Speaker
Description
Both vacancies and grain-boundaries (GB) are important defects in materials. The vacancies can interact with the GBs which might lead to a formation of voids, as a result this might start the formation of pores or cracks. From atomistic simulations it is known, that vacancies can be attracted to GBs, which indeed may act as sinks for the vacancies. In this work we use ab-initio methods to study the attraction of vacancies to GBs in tungsten. This is done via calculating the segregation energy that quantifies a defect’s attraction towards GBs. In the study we include 15 different coincident site lattice GBs ranging from $\Sigma3$ to $\Sigma43$ enabling a representative overview of segregation behaviours. In addition to the segregation energies also structural changes of the GB are observed which are characterized by a change of the stress-state of the atomistic structure and the width of the GB. Combining these results allows to estimate overall size-changes due to annealing of vacancies at GB in nano-crystalline materials produced using e.g. high pressure torsion. The results show a few segregation sites towards which the vacancies are strongly attracted, where the segregation energy corresponds to the vacancy formation. Relaxation of the atomistic structure leads to a filling of the vacancy which is accompanied by reduction of the GB width. However, the majority of sites show intermediate segregation energies, where the vacancy remains stable, i.e. is not filled.