K. Vedula

High temperature dispersion strengthening of NiAl

A potential high temperature strengthening mechanism for alloys based on the intermetallic compound NiAl has been investigated. This study forms part of an overall program at NASA Lewis Research Center for exploring the potential of alloys based on NiAl for high temperature applications. An alloy containing 2.26 at% Nb and produced by hot extrusion of blended powders has been examined in detail using optical and electron microscopy. Interdiffusion between the blended niobium and NiAl powders results in the formation of intermediate phases. A fine dispersion of precipitates of a hexagonal, ordered NiAINb phase in a matrix of NiAl can be produced and this results in strengthening of the alloy by interfering with dislocation motion at high temperature. These precipitates are, however, found to coarsen during the high temperature (1300 K) deformation at slow strain rates and this may impose some limitations on the use of this strengthening mechanism.

Modeling of transient and steady-state demixing of oxide solid solutions in an oxygen chemical potential gradient

A numerical technique for predicting the kinetics of demixing of oxide solid solutions in an oxygen chemical potential gradient is presented. The unique advantage of this technique is that it permits handling of transient state conditions, variable diffusion coefficients, and variable boundary conditions. Solutions obtained by this technique agree very well with the steady-state kinetics predicted by analytical methods.

A mathematical model for internal oxidation

A mathematical model for internal oxidation kinetics was developed using numerical methods (finite difference) and computer techniques. The flexibility of the model permitted analysis of semi-infinite and finite situations with planar, cylindrical, and spherical geometries for systems with various amounts of local solute enrichment. Graphical results are presented for subscale thickness as a function of time and local enrichment as a function of position in the subscale. The model is also applied to internal oxidation with a discontinuous change in surface oxygen concentration; a graphical solution encompassing a wide range of possible experimental conditions is presented. The use of the model in analyzing nonisothermal internal oxidation problems is demonstrated.

Spheroidization of binary Fe-C alloys over a range of temperatures

The spheroidization of cementite in binary Fe-C alloys, 0.24, 0.42, and 0.79 wt pct C, was investigated over a range of temperatures, 594°, 649°, and 704°C, for times up to about 106 sec. Quantitative metallography techniques were used to obtain the following microstructural data on the cementite particles: shape, size distribution, mean size, number of particles per unit volume, and growth (and shrinkage) rates of various sizes in the size distribution. The variations of these microstructural parameters were analyzed in terms of existing models for the spheroidization process. The Lifshitz-Wagner analysis is shown to have limited applicability to the spheroidization of cementite in binary steels, since the required steady-state size distribution is not attained in times less than about 106 sec. An analysis similar to that of Lifshitz and Wagner, but requiring no specification of the shape of the size distribution, is shown to apply and indicates that the observed spheroidization was diffusion-controlled. The effective diffusion coefficient was between the values for the diffusion of carbon and iron in ferrite and approximated the coupled diffusion coefficients developed by Oriani and Li, Blakely, and Feingold.

Some effects of minor alloying additions on the development and breakdown of protective oxide scales

The development and breakdown of scales formed on Fe—25Cr and Fe—25Cr—20Ni alloys have been studied in O—S environments, mainly at 700°C. The chromia scales formed on Fe-25Cr were markedly more eff...

Fabrication of composites by diffusion processing

Abstract : Composite materials containing continuous NiAl3 fibers in an aluminum matrix were grown by solid state diffusion at 600C from nickel wires embedded in aluminum (hot-pressed). The kinetics of formation of the NiAl3 and Ni2Al3 phases and the ultimate solid of the nickel terminal solid solution and Ni2Al3 phases were studied. The NiAl3-aluminum composite structure was developed by a diffusion treatment of 72 hours at 600C when 0.005 inch nickel wires were used in the starting structure. Although strengths of these composites were low due to grain boundary fracture of the NiAl3, this method of composite fabrication may prove applicable to the fabrication of composites in other alloy systems. (Author)

Microstructural rearrangement during vacancy annihilation in Co1−δO

When the equilibrium defect concentration of an oxide is lowered, the defects must become annihilated to obtain the lower equilibrium concentrations. Classical point defect theory states that this annihilation reaction must occur at a free surface. If the extent of the annihilation is large enough then structural rearrangement can occur at this free surface. Experiments of this type have been performed on the cation deficient oxides Ni[sub 1][minus][delta] and CO [sub 1][minus][delta]O where the equilibrium cation vacancy concentration was lowered by lowering the temperature and/or oxygen pressure of the system. These experiments showed that structural rearrangements occurred at the free surfaces of the oxides and that the type of rearrangement depended upon the manner in which the defect concentrations were lowered. The authors have also performed vacancy annihilation experiments on Co[sub 1][minus][delta]O, and this communication will show that their results are consistent with the previous investigations and that the microstructural rearrangement that takes place at the free surfaces of these oxides is very sensitive to the extent of the annihilation process.

Alteration of CoO Wafers in an Oxygen Chemical Potential Gradient

The Wagner Oxidation Theory, and its later developments, allow the parabolic rate constant to be predicted from fundamental transport coefficients and point defect thermodynamics, provided that the oxidation reaction is controlled by well-understood lattice diffusion mechanisms. In many practical cases, however, diffusion along fast paths such as grain boundaries is dominant and the actual oxidation rate is strongly influenced by the microstructure of the oxide scale. Moreover, the oxide microstructure is not static, but rather continuously evolves as new oxide grows and as the existing oxide re-structures. The growth mechanism depends upon the oxide microstructure, while the oxide microstructure is determined by the details of the growth mechanism. This creates what is essentially a feedback loop system in which the scale microstructure both influences and is influenced by the growth mechanism. A description of this loop system seems to be necessary to develop a richer understanding of oxidation.