Subgrains

The annealing curves for several copper alloys are compared to copper in Figure 4. Solute additions affect the annealing response, as they do for cold working, by interaction with microplastic processes that account for softening as dislocations and subgrains are eliminated during the anneal. 

To answer questions regarding dislocation multiplication in Mg-doped LiF single crystals, Vorthman and Duvall [19] describe soft-recovery experiments on <100)-oriented crystals shock loaded above the critical shear stress necessary for rapid precursor decay. Postshock analysis of the samples indicate that the dislocation density in recovered samples is not significantly greater than the preshock value. The predicted dislocation density (using precursor-decay analysis) is not observed. It is found, however, that the critical shear stress, above which the precursor amplitude decays rapidly, corresponds to the shear stress required to disturb grown-in dislocations which make up subgrain boundaries. 

Meir and Clifton [12] study shocked <100) LiF (high purity) with peak longitudinal stress amplitudes 0.5 GPa. A series of experiments is reported in which surface damage is gradually eliminated. They find that, while at low-impact velocities the dislocations in subgrain boundaries are immobile and do not affect the dislocation concentration in their vicinity, at high-impact velocities ( 0.1 km/s) dislocations emitted from subgrain boundaries appear to account for most of the mobile dislocations. 

Introduction of the surface-nucleation mechanism in numerical computation of elastic-plastic wave evolution leads to enhanced precursor attenuation in thin specimens, but not in thicker ones. Inclusion of dislocation nucleation at subgrain boundaries indicates that a relatively low concentration of subgrain boundaries ( 2/mm) and nucleation density (10"-10 m ) is sufficient to obtain predicted precursor decay rates which are comparable to those obtained from the experiments. These experiments are only slightly above the threshold necessary to produce enhanced elastic-precursor decay. 

G. Meir and R.J. Clifton, Effects of Dislocation Generation at Surfaces and Subgrain Boundaries in Precursor Decay in High-Purity LiF, J. Appl. Phys. 59, 124-148 (1986). 

All real surfaces will contain defects of some kind. A crystalline surface must at the very least contain vacancies. In addition, atomic steps, facets, strain, and crystalline subgrain boundaries all can be present, and each will limit the long-range order on the surface. In practice, it is quite difficult to prepare an atomically flat surface. 

Film rearrangement resulting in the formation of oxide subgrain and grain boundaries these paths of easy ion migration promote the formation of oxide islands and result in an increase in the growth rate of the oxide. 

The formation of pores appears to start along the sub-grain boundaries of the metal, followed by the development of additional pores within the subgrains. Growth of oxide continues on a series of hemispherical fronts centred on the pore bases, provided that the effective barrier-layer thickness between the metal surface and the electrolyte within the pores, represented by the hemisphere radius, is less than 1-4 nm/V. As anodic oxidation proceeds at... 

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)...

Extended defects interrupt the continuity of the crystal, generating crystal subgrains whose dimensions depend, in a complex fashion, on the density of extended defects per unit area. Table 4.1 gives examples of reported dislocation densities and subgrain dimensions in olivine crystals from the San Carlos perido-tite nodules (Australia). Assuming a mean dislocation density within 1.2 X 10 and 6 X 10 cm , Kirby and Wegner (1978) deduced that a directional strain pressure of 35 to 75 bar acted on the crystals prior to their transport to the surface by the enclosing lavas. 

Table 4J Dislocation densities (cm and mean subgrain dimensions (cm) in San Carlos olivines (from Kirby and Wegner, 1978). n.d. = not determined.

Silicon carbide grains are known to contain subgrains of titanium carbide. Equilibrium thermodynamics predicts that titanium carbide will condense before silicon carbide (Fig. 5.13). The titanium carbide grains were apparently accreted by the growing silicon carbide grains and were enclosed as the silicon carbide grains continued to grow. 

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. 

Figure 2.13. Schematic drawing of subgrain magnetic domains. Each of the three domains shown has a different net magnetic moment.

Etch-pit formation techniques have been extensively developed since the first observations on A1 by Lacombe and Beaujard (6) and on semiconductors (Ge)by Vogel et al (7). Besides their seemingly random distribution etch pits are frequently aligned on intragranular boundaries of subgrain boundaries, which are the boundaries of polygonization. 

A comparison between the linear density of the etch pits in these polygonization boundaries and the degree of misorienta-tion of two adjacent subgrains has permitted the unambiguous... 

Fig. 14. Twins produced by plastic deformation and broadened by annealing at 650°C in an imperfect single crystal of a - U grown by progressive phase-change from j3 to a. The bands with different shades correspond to the recovery of the subgrains of the initial crystal. Electroetching reveals small disorientations between the subgrains X150. (D. Calais, P. Lacombe and Mme Simenel, Rev, met., 56, 261 (1959)).

Dislocations of the same Burgers vector climb into stable arrays of parallel dislocations, forming low-angle boundaries between subgrains of relatively low dislocation density. 

The most obvious microstructural characteristics of recovery are probably subgrain boundaries consisting of arrays of parallel dislocations or dislocation networks. 

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