The Correlation of Slip between Adjacent Lamellae in TiAl.
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The Correlation of Slip between Adjacent Lamellae in TiAl. G. Molénat1, A. Couret1, M. Sundararaman2, J. B. Singh2, G. Saada and P. Veyssière LEM, CNRS-ONERA, Chatillon, France; 1 Materials Science Division, Bhabha Atomic Research Centre, Mumbai, India; 2 CEMES, CNRS, Toulouse, France. ABSTRACT In lamellar TiAl, concerted deformation occurs in adjacent true-twin related variants (O/OT). When deformation takes place in a given variant, O, by activation of the slip system with highest shear stress, slip proceeds in the companion variant, OT, by operation of a slip system of the same family in mirror orientation, including the slip direction. Depending on load orientation, the latter system may not necessarily exhibit the highest resolved shear stress. The correlation does not depend upon Burgers vector orientation with respect to the interface. This property applies to ordinary dislocations and twinning. The variant that conforms the Schmid law is referred to as the pilot variant, the twin-related variant is dubbed driven. INTRODUCTION Processed under adequate thermal conditions [1] Ti-rich TiAl alloys exhibit a well-documented lamellar structure that consists of an arrangement of layers of γ-TiAl (L10) and hexagonal α2-Ti3Al phases such that the basal plane of the latter is in coherency with one {111} plane of the γ phase which is tetragonal (c/a = 1.02). Due to the peculiar atomic ordering in γ-TiAl, this results in 6 different orientation variants and several types of interfaces, of which only those separating truetwin related variants are perfectly coherent [2]. A fully lamellar grain is often dominated by one particular orientation together with its twin-related variant whose density is, however, much less [3]. Slip transmission is hampered by geometrically necessary dislocations at interfaces and by the fact that unit translations do not match between two contiguous variants unless these are truetwin-related. Perfect Burgers vectors in one variant may become partial unit translations in the adjacent variant. As an example, 1/2
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