Strengthening Mechanism of Al/Ni Multilayers with Negative Enthalpy of Mixing

The interface strengthening effect in nanoscale metallic multilayers is influenced by the enthalpy of mixing, which governs the chemical distribution and interface microstructure. In this study, Al/Ni multilayers were fabricated by magnetron sputter deposition, exhibiting an ultrahigh peak hardness of 9.5 GPa─the highest reported for face-centered cubic multilayer systems. Advanced electron microscopy revealed extensive interdiffusion at the Al/Ni interfaces and the formation of intermetallic bonds at both interfaces and grain boundaries. A modified confined layer slip model is proposed, accounting for energy changes associated with trailing dislocations propagating through interfaces or grain boundaries due to intermetallic bond formation. The model aligns closely with experimental data, demonstrating that intermetallic bond formation in Al/Ni multilayers significantly enhances interface strengthening, counteracting the weakening effects of interface diffusion. This mechanism may also account for the high peak hardness observed in other multilayer systems with large negative enthalpies of mixing.