The mechanical properties of Compositionally graded nanocrystalline materials (CGNMs) are studied via molecular dynamics simulation. Similarly to metal and alloys, these materials' grain size significantly affects their mechanical properties. However, achieving a complete understanding of the mechanical behavior of CGNMs with different grain sizes, particularly at the atomic level, has remained indefinable. This article uses molecular dynamics (MD) simulations to investigate the tensile mechanical properties of CuNi CGNMs with varying grain sizes. The CGNMs exhibit a dual dependence on grain sizes. The findings demonstrate that the yielding stress of CGNMs increases with a decrease in the grain sizes. Research shows that the critical value of the average grain diameter available to transform the positive Hall-Petch relationship to inverse one is dc equals 11.09 nm, at this size, the yield strength (YS) is 2.7 GPa and is the largest value in the samples. This is explained when the average grain diameter has not reached the critical value, the dislocations moving during plastic deformation will be accumulated at grain boundaries to form dislocation clusters (or walls) that prevent the further movement of other dislocations, and at the same time create a dislocation stress field around the grain boundaries, causing the materials strengthening.
The yield stress decreases if the average grain diameter is smaller than 11.09 nm. When the grain size is smaller than the critical value, the grain volume is too small to contain enough dislocations. Therefore, they glide across the boundary very quickly, decreasing the YS of the material, which means materials softening due to rotation or gliding of grain boundaries. This change in yield stress is consistent with the inverse Hall-Petch relationship. These conclusions have positive significance for the design of these compositionally graded nanocrystalline CuNi.
