Low-angle grain boundary structures and size effects of nickel nanolaminated structures

Recent experiments have demonstrated the fabrication of nickel nanolaminated structures with low-angle grain boundaries and that these nanolaminated structures exhibit an ultrahigh hardness of 6.4 GPa and excellent thermal stability. However, the detailed microstructure of the low-angle grain boundary in these nanolaminated structures and plastic deformation mechanisms underlying their ultrahigh hardness have remained elusive to date. Thus, we performed a series of large-scale atomistic simulations to explore the microstructures, thermal stability and energies of the low-angle grain boundary in nanolaminated nickel and to further mimic the plastic deformation of nanolaminated nanopillars. First, we examined the thermal stability of low-angle grain boundaries in nanolaminated structures with three typical textures observed in previous experiments, and we found that the {111} -textured nanolaminated structure is the most stable and has a critical temperature of approximately 750 K for stability. The low-angle grain boundary in such nanolaminated structures consists of a periodic arrangement of three types of dislocations with Burgers vectors of 1/2[101], 1/3[11¯1] and 1/6[14¯1]. Our simulations also showed that the grain boundary energy of the low-angle grain boundary with misorientation of 5° is approximately 0.70 J/m2 and that the interaction energy between two low-angle grain boundaries follows a power law as the lamellar thickness, with an exponent of −1.14. We further simulated the uniaxial compression and tension of nanolaminated nanopillars with a texture of {111} and different lamellar thicknesses. The simulation results showed that as the lamellar thickness decreases, both compressive and tensile yield stresses significantly decrease, while their flow stresses slightly increase. The size effect on yield stress is attributed to dislocation escape from the low-angle grain boundary due to some typical reactions of constituent dislocations, while the size effect on flow stress is related to a complex reaction and interaction of high-density dislocations. Our study provides a fundamental understanding of the grain boundary structure and plastic deformation in textured nanolaminated metals, which offers insights for the design of stronger nanolaminated structures.