Due to the high density of grain boundaries (GBs), nanocrystalline metals possess superior properties, including enhanced strength, work hardening, and fatigue resistance, in comparison to their conventional counterparts. The expectation of GB migration is critical for grain coarsening and GB annihilation in these materials, significantly affecting the polycrystalline network and mechanical behavior. Here, we perform molecular dynamics (MD) simulations on gold (Au) nanocrystals containing multiple parallelly arranged GBs, with a focus on the investigation of annihilation mechanisms of low-angle grain boundaries (LAGBs). It is observed that the shear-coupled motion of LAGBs, consisting of dislocations, gives rise to their preliminary migration with the reduced separation distance between GBs. With subsequent GB motion, the LAGBs encountered with neighboring GBs, and can be annihilated by various mechanisms, including dislocations interpenetration, dislocations interaction, or dislocations absorption, depending on the specific configuration of the neighboring GB. These findings enhance our understanding of GB interactions and shed light on the controlled fabrication of high-performance nanocrystalline metals.

To reduce costs and improve payload capacity, modern solid launch vehicles usually use the depleted shutdown solid motors. Because of the incapacity to control thrust, fuel burning rate and shutdown time, the energy management guidance method is the key technology for this kind of launch vehicles to complete high precision orbit injection and overcome various dispersions in the meantime. To solve this problem, a guidance method with iteration is proposed for launch vehicles with depleted shutdown solid motors by energy management attitude in yaw channel, the method is able to assess the vehicle's orbit injection capability in real-time before the injection stage ignition and implement direct injection. Numerical simulation result showed its adequately high accuracy of orbit injection and considerable value for the depleted shutdown solid launch vehicles.