Surface Energies of Planar Defects and Dislocation Processes in Al 3 Ti and Tial
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SURFACE ENERGIES OF PLANAR DEFECTS AND DISLOCATION PROCESSES IN AL 3 TI AND TIAL HUG G., DOUIN J. AND VEYSSIERE P. LEM ONERA-CNRS, BP 72,92322 Chitillon Cedex, France. ABSTRACT Information on APB energy in ordered alloys is important not only to understand their mechanical properties but also to analyse conditions for phase transformation. APB surface energies
are determined from dissociation distances of superdislocations both inTiA1 and Al Ti Medium and
high APB energy anisotropies are found in TiAl and Al 3Ti respectively. These results are consistent with phase diagram studies which predict that the APB energy vanishes in the (001) plane near the AI3Ti composition. Examples of decompositions of perfect dislocations into perfect dislocations are discussed. INTRODUCTION Ordered structures often exhibits specific mechanical properties such as an anomalous increases of the flow stress and/or a dramatic lack of ductility which is very common in these materials. It is well known that the flow stress anomaly originates from the dislocation fine structure itself, which is in turn dependent on APB energy. In the L12 structure, it has been argued1 that the APB energy on ( 100) is low since there is no first neighbour violation. On the other hand, APBs on (111 ) planes should, on a hard sphere model, exhibit a higher surface energy because of first neighbours modifications. Such an energy anisotropy of APBs has important consequences on properties of superdislocations since the ( 100) low energy dissociation plane does not coincide with the ( 111 ) easy glide plane of face centrered cubic crystals. This constitutes the foundation of the well-4 2 known Kear-Wilsdorf (KW) mechanism . In fact, APB surface energy measurements in Ni3AI3,
based on dissociation distance are in favour of a reduced anisotropy. Atomic simulations of APB on different planes5 confirm that the APB energy is much more isotropic than predictable from a hard sphere model, and this is due to atomic relaxation at the defected surface. More recently, a study of the statistical distribution of APB planes in rapidly quenched superalloys 6 brought further support to the fact that the APB surface energy is relatively isotropic in these materials. At least it is isotropic enough for the criterion defined by Paidar, Pope and Vitek7 (PPV) for the cross-slip from ( 111 ) to (100) not to be fulfilled (y,00 < 7r1/ v3). The Ti-Al system offers a rather different situation. Near the A3B composition an intermetallic alloy may crystallize under several long range ordered structures that can be related to theLl 2structure. By introducing an APB every other (00) atomic cube plane in the L12 structure, one gets the DO22 structure under which Al 3Ti crystallizes. In the Ti-Al phase diagram, near the Tli+xAl3_x composition, it is thought the APB energy vanishes for the Al3Ti composition8 .Recently, the possibility of transforming the DO22 structure intoLl 2 by modifying the sign of the APB energy in A1Ti with the addition minorelements such as copperornickel, has raised co
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