In-Situ TEM Studies of the Deformation Mechanism of Nanocrystaline Ni

Abstract

Nanocrystalline metals exhibit remarkable mechanical properties relative to their coarsegrained counterparts, including high strength, high hardness and an enhanced ability to deform superplastically. Despite this fact, an understanding of the physical mechanisms responsible for these basic properties remains elusive. Based on abundant indirect evidence and physical insight from molecular dynamics simulations, it has been largely accepted that with the decrease of grain size, a crossover regime exists, where the deformation mechanism of nanocrystalline materials will change continuously from dislocation nucleation and motion to grain boundary (GB) mediated plasticity, i.e. from an intragranular process to an intergranular one. However, to date no conclusive direct experimental evidence for the operation of non-dislocation based plasticity mechanism has been detected.Here we show by solving the challenging problems encountered in previous studies, in situ dynamics dark-field transmission electron microscopy (DFTEM) has been done successfully which reveal conclusive experimental evidence that GB mediated plasticity, such as grain boundary sliding and grain rotation, become a prominent deformation mode for as-deposited Ni with an average grain size of about 10 nm [ Fig. 1]. Moreover, no deformation twins were found in grains that are still in a strained state. Theoretical analysis suggested that the deformation mechanism crossover resulted from the competition between the deformation controlled by nucleation and motion of dislocations and the deformation controlled by GB related deformation accommodated mainly by GB diffusion with decreasing grain size [1, 2] We have also used nano-beam electron diffraction in the TEM to study the behavior of individual grains in nanocrystalline Ni during deformation under low local strain rate conditions. Direct measurement of lattice distortions during straining reveals that grain interiors may experience ultra-high elastic distortions during tensile deformation. These observations allow us to critically evaluate two highly cited models, which predict a critical grain size below which dislocation pile up or multiplication cease operation. We find that only the dislocation multiplication model can reasonably predict this critical grain size. Based on our application on dislocation multiplication model, we find the critical grain size in nickel to lie between 3.4~10.5 nm, depending on the type of dislocation [3]. This agrees with our observations surprisingly well [4].

Effects of solute Mg on grain boundary and dislocation dynamics during nanoindentation of Al–Mg thin films

“…However, because of the high activation energy required for grain boundary mobility, it is not commonly considered an important deformation mechanism at room temperature. The occurrence of grain boundary motion in room temperature deformation of nanocrystalline fcc metals was anticipated recently by molecular dynamics simulations [9] and a simple bubble raft model [10], and confirmed by in situ observations of grain coarsening during indentation of nanocrystalline Al thin films [11] and grain rotation during straining of nanocrystalline Ni thin films [12]. The fact that in situ nanoindentation experiments also showed grain boundary motion at much larger grain sizes (on the order of 0.1 lm) is attributed to the highly inhomogeneous stress induced by the indenter.…”

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.…”

Partial-mediated slips in nanocrystalline Ni at high strain rate

“…© 2007 American Institute of Physics. ͓DOI: 10.1063/1.2745250͔To design nanocrystalline ͑nc͒ materials for superior mechanical properties, it is critical to understand their deformation physics.1 Face-centered-cubic ͑fcc͒ nc metals have been reported to deform by partial dislocation emission from grain boundaries ͑GBs͒, 2-10 full dislocation, 3,5,7 GB sliding,3,[11][12][13] grain rotation, 11,14 and deformation twinning. 2,3,[6][7][8][9][13][14][15][16][17][18][19][20] Significantly, recent molecular dynamics ͑MD͒ simulations ͑Ref.…”