We present direct observations of room temperature dislocation plasticity in single crystalline silicon. Previous studies have shown that phase transformation and fracture are the relevant mechanisms of deformation in silicon. In contrast, using in-situ nanoindentation in a transmission electron microscope we find dislocation nucleation and metal-like flow. The results of finite-element modelling suggest that the presence of free surfaces in our unique sample geometry leads to preferential surface nucleation of dislocations and the suppression of phase transformation.Over fifty years of research on dislocation behaviour in silicon supports the conclusion that dislocations do not move during conventional mechanical testing at temperatures below 450 C [1,2]. Under extreme conditions such as indentation loading, where localized stresses can approach the theoretical shear strength of the material, dislocation structures have been observed in silicon. Traditionally, these are thought to result from either block slip [3,4] or phase transformations [5][6][7][8], rather than the nucleation and propagation mechanisms associated with conventional dislocation plasticity. However, recent results have shown evidence of room temperature dislocation plasticity in silicon in the absence of phase transformations through post mortem transmission electron microscopy (TEM) of shallow indentations [9]. Here we present the first in-situ observations of dislocation formation and propagation in silicon at room temperature.While indentation-induced dislocation nucleation has been widely documented for metallic systems [10][11][12], the high Peierl's barrier associated with covalently bonded materials tends to suppress dislocation activity. Prior research suggests that the indentation of silicon causes plastic deformation through a series of phase transformations [13]. During conventional indentation loading, silicon transforms
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].
The articles in this issue of MRS Bulletin provide a sample of what is novel and unique in the field of in situ transmission electron microscopy (TEM). The advent of improved cameras and continued developments in electron optics and stage designs have enabled scientists and engineers to enhance the capabilities of previous TEM analyses. Currently, novel in situ experiments observe and record the behavior of materials in various heating, cooling, straining, or growth environments. In situ TEM techniques are invaluable for understanding and characterizing dynamic microstructural changes. They can validate static TEM experiments and inspire new experimental approaches and new theories.
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