Probing the size- and constituent-mediated mechanical properties and deformation behavior in crystalline/amorphous...

Two kinds of crystalline/amorphous nanolaminates (C/ANLs), i.e., Ag/Cu-Zr and Mo/Cu-Zr, with a wide range of modulation ratios η (thickness ratio of the amorphous layer to the crystalline layer) from 0.1 up to 9.0 were, respectively, prepared using magnetron sputtering. The hardness and the strain rate sensitivity m were measured for comparison through nanoindentation testing. The mechanical properties displayed a strong η-dependence, which was tuned by the crystalline phases. With the increase of η, the hardness increased in the Ag/Cu-Zr nanolaminates while it decreased in the Mo/Cu-Zr ones. However, the two C/ANLs showed similar variations in m that was reduced gradually from positive values at small η to negative values at large η. Microstructural examination demonstrated that the amorphous Cu-Zr layers in both the C/ANLs showed a deformation-induced crystallization (DIC) phenomenon within the nanoindentation deformation zone. The DIC was highly dependent on η and became more intense in the Mo/Cu-Zr than in the Ag/Cu-Zr C/ANLs. The η- and constituent-dependent DIC behaviors were rationalized in light of the stress field applied on the amorphous layers that is sensitive to both the amorphous layer thickness and the crystalline constituents. This DIC-induced negative m in amorphous layers competed with the positive m in crystalline layers, leading to a negative-to-positive change in m on reducing η. The underlying deformation mechanism was revealed to be the cooperation between dislocation activities in the crystalline layers and shear transformation zone motions in the amorphous layers. Furthermore, a modified mechanistic model was utilized to quantitatively describe the η-dependent hardness at different crystalline constituents.