Synthesis and Characterization of Ti ₃ AlC ₂

Abstract

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 ∼4 vol% Al 2 O 3 ), fully dense, and have a grain size of ∼25 μm. 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 ∼3.5 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 μΩ·m 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 room‐temperature 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 ∼45° 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 post‐indentation flexural strength by ∼7%. 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).