Browsing by Author "Murphy, ST"

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    Hydrogen accommodation in Zr second phase particles: implications for H pick-up and hydriding of zircaloy-2 and zircaloy-4
    (Elsevier, 2013-04-01) Burr, PA; Murphy, ST; Lumley, SC; Wenman, MR; Grimes, RW
    Ab-initio computer simulations have been used to predict the energies associated with the accommodation of H atoms at interstitial sites in α, β-Zr and Zr–M intermetallics formed with common alloying additions (M = Cr, Fe, Ni). Intermetallics that relate to the Zr2(Ni,Fe) second phase particles (SPPs) found in Zircaloy-2 exhibit favourable solution enthalpies for H. The intermetallic phases that relate to the Zr(Cr,Fe)2 SPPs, found predominantly in Zircaloy-4, do not offer favourable sites for interstitial H. It is proposed that Zr(Cr,Fe)2 particles may act as bridges for the migration of H through the oxide layer, whilst the Zr2(Ni,Fe)-type particles will trap the migrating H until these are dissolved or fully oxidised. © 2012, Elsevier Ltd.
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    Hydrogen solubility in zirconium intermetallic second phase particles
    (Elsevier, 2013-11-01) Burr, PA; Murphy, ST; Lumley, SC; Wenman, MR; Grimes, RW
    The enthalpies of solution of H in Zr binary intermetallic compounds formed with Cu, Cr, Fe, Mo, Ni, Nb, Sn and V were calculated by means of density functional theory simulations and compared to that of H in α-Zr. It is predicted that all Zr-rich phases (formed with Cu, Fe, Ni and Sn), and those phases formed with Nb and V, offer lower energy, more stable sites for H than α-Zr. Conversely, Mo and Cr containing phases do not provide preferential solution sites for H. In all cases the most stable site for H are those that offer the highest coordination fraction of Zr atoms. Often these are four Zr tetrahedra but not always. Implications with respect to H-trapping properties of commonly observed ternary phases such as Zr(Cr, Fe)2, Zr2(Fe, Ni) and Zr(Nb, Fe)2 are also discussed. © 2013, Elsevier B.V.
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    The stability of alloying additions in zirconium
    (Elsevier, 2013-06-01) Lumley, SC; Murphy, ST; Burr, PA; Grimes, RW; Chard-Tuckey, PR; Wenman, MR
    The interactions of Cr, Fe, Nb, Ni, Sn, V and Y with Zr are simulated using density functional theory. Thermodynamic stabilities of various different Zr based intermetallic compounds, including multiple Laves phase structures and solutions of alloying additions in both α and β -Zr were investigated. The thermodynamic driving forces in this system can be correlated with trends in atomic radii and the relative electronegativities of the different species. Formation energies of Fe, Ni and Sn based intermetallic compounds were found to be negative, and the Zr2FeZr2Fe and Zr2NiZr2Ni intermetallics were metastable. Most elements displayed negative energies of solution in β-Zr but positive energies in the α-phase, with the exception of Sn (which was negative for both) and Y (which was positive for both). Solutions formed from intermetallics showed a similar trend. © 2013, Elsevier B.V.
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    The thermodynamics of hydride precipitation: the importance of entropy, enthalpy and disorder
    (Elsevier, 2014-10-15) Lumley, SW; Grimes, RW; Murphy, ST; Burr, PA; Chroneos, A; Chard-Tuckey, PR; Wenmam, MR
    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 ζζ phase. The configurational entropy change during precipitation favours face-centred cubic hydrides. Given this, at concentrations below 300 ppm 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 γγ hydride is the most stable phase, it is very close in energy to the δδ phase. © 2014 Acta Materialia Inc.