The deformation microstructures have been investigated from two aspects by high-resolution electron microscopy: one is deformation twin and twin intersection; the other is deformation-induced phase transformation. The detailed jobs are as following:
1.Deformation Twin and Twin Intersection
Three kinds of nucleation regionshave been identified: γ/γinterface, γ/α2interface and the middle of γ lath. The former two represent the nonhomogeneous nucleation of deformation twin of TiAl, and the last one represents the homogeneous nucleation of deformation twin of TiAl. The propagation of deformation twin is fulfilled by the movement of a/6<11-2] dislocations on the successive {111}γ planes. The decomposition of incident twin in the twin intersection has been studied. It is a process related to the dislocation dissociation. The research also revealed that the intersection of deformation twin can cause the formation of nano-twins band.
2.The Deformation-induced Phase Transformations
The deformation-induced γ→DI-α2 phase transformation can occur in the twin-intersection regions during both hot-temperature and room-temperature deformations. The productions of these phase transformations are the same one during these two kinds of deformations. Compositional analysis suggested that there is no compositional diffusion during this phase transformation and the DI-α2 phase has the same composition as the γ matrix. The mechanism of this phase transformation can be explained by the model of dislocation slip during FCC→HCP transformation. The detailed structure has been identified through the analysis of diffraction patterns and the contrast of simulated HREM with the experimental one. The DI-α2 phase is not the standard α2 phase. The different composition and order of atomic arrangement are their two major differences. The calculation of formation energy suggests that it is favorable in the energy for the deformation-induced γ→DI-α2phase transformation, and the DI-α2 phase will not transform to the standard α2 phase.
The deformation-inducedα2→γ phase transformation can occur during both hot-temperature and room-temperature deformation. Since there is no compositional diffusion during room-temperature deformation, the DI-γ is not the standard γ phase and its composition is the same as the α2 matrix. However, because it is easy for compositional diffusion and the change of atomic arrangement during high-temperature deformation, the DI-γ phase is the standard γ phase. The deformation-inducedα2→γ phase transformation nucleates at the stacking faults of α2 phase, and the propagation of γ phase is completed by the glide of a/3[1-100] dislocations on the alternate (0001)α2 planes.
The imaging of overlapped structures in TiAl has been studied through the comparison of simulated HREM images with experimental ones. The so-call 9R structure reported before is not the true 9R structure. It is the Moire fringe caused by the overlapping of two twin-related γ laths. In the precipitation of γ phase from α2 phase or the opposite precipitation, there will appear fringes in the tips of precipitates. These fringes were once considered by mistake as the stacking faults fringes during the α2↔γ transformation. This work revealed that they are Moire fringes caused by the overlapping of α2 and γ phases. Through the study of imaging of overlapped structures, we may have a better understanding of the deformation-induced phase transformation.
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