Strengthening and microstructure modification associated with moving twin boundaries in hcp metals

The interaction of a moving twin boundary with clusters of self-interstitial atoms and vacancies, containing up to 35 point defects, has been studied by atomic computer simulation in a model crystal of α-zirconium. Conservative movement of the boundary has been achieved by glide of twinning dislocations of edge character under applied shear stress. Several reactions were observed, the result depending on cluster orientation and location relative to the glide plane of the twinning dislocation. They include: (i) restriction of twin boundary mobility; (ii) change of cluster orientation and shape; (iii) glissile cluster drag by the boundary without contact; and (iv) total or partial absorption of a cluster by the twin boundary and cluster drag, together with simultaneous glide along the interface. It is concluded that the applied shear stress for motion of twin boundaries is raised by interaction with point defect clusters. Furthermore, moving twin boundaries act as defect sinks or recombination centres and provide a means for removing defects from regions of radiation damage.