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摘要
The microstructural evolution in mushy zones of alloys due to temperature gradient zone melting (TGZM) is studied by simulations using a two-dimensional quantitative cellular automaton (CA) model and in situ observations of directional solidification with a transparent organic SCN-ACE alloy. The present model is an extension of a previous CA model by involving the mechanisms of both solidification and melting. The present CA model is adopted to simulate the temporal evolution of the position and velocity of a liquid pool migrating in the solid matrix of a SCN-0.3 wt% ACE alloy under conditions that the pulling velocity is either lower or higher than the critical pulling velocity. The CA simulated position and velocity curves agree well with analytical solutions. Simulations are also performed for the microstructural evolution of columnar dendrites in a SCN-2.0 wt% ACE alloy held in a stationary temperature gradient using the present CA model and a previous CA model that does not include the melting mechanism under otherwise identical conditions for comparison. The results show how melting is essential to dendrite arm migration in a temperature gradient. The time-averaged velocities of arm migration obtained from the present CA simulations increase with increasing temperature gradient and with decreasing the length between the initial arm position and the liquidus. This agrees reasonably well with experimental measurements and analytical predictions. The mechanisms of dendrite arm migration are investigated in detail by comparing the local equilibrium and actual liquid compositions at solid/ liquid interfaces. The simulations render visualizing the complex interactions among local temperature, solute distribution/diffusion, and solidification/ melting during the TGZM process.
