Real-time atomic-resolution observation of coherent twin boundary migration in CrN

Although coherent twin boundary (CTB) migration in fcc metals has been widely studied, little is known about the CTB migration behavior in the binary transition-metal nitrides system (e.g. rock-salt CrN). Using in-situ atomic-resolution electron microscopy, we report two different twin boundary defect (TD) nucleation and CTB migration modes at the CTB/ITB (incoherent twin boundary) and CTB/surface junctions. A new twin defect nucleation and CTB migration mode are observed from the CTB/surface junction. We show that such CTB migration is associated with a boundary structure alternating from an N-terminated to Cr-terminated, involving Cr and N atom respective motion, i.e., asynchronous CTB migration. We further reveal the dynamic and thermodynamic mechanism of such asynchronous migration through strain analysis and DFT simulations. Our findings uncover an atomic-scale dynamic process of defect nucleation and CTB migration in a binary system, which provides new insight into the atomic-scale deformation mechanism in complex materials.