High entropy alloy has attracted extensive attention in nuclear energy due to outstanding irradiation resistance, partially due to sluggish diffusion. The mechanism from a defect-generation perspective, however, has received much less attention. In this paper, the formation of dislocation loops, and migration of interstitials and vacancies in CoNiCrFeMn high entropy alloy under consecutive bombardments were studied by molecular dynamics simulations. Compared to pure Ni, less defects were produced in the CoNiCrFeMn. Only a few small dislocation loops were observed, and the length of dislocation was small. The dislocation loops in Ni matrix were obviously longer and so was the length of dislocation. The interstitial clusters had much smaller mean free path during migration in the CoNiCrFeMn. The mean free path of 10-interstitial clusters in CoNiCrFeMn was reduced over 40 times compared to that in pure Ni. In addition, CoNiCrFeMn had a smaller difference of migration energy between interstitial and vacancy, which increased the opportunity of recombination of defects, therefore, led to less defects and much fewer dislocation loops. Our results provide insights into the mechanism of irradiation resistance in the high entropy alloy and could be useful in material design for irradiation tolerance and accident tolerance materials in nuclear energy.

Magnetic microrobots have garnered increasing attention in the biomedical field due to their wireless motion control and noninvasive therapeutic potential. However, current microrobots fabricated from soft magnetic materials exhibit critical limitations, including weak magnetic response and uncontrollable locomotion in high-rate blood flow, which hinder their practical applications. Here, we develop a fabrication method for three-dimensional (3D) hard magnetic microrobots (HM-microrobots) containing neodymium-iron-boron particles. HM-microrobots, featuring programmable 3D helical morphologies, are fabricated by integrating two-photon polymerization printing of molds in positive photoresist with subsequent vacuum infusion molding of hard magnetic slurries. The microrobots achieve a maximum swimming velocity of 22.6 body lengths per second (3 millimeters per second) in a weak rotating magnetic field of 2 millitesla, which represents a notable improvement over their soft magnetic counterparts. Furthermore, they demonstrate controllable upstream propulsion in subcentimeter-per-second blood flow, showing great potential for endovascular applications.

The defects in different materials after bombardment.

In this paper, we investigate the friction behaviors of graphene flakes sliding on a gold substrate using molecular dynamics simulations. The effects of flake size, flake shape, relative rotation angle between flake and substrate, and crystal orientation of substrate on the friction process are thoroughly studied. It is found that under the same load, the average friction forces per atom are smaller for a bigger graphene flake, which exhibits an obvious size effect. It is also shown that flake shape is critical in determining the friction in the sliding process. The average friction forces per atom for the square flake are much bigger than those for the triangular and round flakes. Moreover, the average friction forces per atom for the triangular flake are the smallest. We also find that the orientation of graphene flake relative to gold substrate plays a vital role in the friction process. The friction forces for the graphene flake sliding along the armchair direction are much bigger than those for the flakes with rotation. In addition, it is also found that single crystalline gold substrate exhibits a significant anisotropic effect of friction, which is attributed to the anisotropic effect of potential energy corrugation. These understandings not only shed light on the underlying mechanisms of graphene flake sliding on the gold substrates but also may guide the design and fabrication of nanoscale graphene-based devices.