Фазовая диаграмма системы Cr-H

К оглавлению: Другие диаграммы (Others phase diargams)

Cr-H (Chromium-Hydrogen) M. Venkatraman and J.P. Neumann Despite the technical importance of the Cr-H system, the available information concerning the phase diagram is limited and not very reliable. Based on the few investigations that have been carried out in the Cr-rich region, a partial phase diagram was constructed. The occurrence of three intermediate phases-CrH, CrH2, and CrH3-has been reported in the literature, but the existence of CrH3 is doubtful. The hydrides are stable only at elevated H2 pressures, but they can be retained in a metastable state at room temperature for limited times. The need for high-pressure studies and the occurrence of nonequilibrium states make it difficult to establish a reliable Cr-H phase diagram. The liquidus and solidus curves of the assessed diagram are based on the data of [81Sha]. Because the melting temperature of Cr reported by [81Sha] is 90 C lower than the adopted value of 1863 C [Melt], the temperatures of both curves were raised by 90 C. The isobar p(H2) = 1 bar in the liquid state is taken from [71Pet]: log (at.% H) = 1.970 - 5330/T(K). The equation is based on the measurements of the solubility of H in liquid Cr-Si alloys by [71Pet] and on the single measurement of the solubility of H in pure liquid Cr by [63Wei], who reported a value of~0.33 at.% H at 1903 C and 1 bar. The isobar p(H2) = 100 bar was calculated from the 1-bar isobar, assuming that Sieverts' law is obeyed [81Sha]. The isobar p(H2) = 100 bar by [81Sha] is incorrect. It appears that [81Sha] made an error when converting the solubility equation of [71Pet] from the units "atomic fraction hydrogen" to "cm3 H2 per 100 g of Cr." The isobar p (H2) = 1 bar in the solid state in the assessed diagram was obtained by extrapolation of the solubility curve to higher temperatures. Analogous to that for the liquid state, the p (H2) = 100-bar isobar in the solid state is calculated from the 1-bar curve, assuming validity of Sieverts' law. The liquidus and solidus curves in the assessed diagram were drawn to obtain a match of the isobars in the liquid and solid states. The solubility of H in solid Cr for p (H2) = 1 bar was measured by [29Mar], [ 32Luc], [73Arn], and [81Sha]. Above 727 C, the studies are in good agreement. A linear regression analysis was applied to the data of [29Mar], [32Luc], and [ 73Arn]; it yields for the solubility of H for p (H2) = 1 bar log (at.% H) = 0. 120 - 2610/T (1000 to 1700 K). Below 727 C, the measured solubilities are higher than the solubilities predicted. It is suggested that this is due either to metastable conditions or to lattice defects-such as dislocations or grain boundaries-that may affect the solubility at these low H concentrations. No explanation was given by [ 81Sha] for their observation that the solubility of H below 627 C increases with decreasing temperature. According to X-ray [63Kno1] and electrical conductivity measurements [63Kno2], the solubility of H in solid Cr in equilibrium with CrH is approximately 4 at.% H between room temperature and 100 C. However, metastable, supersaturated ( Cr) solutions with up to 15 to 20 at.% H form easily [25Hut, 63Kno1]. The existence of CrH and CrH2 has been clearly established, but their stability as a function of composition, temperature, and pressure requires clarification. However, the hydrides are stable only at elevated H2 pressures. The homogeneity ranges of both hydrides extend over several atomic percent, but the exact location of the phase boundaries is controversial. Based on structural considerations and experimental results, [47Sna] concluded that the homogeneity range of CrH extends from 33 to 50 at.% H, but according to other investigators, the range is considerably smaller. X-ray measurements [63Kno1] and electrical conductivity studies [63Kno2] indicated a homogeneity range from 48.5 to 50.0 at.% H, and [64Alb] prepared the hexagonal hydride at 46.0 and 47.6 at.% H. Magnetic susceptibility and X-ray investigations by [67Roy] placed the homogeneity range from 47.5 to 60.0 at.% H-beyond the equiatomic composition CrH. An approximate homogeneity range of CrH from 47 to 50 at.% H is accepted in this evaluation. The fcc hydride CrH2 was identified at 63 at.% H by [47Sna]. [49Sna] suggested that CrH2 may exist over the composition range 50 to 67 at.