Anomalously high density and thermal stability of nanotwins in Ni(W) thin films: Quantitative analysis by x-ray diffract
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E.J. Mittemeijer Phase Transformations, Thermodynamics and Kinetics, Max Planck Institute for Intelligent Systems (formerly Max Planck Institute for Metals Research), D-70569 Stuttgart, Germany; and Institute for Materials Science, University of Stuttgart, D-70569 Stuttgart, Germany (Received 27 March 2014; accepted 21 July 2014)
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.
I. INTRODUCTION
Nanocrystalline materials have been a focus of research in the last decades due to their exceptional properties,1,2 being also intrinsically different from the corresponding coarse-grained materials.3,4 Nanocrystalline Ni–W films were produced successfully by electrodeposition and were found to exhibit outstanding mechanical properties and corrosion resistance.5–8 In recent years, magnetron-sputtered Ni–W films were investigated,9–12 focusing on the microstructure of these films. The presence of a high density of nanotwins was revealed.11,12 The nanotwinned microstructures of these Ni(W) solid-solution films resemble to some extent those of pure nanotwinned metals such as Cu. The nanotwinned Cu microstructures were shown to be responsible for distinctly enhanced hardness, tensile strength, and resistance against fatigue, even in combination with ductility larger than for their nanocrystalline, untwinned counterparts.13–15 It will be shown that the formation of the nanotwinned microstructure in the case of Ni(W) is mainly W-content driven, in contrast to the apparently deposition-induced nanotwins found in pure metals such as Cu. The nanotwinned nature of the microstructure of
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.202 1642
J. Mater. Res., Vol. 29, No. 15, Aug 14, 2014
http://journals.cambridge.org
Downloaded: 22 Mar 2015
Ni(W) films, as analyzed in this work, has the potential for drastic improvement of the mechanical properties. Preceding work has shown that the magnetron-sputtered Ni(W) films exhibit a columnar grain morphology, being nanocrystalline wi
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