Tensile Stress and Thermal Growth Effects on Grain Boundary Motion in Nanocrystalline Nickel
We report on two studies that involve molecular dynamics (MD) simulations of grain boundary motion in nanocrystalline (nc) nickel. The first study is conducted to examine the effects of an applied tensile stress on the grain boundary motion in 5 nm3 nc-Ni specimens, half of which contain free surfaces, while the other half have periodic boundary conditions. Grain boundary sliding (GBS) and grain rotation are the deformation mechanisms exhibited by the nc-Ni specimens, in contrast to dislocation-mediated deformation mechanisms found in bulk samples. Specimens that contain free surfaces display a lower yield stress and a lower average grain boundary velocity compared to their periodic counterparts. These phenomena are attributed to the higher degree of grain boundary sliding present within the free surface specimens.
The second study examines thermal effects of various annealing temperatures on grain boundary motion in 5 nm3 periodic nc-Ni specimens. It is found that grain growth exhibits a linear relationship with time, as opposed to parabolic grain growth observed in bulk metals. During the annealing process, it is also observed that the average grain boundary energy decreases with t-1/2, as grains oriented themselves in a lower-energy configuration with their neighbors via grain rotation. An Arrhenius plot of average grain boundary velocity and energy per atom within a grain boundary displays identical slopes, and thus, identical activation energies of ~ 53 kJ for both characteristics. This can be attributed to the fact that grain boundary velocity and energy per atom are governed by the same entity, which is grain boundary diffusion. The annealed samples display a grain rotation-coalescence growth mechanism, where adjacent grains rotate concurrently, to decrease the misorientation energy of the grain boundary between them. It is observed that some grains have achieved the same orientation at the end of the growth process, indicating that the grain boundary has been annihilated, and the two grains have coalesced into a single larger grain.