Strong Strain Hardening in Nanocrystalline Nickel

Abstract

Low strain hardening has hitherto been considered an intrinsic behavior for most nanocrystalline (NC) metals, due to their perceived inability to accumulate dislocations. In this Letter, we show strong strain hardening in NC nickel with a grain size of $20 nm under large plastic strains. Contrary to common belief, we have observed significant dislocation accumulation in the grain interior. This is enabled primarily by Lomer-Cottrell locks, which pin the lock-forming dislocations and obstruct dislocation motion. These observations may help with developing strong and ductile NC metals and alloys.

Dislocation–twin interactions in nanocrystalline fcc metals

“…7a. Such a dislocation structure has been observed both experimentally [3,51] and by MD simulation [9,33,37], and it has been assumed to be very effective in blocking other dislocations and, consequently, causing strain hardening [9].…”

“…This could produce a scenario in which the leading partial cross-slipped into the twin boundary plane or was transmitted across the twin boundary, while the trailing partial remains on the original slip plane in the matrix. Such a dislocation structure is believed to be effective in improving the work hardening rate [9].…”

“…Twins have been reported to significantly affect the mechanical properties of nanostructured face-centered cubic (fcc) metals and alloys [1][2][3][4][5][6][7][8][9][10]. Importantly, twins have been shown to be able to simultaneously increase the strength and ductility of nanostructured metals, which is attributed to the dislocation interaction with and accumulation at twin boundaries.…”

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

“…It is interesting to note that the grain size of the NC sample investigated in Ref. [22] was increased from 25 nm to 38 nm while the strain was increased from 0 to 1, which suggests that shearcoupled migration of grain boundaries might have played an important role in enhancing dislocation emission and hence achieving the strong strain hardening characteristic in the sample. This athermal process [20] normally consists of a normal GB migration accompanied by a tangential translation of grains parallel to the GB plane for both low-angle and high-angle boundaries; the process would then produce shear deformation of the lattice traversed by the migrated GB.…”

“…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.…”