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  1. Abstract The resistance to oxidizing environments exhibited by some M n+1 AX n (MAX) phases stems from the formation of stable and protective oxide layers at high operating temperatures. The MAX phases are hexagonally arranged layered nitrides or carbides with general formula M n +1 AX n , n  = 1, 2, 3, where M is early transition elements, A is A block elements, and X is C/N. Previous attempts to model and assess oxide phase stability in these systems has been limited in scope due to higher computational costs. To address the issue, we developed a machine-learning driven high-throughput frameworkmore »for the fast assessment of phase stability and oxygen reactivity of 211 chemistry MAX phase M 2 AX. The proposed scheme combines a sure independence screening sparsifying operator-based machine-learning model in combination with grand-canonical linear programming to assess temperature-dependent Gibbs free energies, reaction products, and elemental chemical activity during the oxidation of MAX phases. The thermodynamic stability, and chemical activity of constituent elements of Ti 2 AlC with respect to oxygen were fully assessed to understand the high-temperature oxidation behavior. The predictions are in good agreement with oxidation experiments performed on Ti 2 AlC. We were also able to explain the metastability of Ti 2 SiC, which could not be synthesized experimentally due to higher stability of competing phases. For generality of the proposed approach, we discuss the oxidation mechanism of Cr 2 AlC. The insights of oxidation behavior will enable more efficient design and accelerated discovery of MAX phases with maintained performance in oxidizing environments at high temperatures.« less
    Free, publicly-accessible full text available December 1, 2022
  2. Herein, we synthesize dense, predominantly single-phase polycrystalline samples of the Mn2AlB2 ternary compound, using reactive hot-pressing of manganese, aluminum, and boron powder mixtures under vacuum. With a Vickers hardness of 8.7 GPa, Mn2AlB2 is relatively soft for a transition metal boride and lacked dominant cracks at the corners of the indentations. With Young’s and shear moduli of 243 GPa and 102 GPa at 300 K, respectively, it is reasonably stiff. The Poisson’s ratio is calculated to be 0.19. With compressive strengths of 1.24 ± 0.1 GPa, the samples were quite strong considering the grain size (1–15 μm). The electrical resistivity at 300 K was ∼5 μΩm and decreased linearlymore »upon cooling. At 0.0036 K−1, the temperature coefficient of resistivity was relatively high compared to MoAlB. The average linear thermal expansion coefficient was also found to be relatively high at 18.6 × 10-6 K−1 from 298 to 1173 K. Mn2AlB2 was not thermally stable above ∼1379 K. While Mn2AlB2 was not machinable with conventional tooling, intriguingly, high-speed carbide tools bits readily penetrate the surface – with no cracking or chipping for a few millimeters – before stopping.« less