Experimental evidence for the role of supersaturated interfacial alloys on the shear elastic softening of Ni/Mo superlattices

The importance of the interfaces in governing the elastic behavior of nanometer-scale metallic superlattices has long been recognized, although their intrinsic nature and the underlying physical mechanisms are still subject to controversy. In the present study, the key role of an interfacial alloying effect on the dramatic softening (-62%) in the shear elastic constant $C_{44}$ of Ni/Mo multilayers is clearly evidenced. By combining x-ray diffraction and Brillouin light scattering experiments on both Ni/Mo superlattices and ${\mathrm{Ni}}_{1-x}{\mathrm{Mo}}_x$ solid solution samples, we show that the formation of metastable alloys, obtained either by cosputtering or stabilized at the interfaces, induces an elastic lattice instability. In addition, the extremely low value of $C_{44}$ observed for the multilayers, at small modulation periods, suggests the stabilization under epitaxial growth of a highly supersaturated interfacial alloyed layer.