Фазовая диаграмма системы H-Zr

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H-Zr (Hydrogen-Zirconium) E. Zuzek, J.P. Abriata, A. San-Martin, and F.D. Manchester The assessed phase diagram for the Zr-H system is of the eutectoid type and consists of: (1) the cph terminal (random interstitial) solid solution, (aZr), or a phase, which shows a maximum hydrogen solubility of X = 0.063 (5.9 at.% H) at 550 C (X = H/Zr is the hydrogen/zirconium atomic ratio); (2) the bcc terminal (random interstitial) solid solution, (bZr), or b phase, which has a eutectoid decomposition at 550 C, at the hydrogen concentration of X = 0.60 ( 37.5 at.% H); (3) the d phase, a fcc hydride; and (4) the e phase, a fct hydride with axial ratio c/a < 1. The assessed diagram is based on review of the experimental data [55Gul, 59Lag, 60Lib, 62Bec, 62Lib, 67Kea, 68Moo1, 68Moo2, 69Moo, 83Rit, 86Zuz]. Joint consideration of the b/(a + b) and b/(b + d) phase boundaries should allow the evaluation of the composition of the b phase to be obtained at the eutectoid temperature. However, probably because of the long equilibration times necessary to reach stable equilibrium near the eutectoid temperature, wide scatter occurred in the experimental data points at and near this temperature. Consequently, only a precision of X = 0.60 с 0.14 (37.5 с 6.0 at.% H) can be assumed for the composition of b phase at the eutectoid temperature. This portion of the Zr-H phase diagram is highly uncertain, and more experimental work is required to determine the equilibrium conditions. Concerning the d + e equilibrium, some discrepancies exist regarding the nature of the d = e transition (whether it is of first or higher order). These may be traced to the experimental difficulties in obtaining genuine stable equilibrium, as well as to the effects of oxygen impurities, which tend to stabilize the d + e two-phase field [63Sin]. The present evaluators accept the existence of a d + e two-phase field at low enough temperatures. [68Moo1] concluded that the d + e region exists up to around 728 K, where it closes to practically a single line for higher temperatures with characteristics similar to those of a second-order transformation. For the d and e phases at temperatures below approximately 200 K, anomalies in the temperature dependence of the specific heat [73Top], elastic moduli [80Byd] , and the proton spin-lattice relaxation time have been observed, and the existence of disorder-order transitions and the formation of superstructures have been suggested. However, firm experimental evidence of structural changes is lacking in the reported anomalies. It has been suggested [83Rat] that one of the anomalies arises from the increased involvement of optical modes in the vibrational spectrum of Zr-H and not from an ordering process. Until there is more substantial evidence for the existence of these ordered structures in the d and e phases of Zr-H, they must be regarded as unconfirmed possibilities. [62Bec] identified a metastable hydride phase, g, of composition X ~ 1.0, which appeared on cooling into the lower temperature range of the (a + d) region. However, [72Mis] disagreed with this result and suggested that the g phase was a stable phase formed at about 528 K through the peritectoid reaction a + d = g. Currently, the existence of the peritectoid reaction has been discarded on the basis of experimental results, and the generally accepted opinion [83Nor] is that the g phase is in fact a metastable phase. Additionally, recent enthalpy calculations [85Ive] support this conclusion. The formation of the g phase appears to be favored by higher rates of quenching, whereas lower rates favor the formation of the stable d phase [ 83Nor]. [81Wea] recognized that the formation of the g phase from the a phase occurs by means of a hybrid process involving a shear of the cph lattice and a simultaneous diffusional migration of hydrogen atoms. A similar situation seems to exist for the d <259> g transformation [80Cas]. [82Dey] further conjectured that the formation of g phase in the a phase involves an intermediate step of b phase formation, where the bcc structure of b phase provides an easy path for the shearing process involved in the a <259> g transition. 55Gul: E.A. Gulbransen and K.F. Andrew, Trans. AIME, 203, 136-144 (1955). 56Vau: D.A. Vaughan and J.R. Bridge, J. Met., 8, 528-531 (1956). 59Lag: L.D. LaGrange, L.J. Dykstra, J.M. Dixon, and U. Merten, J. Phys. Chem., 63, 2035-2041 (1959). 60Kem: C.P. Kemper, R.O. Elliott, and K.A. Gschneidner, J. Chem. Phys., 33, 837-840 (1960). 60Lib: G.G. Libowitz, USAEC Rep. NAA-SR-5015, Atomics International (1960). 62Bec: R.L. Beck, Trans. ASM, 55, 542-555 (1962). 62Lib: G.G. Libowitz, J. Nucl. Mater., 5, 228-233 (1962). 63Sin: K.P. Singh and J.G. Parr, Trans. Faraday Soc., 59, 2256-2259 (1963). 67Kea: J.J. Kearns, J. Nucl. Mater., 22, 292-303 (1968). 68Moo1: K.E. Moore and W.A. Young, J. Nucl. Mater., 27, 316-324 (1968). 68Moo2: K.E. Moore and M.M. Nakata, AI-AEC-12703, 30 Sep (1968). 69Moo: K.E. Moore, J. Nucl. Mater., 32, 46-56 (1969). 70Bar: K.G. Barraclough and C.J. Beever, J. Nucl. Mater., 34, 125-134 (1970). 72Mis: S. Mishra, K.S. Sivaramakrishnan, and M.K. Asundi, J. Nucl. Mater., 45, 235-244 (1972). 73Top: L.S. Topchyan, I.A. Naskidashvili, R.A. Andrievskii, and V.I. Savin, Fiz. Tverd. Tela, 15, 2195-2197 (1973) in Russian; TR: Sov. Phys. Solid State, 15, 1461-1462 (1974). 80Byd: I.N. Bydlinskaya, I.A. Naskidashvili, V.A. 'Melik-Shakhnazarov, and V.I. Savin, Fiz. Tverd. Tela, 22, 886-888 (1980) in Russian; TR: Sov. Phys. Solid State, 22, 517-518 (1980). 80Cas: M.P. Cassidy and C.M. Wayman, Metall. Trans. A, 11, 47-56 (1980). 81Wea: G.C. Weatherly, Acta Metall., 29, 501-512 (1981). 82Dey: G.K. Dey, S. Banerjee, and P. Mukhopadhyay, J. Phys. Coll., 43, 327-332 (1982). 83Nor: D.O. Northwood and O. Kosasih, Int. Met. Rev., 28, 92-121 (1983). 83Rat: I.G. Ratishvili, Fiz. Met. Metalloved., 55, 665-675 (1983) in Russian; TR: Phys. Met. Metallogr., 55, 34-43 (1983). 83Rit: I.G. Ritchie and K.W. Sprungmann, AECL-7806, Dec (1983). 86Zuc: E. Zuzek, Surf. Coat. Technol., 28, 323-338 (1986). Published in Bull. Alloy Phase Diagrams, 11(4), Aug 1990. Complete evaluation contains 6 figures, 11 tables, and 114 references. 1