Dislocation dipoles and grain boundary pre-melting

[1] The transformation of narrow dislocation dipoles in selected fcc metals and in γ-TiAl

H Wang et al

Transformations of vacancy dipoles of dissociated edge dislocations are analyzed in Cu, Ni and γ-TiAl by molecular dynamics. Dipole heights up to 20 {1 1 1} interplanar distances are investigated at temperatures ranging from 0 K to near the melting points of Cu and Ni and slightly below the upper boundary of the single phase γ-TiAl domain. Three model configurations, hollows, vertical compact and inclined dipoles, are considered and their relative stabilities compared. Except for dipoles one interplanar distance high, hollows are either metastable or unstable and they are never formed by mutually approaching dipolar dislocations. The three configurations transform into a variety of height- and temperature-dependent layouts including cores containing ordered free volumes, zigzagged faulted dipoles and agglomerated stacking-fault tetrahedra (SFT). At the highest temperatures, small individual SFTs are formed by short-range pipe-diffusion along the dipole cores. There is no critical height below which small-height dipoles or their debris would just simply disappear.

[2] Thermodynamics of grain boundary premelting in alloys. II. Atomistic simulation

P L Williams and Y Mishin

We apply the semi-grand-canonical Monte Carlo method with an embedded-atom potential to study grain boundary (GB) premelting in Cu-rich Cu–Ag alloys. The Σ5 GB chosen for this study becomes increasingly disordered near the solidus line while its local chemical composition approaches the liquidus composition at the same temperature. This behavior indicates the formation of a thin layer of the liquid phase in the GB when the grain composition approaches the solidus. The thickness of the liquid layer remains finite and the GB can be overheated/oversaturated to metastable states slightly above the solidus. The premelting behavior found by the simulations is qualitatively consistent with the phase-field model of the same binary system presented in Part I of this work [Mishin Y, Boettinger WJ, Warren JA, McFadden GB. Acta Mater, in press]. Although this agreement is encouraging, we discuss several problems arising when atomistic simulations are compared with phase-field modeling.