Modelling of liquid film migration in Al-Cu alloys

Microstructural evolution in the presence of liquid film migration (LFM) is simulated for Al-Cu alloys using a cellular automaton (CA) model. Simulations are performed for the microstructural evolution and concentration distribution in an Al-4 wt.%Cu alloy with initially equiaxed grain structures holding in a temperature gradient. A slight deviation from local equilibrium, estimated from experimental data, is considered to be the driving force for LFM. The direction of LFM is triggered by concentration fluctuations setting a concentration gradient as a further driving force. The simulation successfully reproduces the experimentally observed microstructures generated by LFM accompanied by a particle free zone behind the liquid film. The solid concentration in the particle free zone is found to be the equilibrium solid concentration. The simulated concentration profile across the migrating liquid film agrees well with experimental measurements. The simulated grain structure becomes coarser and highly elongated after holding in the temperature gradient. The results reveal that the increase in transversal grain width is mainly controlled by LFM, while the grain elongation in longitudinal direction is attributed to both LFM and temperature gradient zone melting. The solid concentration decreases from the initial (supersaturated) composition to the local equilibrium solid concentration corresponding to the local temperature. This article is part of the theme issue 'Transport phenomena in complex systems (part 2)'.

Simulation of Liquid Film Migration during Melting

Melting of a single-phase polycrystalline material is known to start by the formation of liquid films at the surface and at grain boundaries. The internal liquid films are not necessarily quiescent, but can migrate to avoid/reduce supersaturation in the solid phase. The migration is discussed in the literature to be governed by coherency strains of the solid/liquid interface, by concentration gradients in the liquid or by concentration gradients in the solid phase. A phase transformation model for diffusional phase transformations considering interface thermodynamics (possible deviations from local deviations) has been put up to describe the migration of the solid/liquid (trailing) and the liquid/solid (leading) interfaces of the liquid film. New experimental results on melting in a temperature gradient in combination with simulation calculations reveal that concentration fluctuations in the liquid phase trigger the liquid film migration and determine the migration direction, until after a short time in the order of microseconds the process is governed by diffusion in the solid phase.

Kinetics of Isothermal Phase Transformations by Phase-Field Simulations: An Analogy between the Peritectic and Monotectic Systems

We present phase-field simulations of isothermal phase transformations in the peritectic system below and above the peritectic temperature TP , and in the monotectic system below the monotectic temperature TM. We focus particularly on the Liquid-Film-Migration (LFM) mechanism, which appears to be the generic process for phase transformations above TP . Below TP , we obtain an assymetric LFM, suggesting the existence of a doublon structure in free space. In the monotectic system, the transformation from a liquid L1 to a solid+liquid L2 mixture proceeds via the migration of a L2 film, which is the analogous of the LFM process. When the metastable state consists of a liquid-liquid mixture, a dendritic-like solidification is obtained.