Periodic Layer Formation during Multiphase Diffusion in Silicide Systems

Periodic layered morphology may occur during displacement solid-state reactions in ternary (and higher-order) silicide and other material systems. This periodic layered structure consists of regularly spaced layers (bands) of particles of one reaction product embedded in a matrix phase of another reaction product. The number of systems that is known to produce the periodic layered structure is rather small but increasing and includes metal/metal and metal/ceramic semi-infinite diffusion couples. The experimental results on different systems, where the periodic pattern formation has been observed are systematized and earlier explanations for this peculiar diffusion phenomenon are discussed. Formation of the reaction zone morphologies periodic in time and space can be considered as a manifestation of the Kirkendall effect accompanying interdiffusion in the solid state. The patterning during multiphase diffusion is attributed to diverging vacancy fluxes within the interaction zone. This can generate multiple Kirkendall planes, which by attractingin situ-formed inclusions of “secondary-formed phase” can result in a highly patterned microstructure.

The Ni-Al-Hf Multiphase Diffusion

The generalized Darken method was applied to simulate the diffusion between γ-Ni| γ’-Ni3Al and γ’-Ni3Al|β-NiAl interfaces. The results of calculations were compared with the experimental concentration’s profiles of nickel, aluminum and hafnium in aluminide and hafnium doped aluminide coatings deposited by the CVD and PVD methods on pure nickel. The method deals with the Wagner’s integral diffusion coefficients and thermodynamic data - activities of components. The experimental results agree with the simulated ones.

An Overview of Selected Phenomena in Multicomponent Diffusion

There exist several interesting phenomena and observations reported in literature for isothermal diffusion in multicomponent systems. Such phenomena include uphill diffusion, development of zero-flux planes and flux reversals for individual components, flux reversals at interfaces, and instability at interfaces and multiphase layer development. In addition, uncommon diffusion structures exhibiting unusual diffusion paths can develop in both single phase and multiphase diffusion assemblies. An overview of such phenomena is presented to highlight the role of interactions among diffusing components with the aid of selected diffusion studies carried out in multicomponent alloy systems, aluminides, silicides, and nuclear fuels.

The Ni-Al-Zr Multiphase Diffusion Simulations

The generalized Darken method allows a quantitative description of diffusion mass transport in multi-phase materials. The method characterizes the diffusion zone by phase volume fractions. The results of the calculations are compared with experimental concentration’s profiles of nickel, zirconium and aluminum in zirconium doped aluminide coatings deposited on pure nickel by the PVD and CVD methods.

Anisotropic Diffusion Behaviour of Al and Zn in HCP Mg: Diffusion Couple Experiment Using Mg Single Crystal

Diffusion couple experiments for Mg-Al and Mg-Zn were carried out with Mg single crystal to determine the anisotropic diffusion coefficients of Al and Zn in hcp Mg at the temperature range between 553 and 693 K. Based on the experimental results, anisotropic diffusion coefficients of Al and Zn were calculated using multiphase diffusion simulations. Al diffusion in hcp Mg is slightly faster than Mg self-diffusion itself, but the diffusion of Zn is slightly slower than Mg self-diffusion. The diffusion coefficients of Al and Zn along the a-axis (basal plane) of hcp Mg is slightly higher (1.1-1.4 times) than those along the c-axis (normal to the basal plane), which is also similar to Mg self-diffusion behaviour.