First-principles surface parameters and gradient-dependent plasticity in Cu-Ni FGMs: A molecular dynamics approach

• Investigation into the mechanical properties of functional graded materials (FGMs) with different volume distributions using molecular dynamics (MD) simulations, which is crucial for exploring the feasibility of their applications in micro-nanodevices. • Prediction of mechanical properties such as Young’s modulus through MD simulations, which closely matches results from continuum medium theory. • Calculation of surface parameters from an atomic level for the first time, providing reliable data for the theory of micro-scale surface effects under continuum mechanics. • Examination of dislocation expansion during deformation, offering insights into the micro-mechanical response and deformation mechanism of FGMs from an atomic perspective. As micro- and nanoscale devices become increasingly integrated into MEMS, NEMS, and energy conversion systems, the demand for materials with spatially tunable mechanical properties has grown significantly. Functionally graded materials (FGMs), which exhibit continuous compositional variation, are promising candidates for these applications. However, at the nanoscale, classical continuum models fail to capture size-dependent effects, particularly surface-dominated phenomena. This creates a critical gap in understanding how composition gradients influence mechanical behavior from an atomic perspective. This study investigates the mechanical response of Cu-Ni FGMs with power-law composition gradients using molecular dynamics (MD) simulations. Special attention is given to surface effects, dislocation evolution, and phase transformation mechanisms that are not accessible via continuum approaches. Tensile, compressive, and nanoindentation simulations were performed on Cu-Ni FGMs with varying Ni volume fractions. The Young’s modulus of different components obtained by tensile simulation is close to the results of the continuum medium theory. The error is less than 6%. From pure copper to pure nickel, mechanical properties gradually improve, and the peak proportion of the BCC structure is negatively correlated with the Young’s modulus. We found that when BCC serves as the structural precursor, anomalous phenomena such as an increase in dislocation density may occur.Compression and nano-indentation simulations were performed to derive surface parameters. Notably, these parameters were determined at the atomic level for the first time. The results indicate that the lower stacking fault energy of copper facilitates premature stress relaxation. The macroscopic performance is that the hardness decreases by 9% and the plastic zone expands by 2.4 times.Providing reliable surface parameters from the atomic level for the theory of micro-scale surface effects under the continuum mechanics theory.The error between the three-point micro-bending experiment (Cu-Ni gradient film) and the simulated deflection curve is less than 6%. The dislocation expansion during the deformation process is also investigated,and a large number of dislocation stacking fault activity analysis was carried out.They provide an atomic perspective for the micro-mechanical response and deformation mechanism of functional graded materials.