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On the kinetics of atomic ordering in A15 compounds
Physica 135B (1985) 353-354 North-Holland, Amsterdam
ON THE KINETICS OF A T O M I C O R D E R I N G IN A15 C O M P O U N D S *
D.O. W E L C H Brookhaven National Laboratory, Upton, NY 11973, USA
R. F L U K I G E R Institut fiir Technische Physik, Kernforschungszentrum Karlsruhe, D-7500 Karlsruhe, Fed. Rep. Germany
Extended abstract
Many of the supercoducting critical properties of A15-structure intermetallic compounds depend sensitively on the degree of atomic order. Small deviations from perfect order can cause large changes in the critical temperature and upper critical magnetic field. Deviations from perfect order can be caused by equilibrium thermal fluctuations, radiation damage, or non-equilibrium processes during compound synthesis. Understanding of the kinetics of order-disorder phenomena is thus of some importance in optimizing the superconducting properties of A15 compounds. A theory of ordering kinetics, based on an oversimplified model of interatomic bonding in the A15 structure, was proposed earlier [1] and applied to reordering kinetics in radiation damaged [1] and rapidly solidified A15 compounds [2]. Under certain limiting conditions, the kinetics can be either first- or second-order. The studies above found that second-order kinetics gave a marginally better description of the data. Recently, however, computer simulation studies [3] of disorder and point-defect mobilities in Nb3Sn revealed that the original kinetic model [1] was probably based on inappropriate assumptions regarding interatomic bonding. The calculated results of the point defect jump rates for Nb3Sn
* Research at Brookhaven was supported by the U.S. Department of Energy, Division of Materials Sciences, Office of Basic Energy Sciences under Contract No. DE-AC0276CH00016.
suggest that first-order reordering kinetics should be found for this compound and that the controlling activation energy should be that for the site exchange of a Nb atom on the Sn sublattice with a vacancy on the Nb sublattice. Recent experimental studies [4] of reordering kinetics for rapidlyquenched V3Ga find first-order kinetics, and thus seem to support the theoretical prediction for Nb3Sn. We report here conclusions based on the results of studies of reordering kinetics recently done at Karlsruhe for argon-jet quenched Nb3AI, Nb3Pt, and Nb3Au0.7Pt0.3: (1) simple first-order kinetics are not observed for any of these compounds; (2) second-order kinetics are approximately followed for Nb3Pt (in agreement with earlier results [2]) and for Nb3Au0.7Pt0.3; (3) neither simple first- nor second-order kinetics are followed for Nb3AI, and the kinetics for this compound are not much affected by small deviations from perfect stoichiometry; (4) the time required to anneal out about one-half of the quenched-in disorder scales with approximate activation energies of about 5, 4, and 2 eV for Nb3AI (1000-1050 K), Nb3Au0.vPt0. 3 (10501100 K), and Nb3Pt (1150-1200 K), respectively, in the temperature ranges noted (note that the higher activation energies correspond, anomalously, to the lower temperature ranges). Thus, these studies reveal that the theoretical predictions for reordering kinetics in Nb3Sn do not seem to be obeyed for some other A15 compounds and that the reordering kinetics do not follow any simple universal behavior for these materials.
0378-4363/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
354
D.O. Welch and R. Fliikiger / Atomic ordering in A15 compounds
References [1] D. Dew-Hughes, S. Moehlecke and D.O. Welch, J. Nucl. Mater. 72 (1978) 225. [2] D. Dew-Hughes, J. Phys. Chem. Solids 41 (1980) 851.
[3] D.O. Welch, G.J. Dienes, O.W. Lazareth Jr. and R.D. Hatcher, J. Phys. Chem. Solids 45 (1984) 1225. [4] A. van Winkel, Ph.D. Thesis, Natuurkundig Laboratorium, University of Amsterdam (1985). A. van Winkel and H. Bakker, J. Phys. F15 (1985) 1565.
ON THE KINETICS OF A T O M I C O R D E R I N G IN A15 C O M P O U N D S *
D.O. W E L C H Brookhaven National Laboratory, Upton, NY 11973, USA
R. F L U K I G E R Institut fiir Technische Physik, Kernforschungszentrum Karlsruhe, D-7500 Karlsruhe, Fed. Rep. Germany
Extended abstract
Many of the supercoducting critical properties of A15-structure intermetallic compounds depend sensitively on the degree of atomic order. Small deviations from perfect order can cause large changes in the critical temperature and upper critical magnetic field. Deviations from perfect order can be caused by equilibrium thermal fluctuations, radiation damage, or non-equilibrium processes during compound synthesis. Understanding of the kinetics of order-disorder phenomena is thus of some importance in optimizing the superconducting properties of A15 compounds. A theory of ordering kinetics, based on an oversimplified model of interatomic bonding in the A15 structure, was proposed earlier [1] and applied to reordering kinetics in radiation damaged [1] and rapidly solidified A15 compounds [2]. Under certain limiting conditions, the kinetics can be either first- or second-order. The studies above found that second-order kinetics gave a marginally better description of the data. Recently, however, computer simulation studies [3] of disorder and point-defect mobilities in Nb3Sn revealed that the original kinetic model [1] was probably based on inappropriate assumptions regarding interatomic bonding. The calculated results of the point defect jump rates for Nb3Sn
* Research at Brookhaven was supported by the U.S. Department of Energy, Division of Materials Sciences, Office of Basic Energy Sciences under Contract No. DE-AC0276CH00016.
suggest that first-order reordering kinetics should be found for this compound and that the controlling activation energy should be that for the site exchange of a Nb atom on the Sn sublattice with a vacancy on the Nb sublattice. Recent experimental studies [4] of reordering kinetics for rapidlyquenched V3Ga find first-order kinetics, and thus seem to support the theoretical prediction for Nb3Sn. We report here conclusions based on the results of studies of reordering kinetics recently done at Karlsruhe for argon-jet quenched Nb3AI, Nb3Pt, and Nb3Au0.7Pt0.3: (1) simple first-order kinetics are not observed for any of these compounds; (2) second-order kinetics are approximately followed for Nb3Pt (in agreement with earlier results [2]) and for Nb3Au0.7Pt0.3; (3) neither simple first- nor second-order kinetics are followed for Nb3AI, and the kinetics for this compound are not much affected by small deviations from perfect stoichiometry; (4) the time required to anneal out about one-half of the quenched-in disorder scales with approximate activation energies of about 5, 4, and 2 eV for Nb3AI (1000-1050 K), Nb3Au0.vPt0. 3 (10501100 K), and Nb3Pt (1150-1200 K), respectively, in the temperature ranges noted (note that the higher activation energies correspond, anomalously, to the lower temperature ranges). Thus, these studies reveal that the theoretical predictions for reordering kinetics in Nb3Sn do not seem to be obeyed for some other A15 compounds and that the reordering kinetics do not follow any simple universal behavior for these materials.
0378-4363/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
354
D.O. Welch and R. Fliikiger / Atomic ordering in A15 compounds
References [1] D. Dew-Hughes, S. Moehlecke and D.O. Welch, J. Nucl. Mater. 72 (1978) 225. [2] D. Dew-Hughes, J. Phys. Chem. Solids 41 (1980) 851.
[3] D.O. Welch, G.J. Dienes, O.W. Lazareth Jr. and R.D. Hatcher, J. Phys. Chem. Solids 45 (1984) 1225. [4] A. van Winkel, Ph.D. Thesis, Natuurkundig Laboratorium, University of Amsterdam (1985). A. van Winkel and H. Bakker, J. Phys. F15 (1985) 1565.