Discrete dislocation sim ulation of thin film plasticit y
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Discrete dislocation sim ulation of thin lm plasticit y B. von Blanckenhagen, P. Gumbsch, and E. Arzt Max{Planc k{Institute for Metals Researc h, Seestr. 92, D-70174 Stuttgart, German y
ABSTRACT
A discrete dislocation dynamics sim ulation is used to investigate dislocation motion in the con ned geometry of a polycrystalline thin lm. The repeated activ ation of a Frank{Read source is sim ulated. The stress to activate the sources and to initiate plastic
ow is signi cantly higher than predicted by models where the dislocations extend o ver the entire lm thic kness. An eective source size, which scales with the grain dimensions, yields ow stresses in reasonable agreemen t with experimen ts. The in uence of dislocations deposited at interfaces is investigated by comparing calculations for a lm sandwic hed between a substrate and a capping layer with those for a free standing lm.
INTR ODUCTION
The plastic properties of polycrystalline thin metal lms with thic kness of 1m and below are very dierent from the properties of the corresponding bulk materials. The o w stresses of thin lms can exceed the o w stresses of the bulk materials b y an order of magnitude and increase with decreasing lm thic kness (e. g. [1]). However, the underlying mec hanisms are not y et well understood. Two models are frequen tly used in the literature to explain the observed high ow stresses of thin lms and the dependence on lm thic kness or grain size. The rst balances the energy of dislocations deposited at the interfaces with the work done by the moving dislocation (Nix{Freund model [2, 3]), whic h results in a ow stress that scales with inverse lm thic kness. If one requires the deposition of dislocations at the grain boundaries, the
ow stress scales with the inverse grain size (Thompson model [4]). Both models qualitatively explain the increase in the ow stress with decreasing lm thic kness (or grain size), but the predicted ow stresses are often m uch smaller than the experimen tally measured ones [1, 5]. Interface dislocations deposited by dislocation glide have been observed in transmission electron microscopy [6, 7], but are reported not to be very stable and to vanish after a short time of irradiation with the electron beam [8, 9]. Other authors ev en observe that the interfaces seem to operate as a sink for the dislocations and not as an obstacle [10]. However, in situ transmission electron microscop y studies of thin lm plasticity show that dislocation sources operate in the grain interior and that single sources repeatedly emit dislocation loops [10]. The dislocation evolution in a columnar grain of a thin lm is sim ulated here with a discrete dislocation dynamics method. Grain boundaries and in terfaces are introduced as impenetrable obstacles for the dislocations. The model geometry corresponds to a thin lm deposited on a substrate covered by a capping layer and is contrasted with sim ulations for a free standing lm where the dislocations can lea ve the lm at the surfaces. A pinned dislocation segmen t is positioned in the
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