Size- and constituent-dependent deformation mechanisms and strain rate sensitivity in nanolaminated crystalline Cu/amorphous Cu-Zr films

The hardness, tensile ductility, and strain rate sensitivity of crystalline Cu/amorphous Cu-Zr nanolaminates (Cu/Cu-Zr C/A NLs) have been measured as a function of modulation ratio (η). With reducing η, the tensile ductility first decreased and subsequently increased, leaving a minimum value at η ∼ 1.0. However, the strain rate sensitivity (m) increased monotonically with reducing η and spanned from a negative value at η over ∼1.0 to a positive one at η below ∼1.0, indicating a tunable strain rate sensitivity in engineered C/A NLs. Careful microstructural examinations reveal that a deformation-induced devitrification (DID) in the amorphous nanolayers is the key factor responsible for the aforementioned experimental phenomena. For thinner amorphous nanolayers, the DID becomes more intense. The size-dependent DID drives the pure Cu-Zr amorphous single layer films to (i) exhibit a thickness-dependent tensile ductility opposite to that of pure Cu single layer films, and (ii) have a negative m contrary to the positive m in their pure Cu counterpart. When the two layers are engineered into C/A NLs, a competition exists between the two inverse constituent nanolayers. This competition is strongly η-dependent, resulting in a non-monotonic evolution in tensile ductility and significant change in m when η spans from 9.0 to 0.1. The fracture mode of the C/A NLs transformed from shearing at small η to opening at large η; this can be rationalized by considering the competition between the two constituent nanolayers as a microcrack initiator. In addition, the strengthening mechanisms of the C/A NLs were analyzed and the η-dependent hardness was quantitatively described using a modified mechanistic model.