Фазовая диаграмма системы H-Ni

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H-Ni (Hydrogen-Nickel) M.L. Wayman and G.C. Weatherly The behavior of the Ni-H system depends strongly on the activity (pressure) of hydrogen. In equilibrium with hydrogen pressures of 1 atm or less, Ni exhibits the features of a classical "endothermic occluder" of hydrogen, in that it reacts endothermically with hydrogen. The result is a very low solubility ( interstitial) of hydrogen in Ni at room temperature (H/Ni › 3 x 10-5) and, most characteristically, a positive temperature dependence of solubility. The solubility of hydrogen is an order of magnitude higher at 1000 atm than at 1 atm. The assessed evaluation of the Ni-H system is based primarily on review of the experimental data of [67Jan, 67Maj, 73Rob, 75Mcl, 78Pon, 80Ser, 81Bar, and 84Mcl]. At hydrogen pressures in the range of 10 to 100 MPa, hydrogen appreciably reduces the melting point of Ni, but this effect diminishes above 40 MPa hydrogen pressure. The system exhibits a eutectic point at 1679 K (1406 C) and H/Ni › 0.036 [81Sha]. The behavior at a hydrogen pressure of 100 MPa is comparable [80Ser]. The microstructure of Ni that has been melted and then quenched from the liquid phase in a hydrogen atmosphere (melt composition up to H/Ni = 0.02) consists of a eutectic mixture of solid metal with hydrogen gas bubbles [79Sha]. Nickel hydride is not observed following such treatments. When Ni is exposed to environments in which the hydrogen activity is very high, corresponding to hydrogen pressures well in excess of 100 MPa, the behavior of the Ni-H system changes markedly. Such high hydrogen activity can be obtained by electrochemically (cathodically) charging the Ni in an electrolyte, for example 1N H2SO4 containing a hydrogen recombination poison such as thiourea or arsenic trioxide. Alternatively, Ni can be subjected to high pressures of hydrogen in a pressure cell; cells capable of hydrogenation at pressures as high as 7000 MPa of hydrogen have been reported [78Pon]. The high-pressure cell techniques yield a controlled atmosphere of well- defined hydrogen activity, which can be maintained long enough to ensure that the system has reached equilibrium [81Bar]. The attainment of high hydrogen activity is facilitated by the high fugacity coefficient of hydrogen, which can often exceed a value of 100, thus permitting higher hydrogen activity at lower hydrogen pressure than would otherwise be the case [84Bar]. With the application of pressures of hydrogen in excess of 1000 MPa, the Ni-H system exhibits an equilibrium surface in pressure-temperature-composition space. This surface is markedly similar to the pressure-temperature-vapor surface for a free gas. The critical point, below which the Ni-H solid solution decomposes to form a hydrogen-poor a solid solution and a hydrogen- rich nonstoichiometric bNi hydride, lies in the vicinity of 623 to 703 K and 1600 to 1900 MPa of hydrogen [79Bar]. The strong pressure dependence of the Ni- H system corresponds to a transition from an endothermic reaction with hydrogen at low pressures to an exothermic reaction at high hydrogen pressures. The hydrogen pressure necessary for the formation of bNi hydride-about 600 MPa- is significantly higher than its decomposition pressure of 340 MPa. The decomposition pressure is believed to represent the equilibrium value of pressure for a-b coexistence [63Sch]. As hydrogen is absorbed at room temperature under high pressure conditions, the terminal solid solubility amax is soon reached at H/Ni < 0.1. Further absorption of hydrogen occurs by the formation of the b hydride in equilibrium with the a solid