Atomistic investigation of grain boundary strengthening and composition-dependent mechanical properties in CuNi binary alloys

Abstract

This work examines the influence of grain boundary (GB) and chemical composition on the elastic and mechanical properties of CuNi binary alloys through molecular dynamics simulations. Three configurations, a defect-free single crystal (GB Non) and two symmetric tilt GBs, Σ5 (310) and Σ13 (314), were evaluated across five compositions. The computed elastic constants reveal that C₁₁ consistently exceeds C₁₂ and C₄₄ (C₁₁ = 185.03, C₁₂ = 124.34, and C₄₄ = 88.99 for Cu_0.50Ni_0.50) a characteristic signature of FCC alloys. The introduction of GBs enhances stiffness, indicating a structural strengthening effect attributed to local atomic rearrangements, with the magnitude of enhancement varying by composition. All configurations satisfy the Born–Huang mechanical stability criteria, confirming their elastic stability. Across all compositions, Young's modulus (E) remains the highest among the three moduli, reflecting the alloys pronounced resistance to uniaxial tensile deformation. Stress–strain analysis further highlights the superior performance of GB Σ13, which exhibits the highest peak stress among all configurations, with peak strength increasing progressively with higher Cu content. This correlates with its highest hardness and melting point (1463.43 K) as well as enhanced machinability compared to GB Non and GB Σ5 with highest Cu concentrations. Cauchy pressure, Poisson's ratio, Pugh's ratio, and Frantsevich's criterion consistently classify the alloys as ductile, while direction-dependent properties reveal anisotropic behavior. Furthermore, reversing the Cu/Ni ratio has negligible impact on the elastic response for GB Non and GB Σ5, as observed for Cu_0.375Ni_0.625 and Cu_0.625Ni_0.375. These results provide atomistic insights into the interplay between GB structure, composition, and mechanical performance, offering valuable guidance for optimizing CuNi alloys in structural applications.