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Thermodynamic Stability of MAX Phases in the Zr-Al-C system at High Temperature

Angeliki Poulou

Deaprtment of Materials

Monday 11th November 2019
Time: 12.00
Venue: Room G01, Royal School of Mines
Contact: Ms Hafiza Bibi
Tel: 020 7594 7252

Abstract:

MAX phases are a growing class of layered materials, widely researched because of their interesting combination of metallic and ceramic properties. The synthesis of such materials has however been a challenge, since the purity of the produced phases is difficult to improve. We have focussed on two of the more recently synthesised members of the class, Zr2AlC and Zr3AlC2. Experimental studies suggest improvement to the stability of such phases through the inclusion of impurities in the form of partial substitutions [1,2]. This suggests that a better understanding of the thermodynamic stability of the Zr2AlC phase against other secondary phases in the Zr-Al-C system is important.

In this study Density Functional Theory (DFT) calculations of the free energy were made as a function of temperature by treating the free energy in the quasiharmonic (QHA) framework. The PBEsol exchange-correlation functional was used, since tests have shown it to be more reliable in metals than LDA or PBE. Calculation of formation energies of Zr-Al intermetallics, Zr carbides and ternary phases including Zr2AlC and Zr3AlC2 MAX phases were conducted and their relative stability compared with Zr2AlC was evaluated at a range of temperatures between 0 and 1800 K. The Zr-Al-C stoichiometric ternary phase diagram was computed indicating that the Zr2AlC phase is unstable, but the energy differences are of the order of kT per atom, and there is a tendency to higher stability as the temperature increases.

[1] Lapauw, T. et al. Synthesis of the new MAX phase Zr2AlC. J. Eu Ceram. Soc. 36, 1847-1853 (2016).

[2] Horlait, D., Middleburgh, S. C., Chroneos, A. & Lee, W. E. Synthesis and DFT investigation of new bismuth-containing MAX phases. Scientific Reports 6, 18829 (2016).

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