Modeling solidification microstructure development in a titanium aluminide alloy.

摘要

Constrained dendritic solidification of alloys through preforms of narrow channel has been found to modify the alloy microstructure and reduce the microsegregation. In order to study this reinforcement effect on TiAl alloys, a 2-D growth algorithm coupled with a finite volume calculation of mass diffusion is developed. With the application of an explicit method for calculating mass diffusion, the efficiency of the simulation is improved by about 30% compared with the implicit method. The proposed eight-directional way of picking interface cells that are subjected to phase change does not increase the model efficiency significantly, however it produces results different from those obtained from the more accurate randomly picking cell methods. A Ti-49 at% Al alpha-phase alloy with six-fold symmetry grown through parallel planar channel preforms with various spacing is simulated. As the channel size decreases, a finer microstructure is formed and a higher fraction of liquid phase is left at the L + alpha &rarr gamma peritectic temperature, which leads to a higher than average Al concentration within the preform. This metal matrix composite is used as an alpha-phase seed to grow a thermodynamically beta-phase stable, low composition Ti-47 at% Al alloy without beta-phase nucleation. The model is extended to multi-component alloys in the Ti-Al-Si system. The free dendrite growth of Ti-43Al-3Si (at%) alloy is simulated. The behavior of the model as a function of time step is studied and an operation window for producing reliable results is found.