solution. The b hydride phase has the same fcc crystal structure as Ni and the a solid solution, but with a high concentration of interstitial H atoms and, consequently, a larger lattice parameter [67Jan]. As the temperature is increased, the maximum hydrogen content in the a phase increases, whereas the minimum hydrogen content of the a phase decreases. Thus, the miscibility gap narrows and finally disappears at the critical point. Above the critical point, a continuous range of hydrogen contents occurs; the terms a and b then refer to the same phase. Values of amax and bmin measured in the vicinity of 298 K [67Maj, 78Pon] lie in the range 0.02 њ amax њ 0.1 and 0.6 њ bmin њ 1.0, respectively. At hydrogen contents between amax and bmin (below the critical point) in the region of immiscibility, the a <259> b transformation can be followed by monitoring the electrical resistance, which decreases as b hydride forms. The b hydride is paramagnetic, whereas a is ferromagnetic. Thus, as the a transforms to b, the saturation magnetization falls to zero in direct proportion to the amount of b phase. In a similar manner, the high value of thermoelectric power of the b hydride relative to the a phase has been used to follow the a <259> b and b <259> a phase transformations [71Sko]. These electrical and magnetic effects are associated with the holes in the d band of Ni being filled with electrons from the hydrogen atoms. The Ni-H system is notably similar to the Pd-H system, but because the critical point of the latter is at a much more easily obtainable pressure ( ~2 MPa, a factor of 1000 lower than that of Ni-H), the Pd-H system has been more widely studied. Other than the coordinates of the critical point, the major difference between the two systems is that the formation of the b hydride causes a much greater lattice expansion in Ni (5.8% increase in lattice parameter as compared to 3.5% in Pd). This is clearly related to the observation that bmin, the minimum hydrogen content of the b hydride at room temperature, is higher in the Ni-H system than in the Pd-H system. There is also a much greater hysteresis in the Ni-H system. The effect on hydrogen solubility of the transition from the ferromagnetic to the paramagnetic state at the Curie temperature is unclear. Some workers have detected a drop in solubility in the vicinity of the Curie temperature [83Sha], as well as a change in the heat of solution [83Sha, 83Vya]. On the other hand, [84Mcl] and others indicated that there is no discernible discontinuity at the Curie temperature. Dislocations and grain boundaries are potential trapping sites, and studies have been carried out on the effects of cold working and grain size on hydrogen solubility. The results of these studies are inconclusive. By comparing single-crystal behavior with that of polycrystals, or by studying polycrystals of varying grain size, [83Vya] found no evidence for the trapping of hydrogen at grain boundaries. [84Mcl] has stated that hydrogen grain boundary or hydrogen dislocation interactions should not cause solubility anomalies. [74Sta] and [86Lee], however, showed an apparent trapping of hydrogen at grain boundaries. Similarly, there are reports of enhanced solubility in cold worked Ni, as a result of hydrogen trapping by dislocations [86Lee], as well as reports of the absence of such an effect [ 59Gri]. Thus, at present, the effects of hydrogen traps on solubility remain uncertain. At hydrogen pressures high enough that the b hydride phase forms, terminal solubility is synonymous with amax, the maximum hydrogen content in the a solid solution in equilibrium with b hydride. The shapes of the absorption