Atomistic Insights into Nanoindentation Behavior of Gradient Nanocrystalline FeMnCoCrNi High-Entropy Alloys

To investigate the mechanical response and atomic-scale deformation mechanisms of gradient nanostructured FeMnCoCrNi high-entropy alloys under nanoindentation, molecular dynamics simulations were utilized to systematically explore the synergistic regulatory effects of indentation direction, gradient rate, temperature, Fe and Mn concentrations, and indentation velocity on their indentation behavior. The results demonstrate that, at a given gradient rate, both the load and hardness during indentation from the large-grain surface are higher than those from the small-grain surface, which is closely associated with the grain boundary distribution characteristics and dislocation motion behaviors in regions with different grain sizes. When the temperature increases from 300 to 1300 K, high temperature induces the formation and diffusion of disordered structures at grain boundaries. With the decrease in Mn content and the increase in Fe content, the indentation load and hardness of the alloy are significantly enhanced, and the mechanical properties reach a peak when the Mn content is 0% and the Fe content is 40%. This phenomenon is mainly attributed to the disappearance of the softening effect of Mn and the prominent solid-solution strengthening effect of Fe. Under high-speed indentation (100 m/s), the load-displacement curve exhibits a continuous upward trend; in contrast, under low-speed indentation (25 m/s), the curve shows obvious fluctuations. This discrepancy arises from the mismatch between the indentation rate and the evolution rate of the alloy's internal microstructure. In addition, the difference in grain size results in distinct phase transformation and dislocation behaviors: in fine-grain regions, high-density grain boundaries impede dislocation slip and facilitate the formation of dispersed HCP phases, whereas in coarse-grain regions, dislocations can achieve long-range slip, thereby inducing the formation of banded HCP phases. This study clarifies the intrinsic mechanism underlying the multi-factor synergistic regulation of the nanoindentation mechanical behavior of the alloy, providing atomic-scale theoretical support for the design and performance optimization of gradient nanostructured high-entropy alloys.