Diffusion-induced liquid film migration in the aluminum-copper system

Abstract

<p>An investigation of liquid film migration (LFM) in the aluminum-copper system is presented in this thesis. Experimental and numerical modelling results are discussed for the migration of liquid lenses, formed on melting of grain-boundary [straight theta] precipitates in Al-3.4 wt% Cu alloys which had been previously equilibrated at 400°C. Liquid films of comparable thickness, formed upon the melting of intragranular Widmanstätten precipitates were found not to migrate. Volume fraction studies indicate that after the initial liquation of the [straight theta] precipitates, the liquid phase undergoes an isothermal solidification. Three-dimensional reconstructions of LFM events were created using a combined SEM/microindenting/polishing method. The results indicate the difficulty in obtaining reasonable measurements of migration distances from two-dimensional cross-sections; a method was developed to obtain appropriate two-dimensional measurements based upon these results. The modelling studies are based on the hypothesis that the leading and trailing solid-liquid interfaces are each characterized by a constrained local equilibrium, as determined by chemical, coherency and capillary terms, and that the process is driven by the liquid concentration gradient resulting from different equilibrium concentrations at the two interfaces. The model is utilized to explore the sensitivity of the process to variations in physico-chemical parameters, and ultimately as an aid in the understanding of LFM and its subsequent cessation. It is determined that the capillary term has a strong influence on the migration process, even for liquid pool dimensions of several microns. From model simulations and an analysis of coherency loss in the leading solid it is concluded that the migration of liquid films in the Al-Cu system is driven by coherency strain energy in the early stages of the process. As migration continues, the increasing energy due to curvature is sufficient to decrease the overall driving force to a point where the film. slows down and coherency is lost. The liquid films become immobile (no reversal of migration) once loss of coherency occurs.</p>