In Situ Study of Deformation Mechanisms in Sputtered Free-Standing Nanocrystalline Nickel Films
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Nickel films of 1.5â10-âŪm thickness, produced by dc magnetron sputtering and with disperse grain size distributions peaking in the 30â60-nm range, were subject to in situ tensile straining in a transmission electron microscope. The deformation was stopped frequently, while keeping the load applied, for transmission electron microscopy observation of the internal structure. Contrast changes occurred in many of the grains between strain increments. Ample evidence was seen of dislocation activity, which appears to be the major mechanism for deformation of the samples. Dislocations were seen in grains as small as 20 nm. Parallel arrays of roughly equally spaced dislocations were observed, spaced about 5â10-nm apart. Intergranular nanovoids were found to form and grow with accompanying strain relief in neighboring grains. The results of the current study are generally consistent with previous in situ investigations and contribute to the understanding of deformation mechanisms in free-standing thin films, which may differ somewhat from those in bulk nanocrystalline materials or in films attached to a substrate.
I. INTRODUCTION
The mechanical behavior of nanocrystalline metals has evoked intense interest in recent years, both because of their high strength and the possibility of new deformation mechanisms coming into operation. Though dislocation activity and the buildup of dislocation networks dominate in metals with grain size in the micrometer or larger regime, molecular dynamics (MD) simulations have indicated that at very small grain sizes (less than approximately 30 nm in Ni), plastic deformation is accommodated mainly in the grain boundaries (GBs).1 At the smallest grain sizes (
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