Nano-indentation

As a powerful tool to analyse microstructural evolution under irradiation and the interaction of moving dislocations with radiation-induced defects, a focused ion beam (FIB) method was applied to ion-irradiated SiC followed by the nano-indentation test. An FIB method has excellent capability to prepare thin foils from the area of interest with a high accuracy of location and wide flexibility in the sampling direction.

....Where effects of ion irradiation on mechanical properties are important issues, indentation techniques have been utilized in many aspects. The application of a focused ion beam (FIB) to the microstructural analysis of indented samples has been reported and has provided information on the behaviour of cracks and dislocations underneath an indentation.

...The incident ions penetrated from the surface to a depth of 2.5 μ m. TRIM codes have been used to estimate the depth profile of damage rate in a thin foil... The amount of damage increases with the depth from the surface and has a peak near the bottom of the damaged range. The damage rate, averaged from the surface to the bottom of the damaged range, was defined as the ‘damage rate’ for the investigation of mechanical-property changes using an indentation test. The reason for this is that the stress field of an indentation extends out of the damaged range, thus, the mechanical-property changes should be correlated with the effects of the whole damages in the damaged range.

...The microstructures were probed under a nano-indentation in unirradiated and irradiated SiC. In the unirradiated SiC, the deformation area extends up to 2000 nm depth and the dislocation networks are formed in the deformed area. In the ion-irradiated specimen, black-dot defects are observed instead of the dislocation networks in the deformed area. The deformation area in the irradiated SiC is apparently smaller than that in the unirradiated SiC.

...The depth of deformation range is <1000 nm and the impression of the nano-indenter is shallower. These results show definitely that the irradiation hardening occurs in the irradiated SiC.

Microstructural analysis of the deformation range under nano-indentation in b-SiC, 2004; www.oxfordjournals.org






....The result of the TEM observation revealed that point defects were introduced to the sample finally milled at 40 kV but not at 10 kV. However, crystal lattice images and electron diffraction patterns were clearly observed on both the samples. The typical influence of the FIB energy was indicated in the elemental analysis.

.....Point defects are observed at a considerably high density in the sample finally milled at 40 kV; however, no defects are observed in the sample finally milled at 10 kV. The electron diffraction patterns of the single crystal grains of the Mg 9wt% Al alloy samples were also observed to check if the incidental crystallographic defect occurred during the final milling.


Evaluation of TEM samples of an Mg-Al alloy prepared using FIB milling at the operating voltages of 10 kV and 40 kV; www.oxfordjournals.org




Dislocations in Complex Materials


Deformation of metals and alloys by dislocations gliding between well-separated slip planes is a well-understood process, but most crystal structures do not possess such simple geometric arrangements. Examples are the Laves phases, the most common class of intermetallic compounds and exist with ordered cubic, hexagonal, and rhombohedral structures. These compounds are usually brittle at low temperatures, and transformation from one structure to another is slow. On the basis of geometric and energetic considerations, a dislocation-based mechanism consisting of two shears in different directions on adjacent atomic planes has been used to explain both deformation and phase transformations in this class of materials. We report direct observations made by Z-contrast atomic resolution microscopy of stacking faults and dislocation cores in the Laves phase Cr2Hf. These results show that this complex dislocation scheme does indeed operate in this material. Knowledge gained of the dislocation core structure will enable improved understanding of deformation mechanisms and phase transformation kinetics in this and other complex structures.


.......plastic deformation in a crystal occurs by the processes of slip and twinning. These processes are accomplished by the motion of dislocations whose character is closely related to the structure of the crystal. When these dislocations produce displacements that are less than a unit lattice translation vector in the crystal, they are called partial dislocations and they bound stacking faults. The motion of a partial dislocation can also produce certain types of phase transformations. The partial dislocation associated with the sliding of close-packed planes of atoms over each other during slip, twinning, or shear transformations in face-centered cubic (fcc) metals is the Shockley partial. In slip, a pair of partial dislocations bounding a stacking fault moves on the slip plane in response to an applied stress to produce plastic deformation. In twinning, a Shockley dislocation sweeps every slip plane, whereas when a Shockley dislocation sweeps alternate slip planes it converts an fcc structure into a hexagonal close-packed (hcp) structure. Observations on Laves phases show that analogous mechanisms could operate in these more complex structures.



http://www.sciencemag.org/cgi/content/full/307/5710/701