Effect of microstructure on crack behavior in nanocrystalline nickel using molecular dynamics simulation

The crack growth process in columnar nanocrystalline samples is simulated using the molecular dynamics method. The effects of grain size, grain boundary, crystallographic orientation and crack tip position on the crack growth behavior are investigated. Different sets of samples with mean grain sizes ranging from 4 nm to 14 nm are prepared. Samples with a similar number of grains and identical dimensions are considered for examining the impact of grain boundary and crystallographic orientation. To assess the effect of the grain boundary, no constraint is considered on the position and orientation of grains, while only the grain orientations are changed to examine the effect of crystallographic orientation. Also, samples with scaled dimensions are created to study the effect of grain size on the crack growth process. In such samples, the relative position of seeds and crystallographic orientation of grains remain constant. The results show that as the mean grain size is reduced, a higher level of stress is required for the initiation of crack growth. Moreover, the reduction of grain size from 14.24 nm to 11.65 nm increases the critical stress intensity factor. Further reduction of grain size decreases the critical stress intensity factor. For grain sizes smaller than 11.65 nm, the volume of stacking fault in the grains near the crack-tip is not significant. Grain size and grain boundary volume have a simultaneous impact on the initial slope which can be observed around the grain size of 11.65 rim. As a result, in nanocrystals with mean grain sizes of less than 11.65 nm, the crack grows in a brittle manner to reach the grain boundary, then progresses intergranularly with high speed. Consequently, grain boundaries and crystallographic orientations are not effective on the crack growth mechanism in nanocrystals with grain sizes less than 11.65 nm, whereas these factors can blunt or arrest the crack in nanocrystals with mean grain sizes of larger than 11.65 nm.