Plastic Deformation of Thin Metal Foils without Dislocations and Formation of Point Defects and Point Defect Clusters
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Plastic Deformation of Thin Metal Foils without Dislocations and Formation of Point Defects and Point Defect Clusters Michio Kiritani, Kazufumi Yasunaga, Yoshitaka Matsukawa and Masao Komatsu Academic Frontier Research Center for Ultra-high Speed Plastic Deformation, Hiroshima Institute of Technology, Miyake 2-1-1, Saeki-ku, Hiroshima 731-5391, Japan ABSTRACT Evidence for plastic deformation of crystalline metal thin foils without dislocations is presented. Direct observation during deformation under an electron microscope confirmed the absence of the operation of dislocations even for heavy deformation. In fcc metals including aluminum, deformation leads to the formation of an anomalously high density of vacancy clusters, in the form of stacking fault tetrahedra. The dependency of vacancy cluster formation on temperature and deformation speed indicates that the clusters are formed by the aggregation of deformation-induced vacancies. Conditions required for the absence of the dislocation mechanism are explained, and a new atomistic model for plastic deformation of crystalline metals is proposed. INTRODUCTION When thin films were pulled out from bulk material by plastic deformation until fracture, the production of an anomalously high density of vacancy clusters was discovered [1]. Other than these point defect clusters, no dislocations were observed in the vicinity. The authors have proposed that the plastic deformation of crystalline metals proceeds by a mechanism that is entirely different from the widely accepted dislocation mechanism. The proposal has not been readily accepted by the research community that has been deeply involved in studying the dislocation mechanism [2]. In the present paper, the authors present evidences for deformation without involving dislocations, and report a variety of experimental observations of the production and annihilation of point defect clusters, providing useful information for establishing a new model of plastic deformation. THINNING PROCESS OF DUCTILE METAL THIN FILMS BY ELONGATION Annealed metal films (Al, Au, Cu and Ni) of ribbon shape (20 m thick, 3 mm wide, and 10 mm long) were cut halfway in the lengthwise central region in order to initiate deformation at that position. The elongation speed of the films varied over a wide range; 10-9 – 1 m/s. Morphology of the Thinning and Fracture: Localized deformation initiates at the notch-cut, and that position of the film becomes progressively thinner and finally the two sides separate, leaving very thin areas at their tips. These areas are sufficiently thin for observation by transmission electron microscopy [3]. Morphology of the fracture was investigated in detail by Wilsdorf [4], and two typical cases are shown below as the required introduction of this paper. The fractured tips often exhibit sawtooth shapes as shown in Fig. 1 (a). The fracture often occurs along the trace of the (100) plane as illustrated by AA’ in the figure, and it cannot propagate through the long channel of the thinned part when its direction is not para
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