isotherms indicate that as the hydrogen pressure increases the hydrogen solubility increases. However, both the hydrogen gas and the a solid solution increasingly deviate from ideality, causing strong positive deviations from Sieverts' law as amax is approached. Values of amax do not appear to be strongly dependent on temperature below 600 K, but may increase as the critical temperature is approached. Data from above the critical point, where the terminal solid solubility is simply the hydrogen content in equilibrium with gaseous hydrogen (because a and b are no longer distinct entities), are not yet available. Increasing the hydrogen pressure has also been found to lower the Curie temperature of the a phase from about 632 K in pure Ni to about 550 K at a hydrogen pressure of about 1200 MPa [77Ant, 78Pon], where amax is reached at H/ Ni ~ 0.02 [77Ant]. The bNi hydride phase exists in equilibrium with high pressures of hydrogen at temperatures below the critical point. It consists of a fcc lattice of Ni atoms with hydrogen atoms located in the octahedral interstices [63Wol]. The bNi hydride phase is paramagnetic [81Bau]. It has a metallic character, with a positive temperature dependence of electrical resistivity. 29Sie: A. Sieverts, Z. Metallkd., 21, 37-46 (1929). 34Smi: J. Smittenberg, Rec. Trav. Chim. Pays-Bas, 53, 1065-1083 (1934). 43Arm: M.H. Armbruster, J. Am. Chem. Soc., 65, 1043-1054 (1943). 59Gri: H.H. Grimes, Acta Metall., 7, 782-786 (1959). 63Sch: N.A. Scholtus and W.K. Hall, J. Chem. Phys., 39, 868-870 (1963). 63Wol: E.O. Wollan, J.W. Cable, and W.C. Koehler, J. Phys. Chem. Solids, 24, 1141-1143 (1963). 67Jan: A. Janko and J. Pielaszek, Bull. Acad. Pol. Sci., Ser. Sci. Chim., 15, 569-572 (1967). 67Maj: S. Majchrzak, Bull. Acad. Pol. Sci., Ser. Sci. Chim., 15, 485-490 (1967) . 68Bla: J.S. Blakemore, W.A. Oates, and E.O. Hall, Trans. AIME, 242, 332-333 ( 1968). 70Com: P. Combette and P. Azou, Mem. Sci. Rev. Metall., 67, 17-31 (1970). 73Mcl: R.B. McLellan and W.A. Oates, Acta Metall., 21, 181-185 (1973). 73Rob: W.M. Robertson, Z. Metallkd., 64, 436-443 (1973). 74Sta: S.W. Stafford and R.B. McLellan, Acta Metall., 22, 1463-1468 (1974). 75Mcl: R.B. McLellan and C.G. Harkins, Mater. Sci. Eng., 18, 5-35 (1975). 77Ant: V.E. Antonov, I.T. Belash, and E.G. Ponyatovskii, Dokl. Akad. Nauk SSSR, 233, 1114-1117 (1977) in Russian. 78Bar: B. Baranowski, in Topics in Applied Physics, Vol. 29, Hydrogen in Metals II, G. Alefeld and J. Volkl, Ed., Springer-Verlag, Berlin, 157-200 ( 1978). 78Pon: E.G. Ponyatovskii, V.E. Antonov, and I.T. Belash, Neorg. Mater., 14, 1570-1580 (1978) in Russian. 79Bar: B. Baranowski, Z. Phys. Chem. (Neue Folge), 114, 71-93 (1979). 79Sha: V.I. Shapovatov and N.P. Serdyuk, Zh. Fiz. Khim., 53, 2187-2191 (1979) in Russian; TR: Russ. J. Phys. Chem., 53, 1250-1252 (1979). 80Ser: N.P. Serdyuk and A.L. Chuprina, Russ. J. Phys. Chem., 54, 1615-1618 ( 1980). 81Bar: B. Baranowski, in Metal Hydrides, G. Bambakidis, Ed., Plenum Press, New York, 193-213 (1981). 81Bau: H.J. Bauer, in Metal Hydrides, G. Bambakidis, Ed., Plenum Press, New York, 313-327 (1981). 83Sha: V.I. Shapovalov and L.B. Boyko, Fiz. Met. Metalloved., 55, 1220-1221 ( 1983) in Russian. 83Vya: A.F. Vyatkin, P.V. Zhorin, and E.M. Tseitlin, Zh. Fiz. Khim., 57, 419- 422 (1983) in Russian; TR: Russ. J. Phys. Chem., 57, 249-251 (1983). 84Mcl: R.B. McLellan and P.L. Sutter, Acta Metall., 32, 2233-2239 (1984). 86Lee: S.-M. Lee and J.-Y. Lee, Metall. Trans. A, 17 181-187 (1986). Published in Phase Diagrams of Binary Nickel Alloys, 1991, and Bull. Alloy Phase Diagrams, 10(5), Oct 1989. Complete evaluation contains 7 figures, 9 tables, and 80 references. 1