The thermodynamics of hydride precipitation: The importance of entropy, enthalpy and disorder

• Published in: Acta materialia Vol. 79; pp. 351 - 362
• Main Authors: Lumley, S.C., Grimes, R.W., Murphy, S.T., Burr, P.A., Chroneos, A., Chard-Tuckey, P.R., Wenman, M.R.
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier Ltd 15-10-2014; Elsevier
• Subjects: Applied sciences; Density functional theory; Dynamic tests; Enthalpy; Entropy; Exact sciences and technology; Hydrides; Metals. Metallurgy; Precipitation; Solid solutions; Thermodynamics; Zirconium; Zirconium hydride; Thermodynamics; Density functional theory; Precipitation; Zirconium hydride; Enthalpy; Entropy; Hydrides; Density
• ISSN: 1359-6454; 1873-2453
• DOI: 10.1016/j.actamat.2014.07.019

The precipitation of zirconium hydrides from Zr solid solution was investigated using first-principles lattice dynamics simulations. These included the temperature-dependent vibrational enthalpy and vibrational entropy combined with the configurational entropy terms. In contrast with previous approaches, it was found that the latent enthalpy alone is not sufficient to fully describe precipitation of hydrides; a full thermodynamic assessment is required. In particular, the vibrational enthalpy of precipitation assists in stabilizing hexagonal close-packed hydrides and is especially important in forming the metastable ζ phase. The configurational entropy change during precipitation favours face-centred cubic hydrides. Given this, at concentrations below 300ppm H, no hydride precipitation is predicted, suggesting that when hydrides are seen in those materials it is because the local concentration of H is greater than that measured globally. While γ hydride is the most stable phase, it is very close in energy to the δ phase.