In-cascade formation of plain vacancy cluster and its stability in pure Fe: MD study

Previously, molecular dynamic (MD) simulations of displacement cascades up to PKA energy of 40 keV using a recently developed EAM-type interatomic potential describing Fe-Fe interaction [1], have been performed. The formation of big flat vacancy clusters at relatively low simulation temperature (100K) was one of the interesting cascade features; which have been obtained using that potential. Here, the work is continued with the purpose of finding the mechanism responsible for vacancy clustering, since no such high rate of vacancy clustering was observed with other potentials [2]. In the present work the following mechanisms have been considered: formation of cluster during the relaxation stage (gathering during recombination), clustering caused by defect diffusion (the effect of local heating of the cascade region plays an important role) and the effect of molten core in case of formation of dense cascade. The results show that the formation of big vacancy clusters occurs directly within the collision spike which can lead to a presence of local melting of the material. After cooling of damaged region, initial clusters can grow due to the migration of neighboring isolated vacancies at the same time clusters become plan. Thermal stability study of obtained clusters showed that above 900K these objects do not keep their planar shape anymore and become spherical ones within very short period of time.