L. Klinger

Controlled formation of surface layers by pack aluminization

Abstract Aluminum diffusion coatings on Armco iron were produced by a pack aluminization technique. Two different beds were applied: a high-activity bed containing pure aluminum as the coating source, and a low-activity bed containing Fe2Al5 as the source. In this work, the master alloy itself was also prepared by a pack aluminization procedure using aluminum and iron powders. The effect of the bed content on the coating was examined. With the low-activity bed, the desired FeAl was formed as the outermost coating layer. A set of complementary examination methods—XRD, SEM, EDX, RBS and CEMS—were employed, providing detailed information on the coating produced.

In situ processing of TiB2/TiC ceramic composites by thermal explosion under pressure : experimental study and modeling

Abstract A model describing non-isothermal reactive diffusion in B 4 C–Ti powder blends leading to thermal explosion (TE/SHS) in the conditions of enhanced heat transfer into the surrounding ambience was developed. The model explicitly incorporates the reaction kinetics and is based on previously reported experimental kinetics of isothermal growth of diffusion layers at a flat B 4 C/Ti interface. In spite of a number of simplifications (spherical geometry, sequential reactions of TiB and TiB 2 formation, etc.), a good agreement between the model calculations and experimental results on TE/SHS in B 4 C–3Ti dense samples placed between the preheated rams of a press has been obtained. The B 4 C powder particle size was found to significantly affect the ignition of TE, and the combustion process could be controlled by addition of small amounts of very fine powder fraction.

Beyond the fisher model of grain boundary diffusion : Effect of structural inhomogeneity in the bulk

Abstract An extension of the Fisher model of grain boundary diffusion is suggested, in which the diffusion along the short-circuit paths in the bulk of the crystalline grains (dislocations, subgrain boundaries, interphase boundaries in the lamellar structures) is taken into account. In the framework of the suggested model the bulk is treated as a stochastic mixture of defect-free crystalline regions and regions of bad material inside the short-circuit paths. The Harrison classification of the diffusion regimes is extended to the new D -regime, in which the kinetics of the penetration of the diffusing element along the grain boundaries is dominated by diffusion along these short-circuit paths. Three different kinetic modes during the grain boundary diffusion in the D -regime are uncovered: for the short annealing times the penetration kinetics follows the Whipple law, but with the bulk diffusion coefficient substituted by g 2 D d , for the intermediate annealing times the penetration distance along the grain boundary is a weak function of time and for the long times the Whipple law is valid again, but with the bulk diffusion coefficient substituted by gD d , where g and D d are the volume fraction of the material inside the short-circuit paths and the diffusion coefficient along them, respectively. The applications of the suggested model to the analysis of experimental data are discussed.

Grain boundary grooving at the singular surfaces

Abstract The unusual topographies of the grain boundary thermal grooves in Ni-rich NiAl were observed after annealing at 1400°C. One of the surfaces forming the grain boundary groove exhibited no curvature measurable in the atomic force microscope, thus indicating its singular character. The theory of grain boundary grooving at singular surfaces was developed in the small-slope approximation and under assumption of negligible diffusivity on these surfaces. The calculated groove shapes are in good agreement with the experimental data and differ considerably from the shapes predicted by the classical Mullins grooving theory for isotropic surfaces.

Modeling of reactive synthesis in consolidated blends of fine Ni and Al powders

Abstract A model is proposed to describe the kinetics of reactive synthesis of NiAl from consolidated Ni–Al fine powder blends for various synthesis heat treatments. The presence of interfacial oxide in the blend and heat exchange between the sample and the environment have been taken into account to simulate the evolution of temperature and phase composition in the blend. A good agreement was obtained between the model calculations and experimental results. The effect of interfacial oxides and of various external parameters (heating rate, heat exchange conditions, powder size, etc.) on the synthesis kinetics was investigated. The model allows to explain the difference in reaction paths (self-propagating synthesis or solid state synthesis) observed in consolidated powder blends, as well as to describe the shape of the differential thermal analysis signal. Based on the results, the disagreement in previously reported data on growth kinetics of aluminides in diffusion structures with different characteristic diffusion distances is discussed.

Experimental observation of periodic structure formation in SiO2–Mg system

Formation of a periodic structure was observed as a result of interaction between bulk SiO2 and Mg powder. The thickness of individual MgO and Mg2Si layers increased with temperature however varied little with annealing time. Total reaction zone growth was diffusion-controlled and proceeded by increasing the number of periodic layers.

Liquid grooving at grain boundaries

The kinetic dissolution and diffusion regimes of liquid grooving of grain boundaries with zero (or near-zero) dihedral angle at the top are discussed, and their perdominance criteria formulated. A new model of liquid grooving is proposed, according to which the top of the channel propagates in a diffusion regime, while its widening proceeds kinetically. Analytical (for the particular case of low solubility) and numerical solutions are obtained. The linear dimensions (depth and width) of the channel are estimated for the mixed regime.

GRAIN BOUNDARY GROOVING WITH SIMULTANEOUS GRAIN BOUNDARY SLIDING IN Ni-RICH NiAl

Abstract Thermal grooving at grain boundaries in Ni-rich NiAl was studied by atomic force microscopy technique. The determined average ratio of grain boundary to surface energy for large-angle grain boundaries at 1400°C is 0.45, which is in a good agreement with the results of computer simulations. It has been found that in most cases thermal grooving at the grain boundaries is accompanied by relative shift of the adjacent grains. This shift is associated with the grain boundary sliding caused by the relaxation of internal substructure of the specimen. A model of grain boundary grooving with the simultaneous sliding is developed. The calculated grain boundary groove profiles are in a good agreement with the experimentally measured ones.

On velocity and spacing selection in discontinuous precipitation—I. Simplified analytical approach

A new approach is proposed to the problem of pattern selection in discontinuous precipitation. It relies on the simultaneous calculation of growth velocities for both the precipitate lamellae and the interlamellar solid solution phase, and on the recognition that the processes of diffusional transport along grain boundaries and interphase boundaries are fundamentally different. The first is considered governed by lateral gradients of concentration, the second by gradients of curvature. It is shown that the simple hypotheses of thermodynamic equilibrium and diffusive flux continuity at the triple junction between the precipitate lamella and the initial and depleted parent phases (along with the requirement that the two phases grow at the same rate) is sufficient to remove the degeneracy of the problem; the new treatment provides a selection of spacing and transformation front velocity consistent with experimental results in the literature.

On the kinetics of interface-diffusion-controlled peritectoid reactions

Abstract We consider the peritectoid reaction α + β → ω as it occurs at previously existing planar α / β interfaces, and at sufficiently low temperatures that volume diffusion is negligible and interphase boundary diffusion is rate-controlling. With the further assumption that the participating phases have fixed compositions and that the interfacial diffusion process is driven by gradients in interfacial curvature, we obtain a unique solution for the growth of the product ω layer along the α / β interface. Both the layer thickness and the steady lengthening velocity are predicted. The layer thickness is of the same order as the capillary length; it would be difficult to detect by conventional means, but could be revealed for example by T.E.M. The layer growth is determined entirely by the undercooling, the interfacial energies and the relevant kinetic quantities. The presence of such thin layers at parent phase interfaces is expected to exert a profound influence on microstructurally determined properties.