  The mechanical behaviour and deformation mechanisms of magnetron-sputtered
Zr/Nb nanoscale multilayers were analysed as a function of the periodicity (L =
10 and 75 nm) and annealing time at 350C (2-336 hrs). The strength of the
as-deposited multilayers was independent of the layer thickness and it was
controlled by the co-deformation of the Zr and Nb layers. Annealing led to
transformation of the Zr layers into ZrO2 after a few hours, while the Nb
layers oxidised at a much slower rate. The volumetric expansion associated with
the oxidation led to the formation of cracks at the interfaces and within the
ZrO2 layers for the multilayers with L = 75 nm but not in the case of L = 10
nm. The nanoindentation hardness increased significantly after annealing due to
the contribution of the residual stresses associated with the volume increase
due to oxidation and to the higher strength of the oxides. The strength
increase after annealing, as measured by micropillar compression tests, was
smaller than that measured by nanoindentation, as it did not include the
contribution of the residual stresses, which were relieved during micropillar
fabrication. The nanolaminate with L = 10 nm presented the highest strength and
toughness as damage during oxidation was suppressed, and a deformation
mechanism controlled by the formation of shear bands.
