The mechanics of slip transition at intermediate temperatures in < 001 >-oriented NiAl single crystals II. A metastable state for a < 111 >{110} dislocations in NiAl and its role in their decomposition

We show in this paper that there exists a metastable arrangement of product s in the reaction a[111]--> a[110] + a[001] in B2 NiAl. Atomistic calculati ons show that the metastable state can be formed spontaneously by a local s huffle or climb decomposition. Once formed, the 'piggyback' configuration c an glide as a unit, with a restoring force helping to keep it intact. Howev er, if the product lines become separated locally beyond a critical distanc e, the result is decomposition loop. Small dislocation loops (less than 100 AngstromA) collapse, large loops (greater than 3000 Angstrom) evolve into dislocation dipoles, and medium loops can fail to evolve further. These res ults are consistent with detailed observations of the microstructure. The i mplications for this metastable state for the mechanical properties are exa mined.