On the kinetics of diffusion controlled precipitation under microgravity

Abstract

Metal matrix composites have many desirable characteristics for precision components, e.g., light weight, dimensional stability, and high thermal conductivity. The understanding of the mechanisms and processes taking place at their production is of crucial importance, since these factors are important for the properties of the composite and its future application. A theoretical model of the processes at diffusion controlled formation of TiB2 in liquid Al under microgravity has been proposed. The main features of the developed model are: mechanisms controlling the formation of the TiB2; degree of distribution of the dispersed TiB2 particles; and mean size and size distribution of the dispersed TiB2 particles. The results obtained with the aid of the proposed model were verified on our recent experiments at the STS95 space shuttle mission. The model predicts correctly the TiB2 particles distribution along the specimens and the characteristic split of the reaction zone observed experimentally.

Size Control of In Situ Synthesized TiB2 Particles in Molten Aluminum

“…We assume that the diffusion of boron and the reaction rate between boron and titanium are fast, so the growth of the particles is only diffusion-controlled as stated in another paper. [26] We can assume the concentration of Ti is constant far away from and at the interface. Therefore, C Ti;1 and C Ti;i are constant.…”

“…The particle's growth is then divided into three categories: surface reaction-controlled growth, diffusion-controlled growth, and mixed-controlled growth, depending on which process is the limiting factor. [23] The growth of TiB 2 is assumed to be diffusion-controlled, [26] so the particle size from the diffusion-controlled growth model can be written as r = (2DSt) 1/2 . Here, r is the radius, D is the diffusivity of the reactant, S is the supersaturation as a function of concentration, and t is the growth time.…”

Growth rate at first-order phase transformation processes in multicomponent systems

“…As a result the global minimisation [17], as done in some previous attempts, do not produce sufficient number of equations to match the number of unknown variables (x a n and r cr (n ¼ 0...k)). In a previous paper [24], one of us argued that the compositions and the critical radius of the nuclei can be directly obtained from Eqs. (1).…”

Managing Uncertainty in Multiscale Systems via Simulation Model Refinement

“…To create in situ Al / TiB 2 K 2 TiF 6 and KBF 4 are reduced with aluminum. A model proposed by Anestiev and co-authors 17 has been used to investigate the diffusion reactions taking place between the intermediate phases of Al 3 Ti and AlB 2 . A coordinate system dividing a 2-dimensional space into strips of equal length is used, half of which contains Al 3 Ti and the other half AlB 2 dissolved in the Al melt.…”