Processing and characterization of Ti₂AlC, Ti₂AlN, and Ti₂AlC_0.5N_0.5

Abstract

In this article, we report on the fabrication and characterization of Ti₂AlC, Ti₂AlN, and Ti₂AlC_0.5N_0.5. 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 Ti₂AlC_0.5N_0.5, 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 Ti₂AlC_0.5N_0.5 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; Ti₂AlC_0.5N_0.5 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 Ti₂AlC_0.5N_0.5 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 (Θ m)⁻¹) is in between those of Ti₂AlC and Ti₂AlN, which are 2.7 × 10⁶ and 4.0 × 10⁶ Θ ⁻¹ m⁻¹, respectively.