Atomic diffusion in metallic melts and the influence of gravity

Abstract

Atomic mobility is one of the parameters that determines the microstructure formation during solidification. In addition to self- and interdiffusion, mass transport can also be caused by thermodiffusion (also known as Ludwig-Soret effect), which describes the formation of a concentration gradient due to a temperature gradient. Different methods have been developed to measure atomic diffusion in metallic melts. This includes shear cell furnaces where the two parts of the diffusion couple are melted separately and brought into contact after the annealing temperature has been reached. The concentration distribution along the diffusion couple is determined at the end of the diffusion time by separating the liquid sample in several small parts and chemically analysing the solidified parts. This technique avoids measurement errors from melting and solidifying the sample materials. For certain binary alloys the concentration along the diffusion couple can also be measured in-situ by X-ray radiography [1].

Measurements of diffusion and thermodiffusion in liquids generally are very sensitive to convection caused for example by buoyancy. To reduce the impact of buoyancy-driven convection, benchmark experiments are performed in microgravity conditions. The gravitational Péclet number and the gravitational length can be used to assess the influence of gravity on atomic diffusion [2]. They show that the diffusion processes of atoms in a liquid is not affected by Earth’s gravitational force but that the process is dominated by the thermal energy of the atoms. Data from experiments under different gravity conditions ranging from 10-5g to 10 6g are summarized. Only accelerations that are orders of magnitude larger than Earth’s gravity influence the diffusion process itself.