Analysis of the Partial Molar Excess Entropy of Dilute Hydrogen in Liquid Metals and Its Change at the Solid-Liquid Transition Andrew H. Caldwella, Antoine Allanorea,∗ aDepartment of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA, 02139 Abstract mix A systematic change in the partial molar enthalpy of mixing (∆hH ) and partial ex molar excess entropy (∆sH ) for dilute hydrogen-metal systems at the solid- mix ex liquid transition is reported. Expressions for ∆hH and ∆sH are derived from ex the Fowler model of hydrogen solubility, and the change in ∆sH at melting is bounded. The theoretical bound is in agreement with measured data. A ex connection is made between the change in ∆sH and short range order in the metal-hydrogen system. Keywords: Liquids, Hydrogen, Solubility, Statistical mechanics, Thermodynamics 1 1. Introduction 2 Metals processing invariably requires the handling of metals in the liquid 3 state. Such operations rarely occur in inert atmospheres. The mole fraction 4 of dissolved gases, in particular hydrogen (H), are typically in the range of −6 −2 5 10 to 10 for liquid metals, and therefore degassing procedures are routinely 6 employed in process metallurgy. This is done to prevent degradation of the 7 mechanical properties of the solidified product, and such effects are well-studied 8 [1, 2]. The severity of these effects depends on the concentration of H in the 9 metal M, which can be determined from the solution thermodynamics of the M- 10 H system. Calculating and predicting H solubility, defined here as its equilibrium ∗Corresponding author Email address:
[email protected] (Antoine Allanore) Preprint submitted to Elsevier January 25, 2019 11 concentration in the metal, is therefore of considerable importance for metals 12 processing.