Oxidation of Ni — Ta alloys. III. Mechanism of oxidation

The results of an x-ray diffraction and metallographic study of the kinetics of scale formation during the oxidation of Ni(Ta), Ni₃Ta, and NiTa in air at 600–1000°C are analyzed. The free energies, equilibrium oxygen pressures, and mass balances of the oxidation reactions were calculated, and conditions for the formation of NiO·Ta₂O₅ and NiO on the alloy determined. It is shown that the oxidation process is controlled primarily by the diffusion of oxygen and counter-diffusion of Ni⁺² in the scale, and involves oxidation, reduction, and synthesis reactions. A multilayer scale is formed, consisting of an outer layer containing only oxides (NiO, NiO·Ta₂O₅, Ta₂O₅) and an inner one which additionally contains nickel. The protective ability of the outer scale depends upon the concentrations of NiO and NiO·Ta₂O₅ in it. Preferential oxidation of tantalum is responsible for the appearance of a subscale consisting of Ni(Ta) + Ta₂O₅ on the intermetallic Ni₃Ta, and Ni₃Ta + Ni(Ta) + Ta₂O₅ on NiTa. Differences in molar volumes of phases result in the formation of pores and cracks at interphase boundaries, particularly in the inner scale on Ni₃Ta. A change in the oxidation mechanism occurs at T ≥ 850°C as a result of the p → n transition in Ta₂O₅, which leads to retarded oxygen diffusion and the appearance of Ta⁺⁵ diffusion in the intermetallic. This, as well as the diffusion of Ni⁺², promotes the healing of macrodefects in the scale. However, it also results in enrichment of the outer scale in pentoxide, which decreases its protective ability. Translated from Poroshkovaya Metallurgiya, Nos. 5–6(413), pp. 69–78, May–June, 2000.     

Influence of crystallography upon critical nucleus shapes and kinetics of homogeneous f.c.c.-f.c.c. nucleation—III. The influence of elastic strain energy

The influence of crystallography upon critical nucleus shapes and kinetics of homogeneous f.c.c.-f.c.c. nucleation was studied by combining the discrete lattice point non-classical model utilized in paper II of this series with the microscopic theory of strain energy as applied by Cook and de Fontaine to compositional fluctuations in cubic lattices. This permits simultaneous consideration of the influence of anistropic interfacial energy and anisotropic strain energy upon nucleation. This is extremely difficult to do using other models of strain energy because of mathematical problems. The model is applied to f.c.c. nuclei in AlCu and CuCo alloys. Nuclei in AlCu are plates because coherency strain energy predominates in this system. In CuCo alloys, the misfit is smaller and the distortion tensor is cubic; thus the nuclei are essentially spherical. Despite this lesser effect, the influence of strain energy upon the nucleation kinetics of Co-rich precipitates in a Cu-rich CuCo alloy is found to be very important, reducing these kinetics by as much as five orders of magnitude.     

Report of the… .

Asummary of a report containing "information about subdoctoral training programs,… questions the committee deemed relevant and important,… and appropriate recommendations." (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Influence of crystallography upon critical nucleus shapes and kinetics of homogeneous f.c.c.-f.c.c. nucleation—V. The origin of GP zones in AlAg and AlCu alloys

GP zone formation in aluminum-base alloys has been proposed, by previous investigators, to take place by spinodal decomposition or by homogeneous nucleation and growth. The possibilities of these two mechanisms are examined here for two alloy systems, AlAg and AlCu, representing two extreme cases, a narrow and a (potentially) very wide miscibility gap, respectively, resulting in low and high interphase boundary energies. The coherent spinodal and the temperature-composition curve corresponding to abundant homogeneous f.c.c.-f.c.c. nucleation were calculated for both systems. In AlAg alloys, the nucleation barrier is so low that negligible undercooling is needed to achieve abundant nucleation. Hence cooling into the spinodal region with ordinary quenching rates is virtually impossible. In AlCu, however, the inverse situation is present and GP zone formation by (or aided by) spinodal decomposition should be feasible if the diffusivity remains sufficiently high in the spinodal region. Detailed evaluation of the latter proposal is prevented, however, by the incompleteness of thermodynamic information on f.c.c. AlCu alloys.     

