Abstract
Bulk composites made of transforming matrix and nanoinclusions possess exeptional mechanical properties owing to excellent interfacical strain matching as implemented by uniform lattice shear of martensitic constituents. Such an effect remains however largely unexploited in thin films. In this study, NiTi/Nb multilayered thin films with various individual layer thickness (h) were prepared by magnetron sputtering. Strong h-dependence in indentation hardness (23 GPa/nm−1/2) is discovered in NiTi/Nb multilayers, as compared to those (4–18 GPa/nm−1/2) in conventional bimetallic X/Nb multilayers where X = Cu, Ti, Ag, Al etc. Indentation hardness of NiTi/Nb multilayers saturates at 12 GPa, as compared to 2.6–8.5 GPa of X/Nb multilayers. In situ grazing incidence synchrotron X-ray diffraction reveals that the lattice strain of Nb in NiTi/Nb is 60–240 % greater than that in X/Nb. We suggest that such discrepancies may arise from the interfacial strain concentration relief facilitated by the transforming NiTi layers.
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