Anomalously high density and thermal stability of nanotwins in Ni(W) thin films: Quantitative analysis by x-ray diffraction

Magnetron-sputtered Ni(W) films appear to possess a high density of nanotwins oriented parallel to the film surface which highly influences the properties of Ni(W) films. A sophisticated analysis method for describing the stacking sequence of close-packed atomic layers by statistical parameters has been developed which is based on the evaluation of intensity streaks in reciprocal space measured by (x-ray) synchrotron diffraction. In particular, the degree of hexagonality introduced by twinning into these ideally face-centered cubic-stacked films can be quantified. The validity of the proposed analysis has been confirmed by direct observation of the stacking sequences of close-packed layers using (high-resolution) transmission electron microscopy. It has been shown that the degree of hexagonality in the as-deposited state is practically proportional to the W content. Further, the thermal stability of the nanotwins increases with increasing W content which can be understood by the appearance of hexagonal close-packed-like domains exhibiting an intrinsic thermodynamic stability.