Deforming Nanocrystalline Metals: New Insights, New Puzzles

Abstract

Recent experiments have brought new insights into the mechanisms which govern the plasticity of nanocrystalline metals. In particular, new opportunities have arisen from the finding that bulk nanocrystalline samples with extremely small grain size, prepared by the inert gas condensation technique, can be deformed to large true strain. The findings elucidate the roles of creep, partial dislocation activity along with its consequences, faulting and twinning, as well as grain boundary sliding and grain rotation. However, they also rise intriguing new questions, specifically with respect to the mechanisms of dislocation nucleation at grain boundaries, and with respect to slip system selection and alignment in twinned grains. An emerging insight is that there is not ‘the’ deformation mechanism at small grain size; instead, deformation mechanism maps in, for instance, the parameter space spanned by the strain rate and the grain size, are more appropriate representations of the various processes that control the materials behavior.

Detecting grain rotation at the nanoscale

“…The grain rotation of nickel media is likely to account for the texture loss. It is known that dislocation glide on preferred slip systems gives rise to crystallographic texture whereas grain rotation or GB sliding alone randomizes the grain orientation distribution (6,28). It is found that grain rotation is driven by the change of GB energy (28).…”

“…The plastic deformation of nanomaterials has attracted much interest in recent years (6)(7)(8)(9)(10)(11)(12), but many controversies still exist (6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17). Various mechanisms have been reported (8,(11)(12)(13)(14)(15)(16)(17)(18).…”

The Suppression of Instabilities via Biphase Interfaces During Bulk Fabrication of Nanograined Zr

“…Zr's strong texture indicates that interface or grain boundary-mediated processes, which tend to randomize the texture, are not active. [57] More importantly, the textures are remarkably similar (apart from the usual modest variations) over the wide range of h for each phase. Abrupt transitions in texture would have significant changes in the predominant deformation mechanism when h crosses from the micron to nanoscale, such as twinning, partial dislocation-mediated slip, or interfacial sliding.…”

“…[54,55] Compared with other texture measurement techniques, NeD enables statistically relevant data without potential modifications due to surface preparation. Texture measurements also provide a robust indicator of the primary deformation mechanisms, such as when interface-driven plasticity, [28,56] grain boundarymediated processes, [57] deformation twinning, [58][59][60] and partial-mediated slip [61] prevail. Figure 3 shows examples of textures at the micron scale and nanoscale in the form of pole figures, a compact way to present texture, as well as the standard texture presentation for Zr (albeit not for Nb).…”

Work softening and annealing hardening of deformed nanocrystalline nickel

“…[1][2][3][4][5][6][7] It has been found that in face-centered cubic ͑fcc͒ nc Ni, mechanisms such as deformation twinning, formation of extended and full dislocations from the grain boundaries ͑GBs͒, GB sliding, and grain rotation can contribute to plasticity. 6,[8][9][10][11][12][13][14][15][16] Recently, we have reported that for nc Ni, a phase transformation from fcc ͑␥͒ to body-centered cubic ͑bcc, ␣͒ can provide another deformation mechanism to realize plastic deformation to relatively large plastic strain during room-temperature rolling, 17 i.e., a far-fromequilibrium microstructure such as nc Ni can accommodate plastic strain via a change in lattice structure upon mechanical loading. In this letter we report that such a strain-induced phase transformation from ␥ to ␣ leads to work softening during cold-rolling and that subsequent annealing at 423 K results in hardening.…”