Atomic Structures of [01̄10] Symmetric Tilt Grain Boundaries in Hexagonal Close-Packed (hcp) Crystals

Molecular dynamics simulation and interface defect theory are used to determine the relaxed equilibrium atomic structures of symmetric tilt grain boundaries (STGBs) in hexagonal close-packed (hcp) crystals with a [01̄10] tilt axis. STGBs of all possible rotation angles θ from 0 deg to 90 deg are found to have an ordered atomic structure. They correspond either to a coherent, defect-free boundary or to a tilt wall containing an array of distinct and discrete intrinsic grain boundary dislocations (GBDs). The STGBs adopt one of six base structures, PB⁽ⁱ⁾ , i = 1, …, 6, and the Burgers vector of the GBDs is related to the interplanar spacing of the base structure on which it lies. The base structures correspond to the basal plane ( θ = 0 deg, PB⁽¹⁾ ); one of four minimum-energy, coherent boundaries, (2̄111),\;(2̄112),\;(2̄114) , and (2̄116)\;( PB⁽²⁾ - PB⁽⁵⁾ ) ; and the ( 112̄0 ) plane ( θ = 90 deg, PB⁽⁶⁾ ). Based on these features, STGBs can be classified into one of six possible structural sets, wherein STGBs belonging to the same set i contain the same base boundary structure PB⁽ⁱ⁾ and an array of GBDs with the same Burgers vector bGB⁽ⁱ⁾ , which vary only in spacing and sign with θ . This classification is shown to apply to both Mg and Ti, two metals with different c / a ratios and employing different interatomic potentials in simulation. We use a simple model to forecast the misorientation range of each set for hcp crystals of general c / a ratio, the predictions of which are shown to agree well with the molecular dynamics (MD) simulations for Mg and Ti.