Annealing Effects on The Microstructure of Electrodeposited Cu/Ag Multilayered Nanocomposites
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ABSTRACT TEM was used to investigate the annealing effects on the microstructure of two electrodeposited Cu/Ag multilayered nanocomposites with bi-layer thickness values of 10Onm and 330nm, respectively, as well as electrodeposited pure copper. It was found that silver layers limited the lateral growth of columnar copper grains and possibly acted as sites for nucleation of new copper grains in the as-deposited composites. The refinement of copper grains in this condition is one reason for the much higher strength of these materials in comparison to pure copper. Annealing at 100 'C did not change the microstructure significantly. After annealing at 150 °C, recrystallization occurred in all samples. The degree of recrystallization was dependent on the extent of internal stresses. Recrystallization was complete in the 11 Onm bi-layer thick composite, which had the highest internal stresses owning to its large Cu/Ag interfacial area. The recrystallization and growth of copper grains can be one reason for the drastic drop of the composites yield strength after annealing at 150 0C. INTRODUCTION Multilayered nanocomposites have recently attracted much attention because of their unusual properties [1-4]. In our previous studies [5-7], we have reported that the electrodeposited Cu/Ag multilayered composites show a very high yield strength, which varies little after annealing at 100 TC. However, upon annealing at 150 °C, the strength reduced to a level comparable to that of the electrodeposited pure copper. The drop of the strength after annealing at 150'C could be attributed to the following factors: (1) the recrystallization and grain growth of copper layers because of the highly stored strain energy in the deposited structure, and (2) the relaxation of misfit strains at the interface between copper and silver layers. The objective of this study was to investigate the effects of annealing on the grain size and structure of copper in Cu/Ag multilayered composites as well as in pure copper. EXPERIMENTAL PROCEDURES Two Cu/Ag multilayerd nanocomposites with bi-layer thickness values of 330nm and 10Onm were produced by the electrodeposition method using a cyanide solution. An annealed polycrystalline copper disk was used as the substrate with a diameter of 35mm. The silver and copper layers were alternatively deposited by electroless displacement and galvanostatic technique, respectively. The thickness of the silver layers was always kept constant while that of copper layers was adjusted by controlling the plating time. Pure copper samples were also produced by the same plating conditions as the copper layers of 330nm thick composite. A silver layer was first deposited on the substrate before the deposition of copper. The experimental setup and detailed deposition procedures have been described in the references 5 and 6. 457 Mat. Res. Soc. Symp. Proc. Vol. 505 ©1998 Materials Research Society
Both composites and pure copper specimens were heat treated in a silicon oil at 100 °C and 150 °C for 6 hours, respectively. Cross-sect
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