Unraveling the correlation between Hall-Petch slope and peak hardness in metallic nanolaminates

Interfaces often dominate plastic response of nanostructured metallic materials, e.g., nanolaminates (NLs), and thus engineering NLs to introduce controllable interfacial properties is a grand challenge. In this work, we comparatively studied the mechanical properties of both crystalline/crystalline NLs (C/CNLs) and crystalline/amorphous NLs (C/ANLs) in terms of the Hall-Petch slope (k) and the peak hardness (H_peak). To characterize the interfacial properties of Cu-based NLs, we propose an energy factor (χ) in light of the interfacial residual dislocation energy determined by the coupling effects of lattice and moduli mismatch and the increased system energy caused by transmission slip determined by the mixing enthalpy (ΔH_mix). It is found that this energy factor χ can quantitatively capture well the variation of Hall-Petch slope k and peak hardness H_peak of NLs with different types of interfaces. There are linear relationships for both k - χ and H_peak - χ plots. For the C/CNLs with positive ΔH_mix, both k and H_peak decrease with increasing χ, whereas for both the C/CNLs with negative ΔH_mix and the C/ANLs, k decreases with decreasing χ while H_peak decrease with increasing χ. The underlying mechanisms for the different mechanical properties of these NLs are elucidated from the perspective of diatomic interactions characterized by the ΔH_mix of a given NL system. These experimental findings provide deep insights into designing tunable interfacial structures in metallic materials to realize their high performance.