TEM Observation of Nanocrystalline Copper During Deformation
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TEM Observation of Nanocrystalline Copper During Deformation Carl J. Youngdahl, Northwestern University, Evanston, IL 60208 USA Richard C. Hugo, Los Alamos National Laboratory, Los Alamos, NM 87545 USA Harriet Kung, Los Alamos National Laboratory, Los Alamos, NM 87545 USA Julia R. Weertman, Northwestern University, Evanston, IL 60208 USA ABSTRACT Nanocrystalline samples of copper were prepared using inert gas condensation and an optimized sequence of powder outgassing and compaction. TEM specimens were cut, electropolished, and mounted in a straining stage. In situ TEM observations including real-time video were captured during straining in the microscope. Areas of presumed increased stress concentration were identified near small cracks around the perimeter of the electropolished hole. Such locations were observed in the TEM while the specimen was pulled in tension. Several microstructural changes were captured during deformation including numerous sudden shifts in contrast of grains and parts of grains, occasional dislocation motion, opening and propagation of the crack. Relationships between grain size and deformation are described. INTRODUCTION The empirical Hall-Petch relation describes the dependence of several mechanical properties, including yield strength and hardness, on grain size. Various theories attempt to explain the dependence in terms of dislocation activity or its suppression. At very small grain sizes (below what is commonly used in structural applications), the relationship predicts strengths beyond the ranges of those attained at conventional grain sizes. As grain size decreases even lower, the relationship predicts values of yield stress that reach the theoretical limit. Possibly the mechanisms responsible for Hall-Petch behavior at conventional grain sizes give way to another mechanism at a certain low "threshold" size. As methods to make materials with smaller and smaller grain sizes have increased in number and effectiveness, it is clear that the measured mechanical properties fall short of the values predicted by the Hall-Petch relation. It would be interesting to discover why, as doing so would lend insight into the microstructural workings of the Hall-Petch relation and could clarify how crystalline materials deform in general. In situ straining experiments carried out in a TEM offer the possibility of examining those deformation mechanisms that may be active [1,2]. Dislocation motion, if present, and displacement between grains may be witnessed and captured in real time. Minute changes in grain orientation (potentially on the order of seconds) can result in changes in contrast. The present paper describes such an in situ straining experiment of a nanocrystalline copper foil carried out at Los Alamos National Laboratory. It must be kept in mind that the deformation behavior observed in thin foils is not necessarily the same as that in the bulk material.
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SAMPLE PREPARATION Samples in this study were compacted from powders made via inert gas condensation [3,4] using a resistive ev
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