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 products in the reaction a[111]--> a[110] + a[001] in B2 NiAl. Atomistic calculations show that the metastable state can be formed spontaneously by a local shuffle or climb decomposition. Once formed, the 'piggyback' configuration can glide as a unit, with a restoring force helping to keep it intact. However, if the product lines become separated locally beyond a critical distance, 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 results are consistent with detailed observations of the microstructure. The implications for this metastable state for the mechanical properties are examined.