Influence of crystallography upon critical nucleus shapes and kinetics of homogeneous F.C.C.-F.C.C. nucleation—II. The non-classical regime

In a previous paper, the shape of the critical nucleus and its influence upon nucleation kinetics were studied at near zero supersaturation, i.e. in the classical nucleation regime, using a discrete lattice plane model of the coherent interphase boundary energy. In order to extend these studies to higher supersaturations, a discrete lattice point model, based on a formalism developed by Cook, de Fontaine and Hilliard, is employed in the present paper and applied to homogeneous nucleation of coherent f.c.c. precipitates in an f.c.c. matrix. Concentration profiles and free energies of formation of critical nuclei are calculated from this model as a function of temperature and supersaturation and compared with results obtained from the Cahn-Hilliard continuum non-classical model and the previously used discrete lattice plane classical model. As predicted in effect by Cahn and Hilliard, the three models converge at very low supersaturations, and the continuum and the discrete lattice point (but not the classical discrete lattice plane) models also do so near the spinodal. Thus the most important differences between the continuum and the discrete lattice point models develop at intermediate supersaturations. The main advantages of the discrete lattice point model are that it allows the influence of crystalline anisotropy to be taken into account, permits treatment of arbitrarily steep variations in composition and provides a more convenient milieu for the incorporation of volume strain energy, as is done in the next paper in this series.     

Overview no. 76: Mechanism of deformation and development of rolling textures in polycrystalline f.c.c. metals—II. Simulation and interpretation of experiments on the basis of Taylor-type theories

Simulations of f.c.c. rolling textures based on homogeneous slip under conditions of full and relaxed constraints (Taylor-type theories) are presented. The characteristic peak and fibre components found in the resultant ODFs for the various relaxed shear possibilities are analysed in great detail and compared with the experimentally observed textures in Al and the CuZn alloys given in Part I. Special attention has been paid to the questions of stability and metastability of different orientations, the position of the fibres in orientation space, the influence of initial textures, the pathways that individual orientations take together with their rates of rotation along and their arrival rates at the fibres. Finally the topological distribution of grains in connection with the compatibility problem of the different stable orientations caused by relaxed shears is discussed.     

A discrete lattice plane analysis of the interfacial energy of coherent F.C.C.: H.C.P. interfaces and its application to the nucleation of γ in AlAg alloys

The discrete lattice plane (DLP) model is used to calculate the chemical interfacial energy (γ) of coherent f.c.c.:h.c.p. interfaces of arbitrary orientation in AlAg alloys. The compositional diffuseness of the interface is neglected. The results of these calculations are used to develop polar γ-plots of an h.c.p. crystal within an f.c.c. matrix, obtained as a function of alloy composition. The Wulff construction on the γ-plot yield the equilibrium shapes taken to correspond to that of homogeneously formed h.c.p. γ′ critical nuclei in an f.c.c. α AlAg matrix. These shapes are then used to calculate the steady state homogeneous nucleation rate of γ′ as a function of alloy composition and reaction temperature. Although these calculations indicated that homogeneous nucleation of coherent γ′ should be feasible in the temperature region between the GP zone solvus and the metastable equilibrium (α/(α+γ′)) solvus, a concurrent TEM investigation showed that the only γ′ present was nucleated on dislocations. While competition with such heterogeneous nucleation may have inhibited homogeneous nucleation, it is probable that the nearest neighbor version of the discrete lattice plane model used significantly underestimated the energy of coherent α:γ′ boundaries in AlAg alloys.     

Influence of crystallography upon critical nucleus shapes and kinetics of homogeneous f.c.c.-f.c.c. nucleation—IV. Comparisons between theory and experiment in CuCo alloys

A discrete lattice point model (Cook, de Fontaine and Hilliard) which incorporates strain energy (Cook and de Fontaine), described in earlier papers, has been used to determine the ranges of temperature and composition at which homogeneous nucleation kinetics of f.c.c. precipitates in CuCo alloys would be neither too fast nor too slow to be measured. These predictions proved successful and it was possible to measure experimentally nucleation kinetics in CuCo alloys containing from 0.5 to 1.0 at.% Co within 50°C temperature ranges. Experimental results were compared with theoretical values obtained from the discrete lattice point, the Cahn-Hilliard continuum and the classical theories of homogeneous nucleation. Very good agreement was obtained between the experiments and all three theories. Although surprising at first, the good matching between classical theory and experiments was explained by showing that the calculated concentration profiles of critical nuclei at the temperatures and alloy compositions experimentally studied did show distinct “volumes” and “interfaces” i.e. the solute concentration did not vary continuously throughout the nuclei. In this case, as pointed out in effect by Cahn and Hilliard, classical nucleation theory indeed applies. These findings provide the first strong support for the essential correctness of homogeneous nucleation theory.     

Results of mass-longitudinal screening in G?rlitz. 1. Morbidity of important infectious diseases

The Authors inform about the degree of infection in children, which are born in G?rlitz in 1957 to 1959. This study aimed at finding out, which infections still called "Classical children's diseases" should be particularly considered in childhood at present. The possibilitiy of vaccination against mumps and rubeolae in the GDR is discussed.