Deformation of Polysynthetically Twinned TiAl Single Crystals with Near-Hard Orientations
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Fig. 1 Geometry of compression specimens. The x axis is the [112] direction.
KK3.1.1 Mat. Res. Soc. Symp. Proc. Vol. 552 © 1999 Materials Research Society
zero, or very near zero, and the other two strains are equal in magnitude but opposite in sign. This suggests that every single domain within the sample deforms in the same way by the above slip mode. Otherwise there would be a non-zero strain in the x-direction. However, as a approaches zero ("A" orientation) or 90 degrees ("N" orientation), the nature of the gross deformation changes markedly: The deformation (in compression) is such that the net slip is still parallel to the lamellar boundaries, even though there are six possible orientations of individual lamellae within the sample, but slip of different kinds occurs in distinct domains of individual lamellae. As shown by Kishida et al [2] the strain in the y direction is now zero, and the other two strains are of equal magnitude but opposite sign. In contrast, as a approaches 90 degrees, the strains in the x and y directions are both equal to one-half of the strain in the z direction, but opposite in sign. As in the near-45 degree orientation, each domain of each individual lamella must conform to these strains, otherwise the net strains would vary in the sample. The strain distribution described above does not occur in a single phase material and, therefore, it must be a consequence of the effect of lamellar boundaries that favors certain combinations of slip. An interesting case is, for example, the situation arising when oais equal to zero. In this case the maximum Schmid factor for any dislocation slip system, whether ordinary or superdislocation, is 0.408, and the maximum Schmid factor for any twin system is 0.236 (see [2]). Kishida et al found that there are only two combinations of slip seen for this orientation: (a) Two of the six domains deform by a combination of ordinary and superdislocation slip, both Burgers vectors of which lie in the lamellar plane and have the same Schmid factor. (b) Four of the six domains deform by a combination of an ordinary slip with the Burgers vector parallel to the lamellar boundary and slip plane inclined to the boundary, and a combination of the ordinary slip and twinning on a different [1111 plane not parallel to the boundary. The geometry of this sample orientation is such that twinning can always occur on the same slip plane as the ordinary dislocation slip, and the sum of the two can, in principle, provide a net slip parallel to the lamellar boundaries. The ordinary slip plus the twinning could be replaced by superdislocation slip. The fact that this does not occur suggests that the critical resolved shear stress (CRSS) for the superdislocation slip is appreciably higher than for ordinary dislocation slip and twinning-this is, in fact, the basis for the usual assumption that the CRSS's are so different. At the same time the combined slip and twinning leading to the shear parallel to the interfacial boundary is a remarkable phenomenon. The dislocations and the
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