Dislocation Core Structures in NiAl
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DISLOCATION CORE STRUCTURES IN NiAI. R. PASIANOT and D. FARKAS. I)cpartment of Materials F7nginccring, Virgini:a l'Polv Blacksburg, VA 24061.
hrc.ii Instiltuie and State University,
ABSTRACT Since the early studies on bce materials, comptiter sinhtila ion ol dislocation core structures has been recognized as a valid means of understanding yicld behavior with temperature. Many of today's high temperature ordered alloys do have yield anomalies and computer simulation of core structures is a basic ingredient inl the corrent theories for the anomalous yield behavior in LI2 alloys. In the case of 112 type alloys there ire %,cry[fe\v of these studies. The present work deals with computer simulation of' core strnctures in 112 NiAI. An embedded atom type potential is used to simulate intterictiois mid ;v'vcral slip systems are considered, h 'xpcriincntally observed. particularly those having 11001 and II III slip vcctors that ir' the INTRODUCTION Current theories on the yield behavior of alloys are always based on the structure of dislocation cores. In the case of the 1.12 structure, for example. the increasing yield strength with temperature can be explained on the basis of the existence )ift\wo different core configurations for the screw [I 101 dislocation dissociated in the 1111l1 pline (I). One of these cores is planar and the therefore glissile. The other is non-plahar and sessile (2). In bcc metals, the yield behavior can be explained on the basis of thc non-planar structi-re o" the dislocation cores for the 1/2111 I1 screw dislocations (3). In the case of 112alloys anomalous yield behavior has been observed in Cu-Zn (4). No detailed models have been developed for this case to the author's knowledge. Slip in Cu-Zn alloys is 1/211111. In nmaterials with higher A1Il1 energies 11001 slip has been observed (5), for instance in FeAl there i a transition to 11001 slip at high temperatures (6) and in NiAI slip is mainly I1001 (7). Yield strength in this material decreases with temperature; < I I I > screw dislocations have also becoi observed almd it was h'ound that they do not dissociate into two 1/2< I II > with ;im AI)I in betwceen biut rather stay as < I II > (7,8). The purpose of the present work is to study dislocalion core configurations in NiAI, in order to contribute to the modeling of the nlcchaiica;l bhli0mior i•i this alloy. INTERA TOMIC POTENTIALS AND S;IA l/l.A TION METHOD We performed atomistic simulations of the dislocation corc area using embedded atom interatomic potentials. These potentials were dcveloped by Voter aid co-workers based on the Ni3AI phase (9). For the NiAI phase only the lattice pjrmm cter and cohesive energy were considered. Our calculations have shown that the 12 phase is indeed stable with respect to the L10. The diffcrence in cohesive energies of thesc two phases is very small (4.369 for the Llo phase ev/atom compared to 4.375 ev/atom for the 132srutcture). This is very reasonable since it is known that off-stochiometry NiAI undergoes a moartensitic transl'ormation to the [lo phase. T
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