  We use atomistic simulations to investigate grain boundary (GB) phase
transitions in el- emental body-centered cubic (bcc) metal tungsten. Motivated
by recent modeling study of grain boundary phase transitions in [100] symmetric
tilt boundaries in face-centered cu- bic (fcc) copper, we perform a systematic
investigation of [100] and [110] symmetric tilt high-angle and low-angle
boundaries in bcc tungsten. The structures of these boundaries have been
investigated previously by atomistic simulations in several different bcc
metals including tungsten using the the {\gamma}-surface method, which has
limitations. In this work we use a recently developed computational tool based
on the USPEX structure prediction code to perform an evolutionary grand
canonical search of GB structure at 0 K. For high-angle [100] tilt boundaries
the ground states generated by the evolutionary algorithm agree with the
predictions of the {\gamma}-surface method. For the [110] tilt boundaries, the
search predicts novel high-density low-energy grain boundary structures and
multiple grain boundary phases within the entire misorientation range.
Molecular dynamics simulation demonstrate that the new structures are more
stable at high temperature. We observe first-order grain boundary phase
transitions and investigate how the structural multiplicity affects the
mechanisms of the point defect absorption. Specifically, we demonstrate a
two-step nucleation process, when initially the point defects are absorbed
through a formation of a metastable GB structure with higher density, followed
by a transformation of this structure into a GB interstitial loop or a
different GB phase.
