Processing and characterization of Ti2AlC, Ti2AlN, and Ti2AlC0.5N0.5

Abstract

In this article, we report on the fabrication and characterization of Ti2AlC, Ti2AlN, and Ti2AlC0.5N0.5. Reactive hot isostatic pressing (hipping) at ≈40 MPa of the appropriate mixtures of Ti, Al4C3 graphite, and/or AlN powders for 15 hours at 1300°C yields predominantly single-phase samples of Ti2AlC0.5N0.5; 30 hours at 1300°C yields predominantly single-phase samples of Ti2AlC. Despite our best efforts, samples of Ti2AlN (hot isostatic pressed (hipped) at 1400°C for 48 hours) contain anywhere between 10 and 15 vol pct of ancillary phases. At ≈25 μm, the average grain sizes of Ti2AlC0.5N0.5 and Ti2AlC are comparable and are significantly smaller than those of Ti2AlN, at ≈100 μm. 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; Ti2AlC0.5N0.5 is stronger in compression, harder, and more brittle than the end-members. Conversely, at temperatures greater than 1200°C, a solid-solution softening effect is occurring. The thermal-expansion coefficients (CTEs) of Ti2AlC, Ti2AlN, and Ti2AlC0.5N0.5 are, respectively, 8.2 × 10-6, 8.8 × 10-6, and 10.5 × 10-6 °C-1, in the temperature range from 25°C to 1300°C. The former two values are in good agreement with the CTEs determined from high-temperature X-ray diffraction (XRD). The electrical conductivity of the solid solution (3.1 × 106 (Ω m)-1) is in between those of Ti2AlC and Ti2AlN, which are 2.7 × 106 and 4.0 × 106 Ω-1 m-1, respectively.