Thermodynamic stabilization of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>γ</mml:mi><mml:mtext>−</mml:mtext><mml:mi mathvariant="normal">U</mml:mi><mml:mo>−</mml:mo><mml:mi>Mo</mml:mi></mml:mrow></mml:math> alloys: Effect of Mo content and temperature

Abstract

The $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{U}\ensuremath{-}\mathrm{Mo}$ alloys have been studied by means of ab initio molecular-dynamic simulations at 900 K as a function of the Mo concentration (0%, 25%, 50%, 75%, and 100%). Using the temperature-dependent effective potential method with the symmetry-imposed force constant extension, we obtained the vibrational, thermodynamic, and elastic properties of U--Mo, including anharmonicity and disorder. We show that the addition of Mo and temperature effects are responsible for the stabilization of the $\ensuremath{\gamma}$ phase of U--Mo alloys in the range of 0--25 % of Mo concentration above 900 K. The vibrational entropy is found to be very large compared to its value in other alloys, about half of the configurational entropy counterpart, and to act against the thermodynamic stabilization of the solid solution. The vibrational and thermodynamic properties are strongly impacted by the disorder, due to the large differences between mass and interatomic bonds of uranium and molybdenum. The increase of elastic shear modulus as a function of the Mo concentration also indicates a mechanical stabilization of the solid solution and validates the thermodynamic findings.

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