Ultrahigh strength and high ductility of bulk nanocrystalline copper

Abstract

We have synthesized artifact-free bulk nanocrystalline copper samples with a narrow grain size distribution (mean grain size of 23nm) that exhibited tensile yield strength about 11 times higher than that of conventional coarse-grained copper, while retaining a 14% uniform tensile elongation. In situ dynamic straining transmission electron microscope observations of the nanocrystalline copper are also reported, which showed individual dislocation motion and dislocation pile-ups. This suggests a dislocation-controlled deformation mechanism that allows for the high strain hardening observed. Trapped dislocations are observed in the individual nanograins.

“Bulk” Nanocrystalline Metals: Review of the Current State of the Art and Future Opportunities for Copper and Copper Alloys

“…[115][116][117][118] At the smallest grain sizes, grain boundary sliding, grain rotation, and grain coalescence become the predominant modes of deformation, again shifting the deformation mechanism and, in some cases, contributing to the so-called inverse Hall-Petch behavior. The maximum magnitude of yield strength can approach or exceed an order of magnitude increase over conventional coarse-grained materials 26,119 and is typically reported in materials with grain sizes of <20 nm.…”

“…In the past, quite frequently porosity from incomplete densification was still an issue with this technique. However, within the last decade, there are increasingly more examples and strategies for obtaining bulk, fully densified nanocrystalline materials produced via ball milling; 133 subsequent consolidation demonstrates that these materials can possess good ductilities while retaining much of the strength of nanocrystalline materials 119,122,134 (i.e., 14% tensile ductility or potentially higher in Refs. 119,122).…”

“…202,203 In many cases, it was found that the grain boundary properties are not simply governed by the crystallographic orientations of the adjacent grains (e.g., see Ref. 119) nor the average properties of the boundary (e.g., energy, excess volume, etc. ), but that the grain boundary structural units, the stress state, 204 and even the free volume within the boundary 112,113,205 plays an important role in the deformation characteristics.…”

Strong Strain Hardening in Nanocrystalline Nickel

“…On the other hand, there are pieces of indirect evidence that limited dislocation accumulation might be possible in NC grains [12][13][14][15]. For example, dislocations have been observed in grains as small as 20 nm [14][15][16][17].…”

Enhancing dislocation emission in nanocrystalline materials through shear-coupled migration of grain boundaries

“…Meanwhile, various NC specimens that possessed both high strength and ductility have been reported [8][9][10][11][12][13][14][15][16][17][18][19]; moreover, it has been proposed that the outstanding balance was due to some grain boundaries (GBs)-mediated deformation mechanisms, such as GB sliding, emission of dislocations from GBs and GB migration [16][17][18][19][20]. Furthermore, enormous dislocation activities were discovered in the grain interiors of some NC samples in experiments [10,[21][22][23] and the accumulation of such dislocations in the grain interior due to the formation of Lomer-Cottrell locks [22] led to their exceptional strain hardening and ductility. However, what remains unclear is the mechanism that leads to the emission of abundant dislocations, which form Lomer-Cottrell locks and, hence, developing a good synergy of high strength and ductility in these NC materials.…”