Polarization of plastic deformation modes in polysynthetically twinned TiAl crystals
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K. Kishida Materials Engineering Laboratory, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
M. Yamaguchi Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan (Received 10 July 2002; accepted 23 December 2002)
Polarization of deformation twinning (its propagation in a certain sense but not in the opposite one) is taken for granted. However, the same phenomenon can occur for a superdislocation glide as well, as is demonstrated in this paper. The consequences for plastic deformation of polysynthetically twinned TiAl crystals with the lamellar interfaces parallel to the loading direction are discussed. It is not the interface itself that is an obstacle for propagating deformation but also the fact that a deformation mode with the parallel Burgers vector cannot be activated in the neighboring lamella due to the directionality of superdislocation motion leading to additional stress increase. I. INTRODUCTION
Great attention has been paid in the last years to titanium–aluminides1 because of their attractive mechanical properties, light weight, and high strength persisting to high temperatures, which make these materials attractive for technological applications. Plastic deformation of polysynthetically twinned (PST) crystals of TiAl is controlled by the deformation modes of diverse types and by their interaction with lamellar interfaces. Temperature and loading-axis-orientation dependencies of single ␥-phase TiAl crystal plastic deformation were systematically studied, and the deformation modes including the slip of ordinary dislocations and of superdislocations, as well as twinning, were identified.2 A most complete analysis of active deformation modes in differently oriented PST TiAl crystals can be found in Ref. 3. Great attention has been paid to the interaction of plastic deformation with grain boundaries in the literature (for example, Refs. 4–10 and references therein). The ␥/␥ interfaces are in fact nothing else than twist grain boundaries of special type, as the rotation angle is a multiple of only 60°. Relatively simple geometrical criteria were proposed as conditions for slip transfer across the interface and were used to interpret the experimental results for bicrystals: (i) The transmission is easier when the angles between the slip planes and slip directions of the incoming and outgoing slip systems are minimal. This criterion can 702
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J. Mater. Res., Vol. 18, No. 3, Mar 2003 Downloaded: 27 Mar 2015
be accepted as correct but has a drawback that it does not take into account the interface, its orientation, and what is happening there. (ii) Obviously, the transmission is easier when the intersection lines of the two slip planes with the interface are parallel, since the rotation of dislocations at the interface for not-parallel intersection lines could not easily take place. (iii) Since the accumulation of residual dislocations can alternate the boundary parameters11 and spends energy (inc
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