Effect of a Surface Constraining Layer on the Plastic Deformation of Au Microspheres
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MRS Advances © 2017 Materials Research Society DOI: 10.1557/adv.2017.644
Effect of a Surface Constraining Layer on the Plastic Deformation of Au Microspheres AZM Ariful Islam1 and Robert J. Klassen1 1
Department of Mechanical and Materials Engineering, Western University, London, Ontario N6A 5B9, Canada.
ABSTRACT
Single crystal Au microspheres, of 3 Pm diameter, with sputter-deposited Ni surface layers, of 40 or 80 nm thickness, were tested in compression at three loading rates to investigate the role of thin passive layers on the mechanisms of plastic deformation of small-volume FCC ductile metal samples. The Ni layer resulted in an increase in the incipient yield force by about 10%. Micro-cracking of the Ni layer was observed to occur with incipient yielding. The estimated apparent activation volume of the incipient plastic deformation process was found to be nearly identical for the Ni-coated and the uncoated Au microspheres. This suggests that, while the stress required to initiate incipient plastic deformation was increased by the constraint imposed by the Ni layer, the subsequent plastic flow occurred by a dislocation nucleation and glide mechanism that is essentially the same as that occurring in an unconstrained Au microsphere.
INTRODUCTION It is well established that small metal samples, with length dimensions in the nanometer or micrometer range, display considerably enhanced mechanical strength compared to their bulk counterparts [1–4]. The data indicate quite conclusively that this increase in strength is due to a transition in the operative deformation mechanisms from one controlled by dislocation-obstacle interactions to one controlled by dislocation nucleation. In addition to this “intrinsic” deformation mechanism transition, the deformation of small samples is often affected by physical constraint imposed by rigid surrounding material. In such situations an increased density of “geometrically necessary” dislocations within the small sample is necessary to accommodate this constraint. Little experimental data exist on the effect of such extrinsic constraint on the activation energy of the dislocation nucleation and glide processes in these small ductile samples. The majority of experiment-based studies of the effect of geometrical constraint on the deformation of micrometer-scale metal samples have been performed with coated thin metal films [5–8]. The flow stress in the coated, also referred to as passivated, thin films is usually higher than the flow stress of the non-passivated films. Plastic deformation occurs by discrete dislocation glide through the thin metal film [9]. If the film is coated by a harder, adherent, layer the threading dislocation cannot leave the
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ductile material and the resulting dislocation pileup effectively hardens the metal film [
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