Determination of Debye-Waller factor and structure factors for Si by quantitative convergent-beam electron diffraction using off-axis multi-beam orientations.

Debye-Waller (DW) factors and structure factors have been measured for Si using convergent-beam electron diffraction (CBED) experiments with a transmission electron microscope equipped with a field-emission gun and a post-column energy-filtering device. Si has been used here to evaluate the accuracy of multi-beam near-zone-axis orientations for the simultaneous refinement of DW factors and multiple structure factors. Strong dynamic interactions among different beams are obtained by tilting the crystal to specific four- or six-beam orientations near major zone axes, which provide sufficient sensitivity to determine accurate DW factors and structure factors. The DW factors of Si were measured using four-beam conditions near the [001] zone axis for temperatures ranging from 96 to 300 K. A comparison of the multi-beam near-zone-axis orientations with other CBED methods for DW and structure factor F(g) refinement is presented.

Simultaneous determination of highly precise Debye-Waller factors and structure factors for chemically ordered NiAl.

Accurate Debye-Waller (DW) factors and several low-index structure factors of chemically ordered ß-NiAl at different temperatures have been measured using an off-zone-axis multi-beam convergent-beam electron diffraction method. The temperature dependences of DW factors of Ni and Al atoms are compared with previous experimental measurements and theoretical calculations. The temperature below which the DW factor of Ni becomes smaller than that of Al was found to be lower than previously reported. Structure factors are determined with an accuracy of 0.05% and compared with prior reports.

Inter- and intra-agglomerate fracture in nanocrystalline nickel.

In situ tensile straining transmission electron microscopy tests have been carried out on nanocrystalline Ni. Grain agglomerates (GAs) were found to form very frequently and rapidly ahead of an advancing crack with sizes much larger than the initial average grain size. High-resolution electron microscopy indicated that the GAs most probably consist of nanograins separated by low-angle grain boundaries. Furthermore, both inter- and intra-GA fractures were observed. The observations suggest that these newly formed GAs may play an important role in the formation of the dimpled fracture surfaces of nanocrystalline materials.

Dislocation dynamics in nanocrystalline nickel.

It is believed that the dynamics of dislocation processes during the deformation of nanocrystalline materials can only be visualized by computational simulations. Here we demonstrate that observations of dislocation processes during the deformation of nanocrystalline Ni with grain sizes as small as 10 nm can be achieved by using a combination of in situ tensile straining and high-resolution transmission electron microscopy. Trapped unit lattice dislocations are observed in strained grains as small as 5 nm, but subsequent relaxation leads to dislocation recombination.

Grain boundary-mediated plasticity in nanocrystalline nickel.

The plastic behavior of crystalline materials is mainly controlled by the nucleation and motion of lattice dislocations. We report in situ dynamic transmission electron microscope observations of nanocrystalline nickel films with an average grain size of about 10 nanometers, which show that grain boundary-mediated processes have become a prominent deformation mode. Additionally, trapped lattice dislocations are observed in individual grains following deformation. This change in the deformation mode arises from the grain size-dependent competition between the deformation controlled by nucleation and motion of dislocations and the deformation controlled by diffusion-assisted grain boundary processes.