Carbon Diffusivity in Multi-Component Austenite

S-J Lee et al

The diffusivity of carbon in multi-component austenite has been investigated using a thermodynamic-based analysis. The activity coefficient of carbon as a function of alloying elements and temperature was used to derive the carbon diffusivity as a function of composition and temperature. The strong influence of chromium (which decreases carbon diffusion rates) was incorporated in the diffusivity equation. The equation for carbon diffusivity was verified by comparing calculated results with measured diffusivity of carbon for various alloys.

Diffusivities of an Al–Fe–Ni melt and their effects on the microstructure during solidification

Lijun Zhang et al

A systematical investigation of the diffusivities in an Al–Fe–Ni melt was presented. Based on the experimental and theoretical data about diffusivities, the temperature- and composition-dependent atomic mobilities were evaluated for the elements in Al–Ni, Al–Fe, Fe–Ni and Al–Fe–Ni melts via an effective approach. Most of the reported diffusivities can be reproduced well by the obtained atomic mobilities. In particular, for the first time the ternary diffusivity of the liquid in a ternary system is described in conjunction with the established atomic mobilities. The effect of the atomic mobilities in a liquid on microstructure and microsegregation during solidification was demonstrated with one Al–Ni binary alloy. The simulation results indicate that accurate databases of mobilities in the liquid phase are much needed for the quantitative simulation of microstructural evolution during solidification by using various approaches, including DICTRA and the phase-field method.

Reactive diffusion

March 31, 2009

Reactive diffusion in nanostructures of spherical symmetry

G Schmitz et al

To investigate reactive diffusion in nanosized spherical geometries, a clear model experiment has been designed. Thin film {Al/Cu/Al} and {Cu/Al/Cu} triple layers were deposited on tips of 25 nm apex radius and investigated by atom probe tomography (APT). At the interfaces within both samples, the growth of the reaction product proceeds parabolically from the very beginning but with remarkably different rates. Growth appears to be always faster if Cu is stacked to the outer side of Al. The complex quantitative analysis of reaction-induced stress, surface tensions and partial mobilities suggests that the different growth rates represent the Darken and the Nernst–Planck limits of interdiffusion. Since the curvature radius of the model samples ranges down to a few tens of nanometers, it is anticipated that an analogous effect may play a role in the oxidation of nanospheres or in chemical reactions of core–shell structures.