Lamellar Orientation Control Of Directionally Solidified γ-TiAl-Based Alloys

γ-TiAl based intermetallics alloys are of many advantages such as low density, high young's modulus, and excellent elevated temperature strength and oxidation resistance. Therefore, they are very promising materials for elevated temperature construction material. Especially those materials with full lamellar structure consisting ofγ-TiAl with a little amount ofα₂ phase indicate fine synthetical mechanical properties both at room temperature and at high temperature. Because of its aeolotropy, lamellar structure is much more appropriate in such applications as aeroengine blades that mainly bear one-dimensional loads. Therefore, many researchers have paid much attention to getting full lamellar structure with designed orientation. Among the significant results, the combination of seeding and directional solidification (DS) is a successful method to get full lamellar structure with a designed orientation.In this paper, a mathematical model was first established to calculate the maximum load bearing direction of lamellar structures with different lamellar orientations. The results suggested that the maximum load bearing direction of aγ+α₂ lamellar structure is parallel with the lamellar orientation when the equivalent volume fraction ofα₂ is more than 30% which is relatively easy to meet for mostγ+α₂-based alloys. This provides a basic theory for lamellar orientation control: the best lamellar orientation is parallel with the growth direction of DS.Experiments using DS technique with seeding in ceramic tube were carried out to control the lamellar orientation. The observation results showed that the lamellar orientation of the obtained DS structure is parallel with the growth direction and keeps that of the seed throughout the DS zone (about 50mm). The structure control using DS technique with seed in ceramic tube is successful.An initial concentration transient zone (CTZ) may appear in a cold crucible DS process with seeding. The existence of CTZ can invalidate the effect of the seed. A composition adjustment (CA) method was proposed in the present work to eliminate the CTZ. The problems in CA method were solved in this paper. The experimental results showed that CTZ can be eliminated using the CA method. With the CA method, a stable growth may be established immediately at the outset of the DS, so that the lamellar structure may grow from the seed to the directionally solidified zone smoothly. The problem that the structure and composition in the initial part of DS defer from those in the stable growth part was solved successfully.Based on the proposed CA method, experiments using cold crucible DS technique with seeding were carried out to investigate the feasibility to control the lamellar orientation. An in situ seeding DS, in which the seed made in the experiment was used directly in the cold crucible DS, was proposed and employed. This method combined cold crucible DS with seed making, which significantly simplified the traditional technique consisting of seed making, seed cutting, seed orientation positioning and DS process. This may be beneficial to the application in production. The results indicate that the CA method should be used to eliminate the CTZ in cold crucible DS with seed; with the CA method, the lamellar structure in the seed can grow into the DS zone and keep its original orientation. The lamellar structure parallel to the growth direction in the DS zone may grow continuously longer than 20mm. Therefore, the lamellar orientation control using cold crucible DS technique with seeding is shown to be feasible.