Free-surface effect on displacement cascades in BCC W: Molecular dynamics study

We investigated the free-surface effect on displacement cascades in bcc tungsten using molecular dynamics simulations. Primary knock-on atom (PKA) is projected at different initial depths with two different projectile directions, inward and outward, to the surface. Compared to the bulk system, a simulation system with free surface contains increased number of remaining point defects and clustered defects at equilibrium. This pronounced defect production near the free surface is caused by the suppression of defect recombination events. The interstitials energetically favor the formation of adatoms at the free surface, and the nonsymmetric feature of interstitial mobility is responsible for active vacancy clustering at the sub-surface. The free surface effect extends to 8 and 4 nm in depth when the PKA projectile direction is outward and inward, respectively, with PKA energy of 30 keV at 400 K. Beyond this characteristic depth, the defect population and clustered fraction become similar to those in bulk system. Clustered vacancy develops into extended defects such as 〈100〉 and 〈111〉/2 vacancy loops. For the first time, immobile 〈111〉/2 dislocation is observed in PKA simulation, which is consistent with the experimental reports of both stable 〈100〉 and 〈111〉/2 dislocations of Mason et al (2014 J. Phys.: Condens. Matter. 26 375701).