Computer-generated Movies Showing the Evolution of Microstructure

Some discussion of the transformations illustrated in these movies can be found in Metals and Alloys lectures.

The following computer simulations of grain growth in two-dimensions have been provided by courtesy of V. Pavlik and U. Dilthey of the ISF-Welding Institute of Aachen University in Germany. The solidification simulations use a technique called "cellular automata" in combination with finite difference methods. Cellular automata allow non-trivial processes and patterns to be computed starting with simple deterministic rules. Solute concentration contours in the parent phase are represented by colours. Look out for the development of secondary dendrite arms, coarsening of these arms, the development of solute segregation due to non-equilibrium solidification. Note how the microstructure changes radically as a function of the solidification parameters (velocity and temperature gradient)

Dendritic solidification in Fe-0.11 wt% C velocity 10 mm/s, temperature gradient 100 K/mm. Have a careful look at the development of the secondary dendrite arms. The initial spacing between the secondary dendrite arms is much finer than in the final microstructure. This is because of coarsening - some of the finer arms dissolve as the coarser ones grow. The later stages also show the coalescence of the dedrite arms (both primary and secondary).

"Seaweed" solidification structure in Fe-0.11 wt% C. velocity 10 mm/s, temperature gradient 300 K/mm

Solidification to a dendritic microstructure where the primary phase to solidify is delta-ferrite, in Fe-0.15 wt% C. velocity 0.1 mm/s, temperature gradient 15 K/mm. The dendrites in this simulation are constrained to grow along particular crystallographic directions determined at the nucleation stage. This is why they grow at an angle even though the temperature gradient is vertical. The simulation illustrates selection during growth, i.e., dendrites which are better oriented to the temperature gradient overgrow those which are not well oriented. This movie has been provided by courtesy of Janin Taiden of Access in Germany.

The following movie is a simulation of eutectic solidification in an Al--Si system. It has been provided by courtesy of Britta Nestler of RWTH in Aachen, Germany. The simulation is based on a technique known as "phase field" modelling. In this method, the boundary is treated as a continuous transition between adjacent grains across a thin layer of finite thickness. The value of a phase-field variable then identifies the location of the boundary and of each grain. The advantage of this method is that the boundary becomes a part of the system so that it does not have to be determined explicitly in the solution. Notice how the eutectic spacing changes as solidification proceeds, and the nature of the solute diffusion field at the solidification front. The diffusion distance is not very large, about equal to the spacing of the lamellae.


The movies presented here have been provided for teaching purposes via the good offices of Dr Vitali Pavlik of Aachen University in Germany.

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