not random, but successively bisect the delaminating sections. The delaminations and associated damage are contained by the kink boundaries. This containment of damage is believed to play a major role in endowing Ti 3 SiC 2 and, by extension, related ternary carbides and nitrides with their damage-tolerant properties.

In this article, the second part of a two-part study, we report on the mechanical behavior of Ti 3 SiC 2 . In particular, we have evaluated the mechanical response of finegrained (3-5 µm) Ti 3 SiC 2 in simple compression and flexure tests, and we have compared the results with those of coarse-grained (100-200 µm) Ti 3 SiC 2 . These tests have been conducted in the 25°-1300°C temperature range. At ambient temperature, the fine-and coarse-grained microstructures exhibit excellent damage-tolerant properties. In both cases, failure is brittle up to ∼1200°C. At 1300°C, both microstructures exhibit plastic deformation (>20%) in flexure and compression. The fine-grained material exhibits higher strength compared with the coarse-grained material at all temperatures. Although the coarse-grained material is not susceptible to thermal shock (up to 1400°C), the finegrained material thermally shocks gradually between 750°a nd 1000°C. The results presented herein provide evidence for two important aspects of the mechanical behavior of Ti 3 SiC 2 : (i) inelastic deformation entails basal slip and damage formation in the form of voids, grain-boundary cracks, kinking, and delamination of individual grains, and (ii) the initiation of damage does not result in catastrophic failure, because Ti 3 SiC 2 can confine the spatial extent of the damage.

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