Dislocation/Twin/Interface Interactions during Deformation of PST TiAl Single Crystals, an AFM Study
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Dislocation/Twin/Interface Interactions during Deformation of PST TiAl Single Crystals, an AFM Study Yali Chen, David P. Pope and Vaclav Vitek Department of Materials Science and Engineering, University of Pennsylvania Philadelphia, PA 19104-6272, U.S.A ABSTRACT PST TiAl crystals oriented such that the deformation axis lies in the (111) interfacial planes have been deformed in compression. This deformation produces so-called "channeled flow" in which the strain perpendicular to the (111) interfaces is zero, while the other two strains are equal and opposite in sign. Thus the sample simply shortens axially and spreads laterally in the channels defined by the (111) interfacial planes. We have examined the fine structure of deformation bands on the free surface of these deformed samples using AFM to see how the deformation processes interact with the boundaries. By measuring the offset angle at the surface we have been able to show that not only is the macroscopic displacement vector parallel to the lamellar boundaries, but the total shear vector in each layer is also parallel to the lamellar boundaries. However these deformation bands have very different characters, requiring complex deformation processes at the boundaries in order to satisfy this requirement. Some consist of either just super dislocations or just ordinary dislocations with Burgers vectors lying in the interface. But others consist of a special combination of twinning and ordinary dislocations in fixed ratio, such that the net shear vector also lies in the boundary, even though the individual twinning and dislocation shear directions are inclined to it. This results in deformation that is homogeneous and completely ‘channeled’ inside each lamella with no shear vector perpendicular to the lamellar boundaries. We have also shown that the cooperative twinning and slip is homogeneous on the nano-scale, i.e., the twinning and slip occurs in the same volume of material. INTRODUCTION Two-phase TiAl intermetallic compounds with lamellar structures are promising candidates to serve as high temperature structural materials. A great deal of effort has been made to improve the toughness and ductility of TiAl materials [1, 2]. The deformation behavior of this material also has attracted a a great deal of attention, but it is difficult to study the deformation mechanisms in random polycrystals due to the complexity of the interfaces. Polysynthetically twinned (PST) crystals, on the other hand, are especially good candidates for studying the interaction of slip and twinning systems with boundaries [3-5] because the lamellar boundaries appear to dramatically control the gross deformation behavior of the crystal. The microstructure of PST crystals is illustrated in figure 1. The two phases, α2-Ti3Al and γTiAl are ordered in such a way that the (0001) plane of the α2 phase is parallel to the {111} planes of the γ phase and the < 1120 > directions of the α2 phase are parallel to the < 110 > directions of the γ phase. Because of the slight tetragonality of the
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