On the Role of Lamellar Interfaces on the Strength and Ductility of Two-Phase Titanium-Aluminum
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ABSTRACT The deformation mechanisms active in hard orientation compressed pst-TiAl have been studied by transmission electron microscopy (TEM). The observed deformation modes involved soft superdislocation and hard ordinary dislocation slip, and hard twin systems. The transfer of these twin shears occurred across all types of (y/y)-interfaces. Transfer mechanisms have been determined on the basis of crystallographic analyses of the TEM data. The active modes of deformation were consistent with macroscopic shape changes observed for the pst-TiAl when the twin shear transfer mechanisms were considered. Based on these observations it has been proposed that the strength and ductility of polycrystalline fully-lamellar TiAl may be improved by reducing the width of both y- and a 2-1amellae. INTRODUCTION Two-phase TiAl alloys with fully-lamellar microstructures exhibit promising mechanical properties and are being considered for high-temperature structural applications in jet-engines [1]. The lamellar grains in these intermetallic alloys consist mainly of lamellae of the y-TiAl phase with a small volume fraction of ct2-Ti3AJ lamellae [2]. The y- and cc2-lamellae obey a crystallographic orientation relationship, [1 i01y//[I 120]ct 2 and (11 l)y//(0001)cz 2, and six variants of y-lamellae, matrix variants (IM to HIM) and the twin related structures (IT to IIIT), coexist with the oa2 -lamellae (Fig. 1) [2]. Lamellar TiAl grains exhibit strongly anisotropic mechanical properties [1, 2]. The yield strength of directionally solidified material comprising only a single lamellar grain, so-called polysynthetically twinned TiAl (pst-TiAl), is very high for loading normal to the lamellar interface plane (4=90, N-orientation), still quite high for loading parallel to the interface (0=0O, A-type orientations), and much lower for intermediate orientations (t0=45', B-type orientations) of the loading direction with respect to the interface [2]. Here 0 is the angle between the interface and the loading direction. Thus hard and soft loading conditions and deformation modes exist for pst-TiAl. The soft deformation modes involve slip of ordinary dislocations (OD), superdislocations (SD) and ordered twinning systems (TD) associated with Burgers shears contained in the lamellar interfaces (11). The hard deformation modes include those slip and twinning systems with Burgers shears with components normal to (111). Hence, the large differences in mechanical properties of pst-TiAl between the soft and hard orientations has been attributed to Hall-Petch effects due to the geometric constraints imposed by the lamellar morphology [2,3]. However, the deformation mechanisms leading to the behavior characteristic of the A-type and the N-orientations, are less well understood. Explanations for the differences between the A-type and N-orientation deformation behavior have to be related to either changes in the behavior of the minority a 2-phase lamellae or in the behavior of the lamellar interfaces or a combination of both. Due to the multitude of differe
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