Subgrain structure

Figure 3 TEM micrographs showing subgrain structure (left) and high density of precipitates (right) in the extruded and T6 tempered SSA018 alloy.

Figure 3. TEM microstructure of the as-received Zr modified 2014 aluminum alloy revealing the very fine subgrain structure and precipitates dispersed within the grains.

The response of a Zr stabilized 2014 aluminum alloy to cyclic loading was studied in the present work. The cycles to failure at different stress amplitude levels were compared with those obtained for the unstabilized alloy. The TEM observation of the modified material showed a very fine subgrain structure characterized by the presence of very fine Al3Zr within the grains, the particles play the role of stabilize the structure, inhibit the recrystallization and increase the strength of the alloy in all the tested conditions. Fracture observations were performed by using a scanning... 

In comparison to skeletal nickel, skeletal copper has a significantly larger crystallite size of about 10-100 nm [32,46,92,96,100,101], Fasman and coworkers [46,100,101] examined the crystal structure more closely and found that it consisted of copper crystals that had agglomerated into granules or precipitated onto oxides. The copper crystal grains and subgrains were of about 10-13 nm in size, while the copper agglomerates were 50-80 nm. 

Fig. 4.9 Stacking of hexagonal basal planes in graphite left). Mechanical activation results in a structure with prohferation of faults in the plane stacking right). The change of stacking sequences in nearby subgrains is marked by arrows. The stacking fault disorder is corroborated from the nonuniform peak broadening or absence of hk indexed peaks in XRD pattern (see Sect. 1.3.3.3)...

As early as 1829, the observation of grain boundaries was reported. But it was more than one hundred years later that the structure of dislocations in crystals was understood. Early ideas on strain-figures that move in elastic bodies date back to the turn of this century. Although the mathematical theory of dislocations in an elastic continuum was summarized by [V. Volterra (1907)], it did not really influence the theory of crystal plasticity. X-ray intensity measurements [C.G. Darwin (1914)] with single crystals indicated their mosaic structure (j.e., subgrain boundaries) formed by dislocation arrays. Prandtl, Masing, and Polanyi, and in particular [U. Dehlinger (1929)] came close to the modern concept of line imperfections, which can move in a crystal lattice and induce plastic deformation. 

Island growth also occurs with polycrystalline films, but in epitaxy, the islands combine to form a continuous single-crystal film, that is, one with no grain boundaries. In reality, nucleation is much more complex in the case of heteroepitaxy. Nucleation errors may result in relatively large areas, or domains, with different crystallographic orientations. The interfaces between domains are regions of structural mismatch called subgrain boundaries and will be visible in the microstructure. 

A polygonized crystal such as this may be regarded as an extreme example of the mosaic structure depicted in Fig. 3-16, extreme in the sense that the average angle between adjacent blocks (subgrains) of the polygonized crystal is much larger than normal. 

Electron microscopy analysis was conducted using carbon replicas and thin foils. The carbon replicas were not of help for quantitative evaluation. Transmission electron microscopy of thin foils offered better results. For all the tested carbon combinations from the A to I labels, thin foils were produced for the heat treatment 450°C/30 h. The A14C3 particle size and the subgrain size were measured using the thin foils. The dispersed phase A14C3 particle size was measured on 200 to 300 thin foil structures, and it was constant and as small as 30 nm. The particle size was influenced neither by the carbon type nor by the heat treatment technology applied. 

A more obvious but perhaps underappreciated problem with surface roughness is the existence of defect sites on a surface, i.e., sites that would not be exposed on a perfectly smooth surface. This type of defect is separate from classical defects like stacking faults, subgrain boundaries and dislocations, and is due just to non-uniform expression of the substrate structure in an uneven surface (Fig. 9) such as could occur with the local development of vicinal faces. As surface characterization methods are generally poor except in the case of a small suite of oxides and silicates, this effect has probably not been fully considered to date. For example, it is possible to imagine a low roughness (hkl) surface that is entirely terminated by small faces with other (hkl) orientations, so that the exposed surface functional groups differ both in density and orientation from what is expected. 

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