% H. X-ray studies by [63Kno1] revealed the presence of both the fcc and the hexagonal hydride in the composition range 50.7 to 52.3 at.% H; [63Kno1] was not able to prepare samples containing more than 52.3 at.% H. Tentatively, it is suggested that the homogeneity range of CrH2 extends from 55 to 67 at.% H. [26Wei] prepared the hydride CrH3 by means of the Grignard reaction, but the results of these experiments were questioned by [55Sar]. At the present time, the existence of CrH3 must be considered doubtful. The stability of CrH at elevated H2 pressures was studied by [56Trz], [75Bar], and [76Pon]. [56Trz] observed that CrH prepared by electrodeposition was not stable at room temperature at an H2 pressure of ~140 bar. [75Bar] and [76Pon] studied the formation and decomposition of CrH from 150 to 400 C and from 3 to 20 kbar. A hysteresis of 5 to 15 kbar between the formation and the decomposition pressures was observed, with the formation of the hydride requiring the higher pressure. According to [76Pon], it is assumed that the equilibrium pressure between (Cr) and CrH is given by the decomposition pressure. Based on the experimental data of [76Pon], a linear regression analysis yields the following expression for the equilibrium H2 pressure between (Cr) and CrH as a function of temperature: log p (H2)/bar = 5.2 - 680/ T (400 to 700 K). The measurement by [75Bar], who reported an equilibrium pressure of 3.2 kbar at 150 C, is in reasonable agreement with the results of [76Pon]. Low-temperature magnetic susceptibility studies of Cr-H alloys ranging from 0 to 58 at.% H were carried out by [64Alb], [65Pro], [66Alb], [67Roy], [74Kha], [ 75Han], and [76Kha]. The hydride CrH was found to be paramagnetic. 25Hut: G.F. HЃttig and F. Brodkorb, Z. Anorg. Chem., 144, 341-348 (1925) in German. 26Bra: A.J. Bradley and E.F. Ollard, Nature, 117, 122 (1926). 26Wei: T. Weichselfelder and B. Thiede, Justus Liebigs Ann. Chem., 447, 64-77 ( 1926) in German. 29Mar: E. Martin, Arch. EisenhЃttenwes., 3, 407-416 (1929) in German. 31Sas: K. Sasaki and S. Sekito, Trans. Electrochem. Soc., 59, 437-444 (1931). 32Luc: L. Luckemeyer-Hasse and H. Schenck, Arch. EisenhЃttenwes., 6(5), 209- 214 (1932) in German. 35Wri: L. Wright, H. Hirst, and J. Riley, Trans. Faraday Soc., 31, 1253-1259 ( 1935). 47Sna: C.A. Snavely, Trans. Electrochem. Soc., 92, 537-576 (1947). 49Sna: C.A. Snavely and D.A. Vaughan, J. Am. Chem. Soc., 71, 313-314 (1949). 55Sar: B. Sarry, Z. Anorg. Allg. Chem., 280, 65-77 (1955) in German. 56Trz: M.J. Trzeciak, D.F. Dilthey, and M.W. Mallett, Battelle Memorial Institute Rep. No. BMI-1112, 32 p (1956). 63Kno1: A. Kn”dler, Metalloberfl„che, 17, 161-168 (1963) in German. 63Kno2: A. Kn”dler, Metalloberfl„che, 17(11), 331-337 (1963) in German. 63Wei: M. Weinstein and J.F. Elliott, Trans. AIME, 227, 285-286 (1963). 64Alb: G. Albrecht and R. Perthel, Phys. Status Solidi, 7, K19-K24 (1964). 65Pro: A.A. Proskurnikov and E.I. Krylov, Zh. Neorg. Khim., 10(5), 1017-1021 ( 1965) in Russian; TR: Russ. J. Inorg. Chem., 10, 551-554 (1965). 66Alb: G. Albrecht and B. Schnabel, Phys. Status Solidi, 15, 141-151 (1966) in German. 67Roy: R.J. Roy and T.R.P. Gibb, J. Inorg. Nucl. Chem., 29, 341-345 (1967). 71Pet: M.S. Petrushevskiy, P.V. Geld, B.A. Baum, and T.K. Kostina, Izv. Akad. Nauk SSSR, Met., 5, 67-72 (1971) in Russian; TR: Russ. Metall., 5, 21-26 (1971) . 73Arn: W.J. Arnoult and R.B. McLellan, Acta Metall., 21, 1397-1403 (1973). 74Kha: H.R. Khan, A. Kn”dler, C.J. Raub, and A.C. Lawson, Mater. Res. Bull., 9, 1191-1198 (1974). 75Bar: B. Baranowski, K. Bojarski, and M. Tkacz, Proc. IV. Int. Conf. on High Pressure, The Physico-Chemical Society of Japan, Kyoto, 577-579 (1975). 75Han: M. Hanson, H.R. Khan, A. Kn”dler, and C.J. Raub, J. Less-Common Met., 43, 93-95 (1975). 75Vis: R. Viswanathan, H.R. Khan, A. Kn”dler, and C.J. Raub, J. Appl. Phys., 46, 4088-4089 (1975). 76Kha: H.R. Khan and Ch.J. Raub, J. Less-Common Met., 49, 399-406 (1976). 76Pon: E.G. Ponyatovskii and I.T. Belash, Dokl. Akad. Nauk SSSR, 229, 1171- 1173 (1976) in Russian; TR: Dokl. Phys. Chem., 229(5), 738-740 (1976). 81Sha: V.T. Shapovalov, N.P. Serdyuk, and V.I. Dolzhenkov, Dop. Akad. Nauk Ukr. RSR A, Fiz. -Mat. Tekh., (3), 87-89 (1981) in Ukrainian. Submitted to the APD Program. Complete evaluation contains 3 figures, 3 tables, and 57 references. 1