Numerical Simulation Of Structural Evolution During Directional Solidification Of Ti-Al Alloy

In this paper, the evolutions of interface morphology and structure are simulated for Ti-Al alloy during its liquid-solid phase transition in directional solidification, which helps us to further understand the directional solidification mechanism of Ti-Al alloy, and it also provides a theorical reference for controlling the microstructures in experiment.During directional solidification of single phase, the model which combines solute diffusion controlled model with Cellular Automation( CA) is presented to simulate microstructure evolution by using, and a diffusion flow method is employed to deal with the discontinuous thermophysical properties in both sides of the solid/liquid interface. And then a microstructure evolution of Al-30wt.%Cu alloy is simulated so as to verify the model, and at the same time, simulated results are compared with experimental and theoretical analysis results. The quantitative analysis of solute distributions along dendritic tips at different growth conditions shows that the simulation results are identified with the classical theory of solute diffusion controlled solidification, which testifies the accuracy of the model. The simulation also reproduces the phenomenon of tip splitting observed often in directionally solidified experiments, and the simulated dendritic morphologies agree well with experimental observation.Using above the solute diffusion controlled model and choosing Ti-(40-50) Al (at.%) alloy , the microstructure evolution of single phase alloy is simulated during its liquid-solid transition of L→βor L→α. The dynamic evolutions of microstructures in directional solidification are derived, and they also match well with experimental observations, such as the coarsening and necking of dendritic arms, the impingments, coalescences and competitional growths etc. Furthermore, a parametric study is performed to investigate the effects of the applied temperature gradient and pulling velocity, the results show that, with increasing pulling velocity, the morphology of the interface varies from plane to cell to dendrite. While in planar growth, whole interface is near planar, and in cell/dendrite transition, the uniform degree of dendrite spacing tends to decrease, and with increasing the numbers of the seed, the uniform degree of dendrite spacing tends to increase.At a fixed pulling velocity, increasing thermal gradient decreses the dendrite arm spacing, and at low thermal gradient, and at cell/dendrite transition zone, the cellular spacing increases to a maximum.Comparision of mcrostructure evolution for directional solidificatedβphase withαphase is done by numerical simulation, the results show that due to different liquidus slope and solute partition coefficient, even at same solidification conditions, cell/dendrite structure is often formed forβphase, and plane/cell structure is often formed forαphase, which is identical with the theory of solidification.Solute diffusion controlled model is presented to simulate the columnar-to-equiaxed transition (CET) by using pre-setted seeds. A parametric study is performed to investigate the effects of the applied temperature gradient and pulling speed, the seed spacing and nucleation undercooling for the equiaxed grains on the CET.The results illustvelocity that the CET depends quantatively on the temperature gradient and pulling velocity, And the present simulations agree with the analytical model of Hunt. And the seed spacings of a cross section has a stronger effect on CET than that of a longitudinal section; for large nucleation undercooling, columnar grain growth is favored; the columnar branch spacing depends not only on the temperature gradient and the pulling velocity, but also on the pre-setted seeds, this hints us that a spacing adjustment can occur through initiation of seeds that develop into new columnar grains.According to criterions of compositional undercooling and the highest interface temperature of steady growth of one phase,the direct growth ofαphase from liquid phase is simulated. The dynamic evolutions of solidification microstructures are derived, and also some phenomena are identical with experimental observations, such as, the peritecticαphase surrounding the primaryβcells. The present simulations also reveal a number of other interesting phenomena related to the growth ofαphase, for example, for large cell/dendrite spacings of a leading primaryβphase or Ti-49at.%Al alloy,αphase tends to grow atβphase interface, and for samll cell/dendrite spacings of a leading primaryβphase or near Ti-47at.%Al alloy, and two phases tend to grow independently.