Subgrain formation

The creep strength of AljNb is comparatively low - a stress of 10 MN/m produces 1 % strain in only 500 h and fracture in 2300 h - whereas the yield stress compares favorably with the superalloys. This illustrates the fact that the difference between the yield stress and the creep strength is much more pronounced for intermetallics than for conventional alloys. Creep of Al3Nb is controlled by dislocation climb which is accompanied by subgrain formation. The observed creep behavior corresponds to that of conventional disordered alloys and the creep rates are described by the known constitutive equations. This will be discussed in more detail with respect to NiAl (Sec. 4.3). The secondary creep rate follows the power law, i.e. Dorn equation for dislocation creep... 

The subgrains formation is related to high plastic deformations ratio and high density of dislocation lines (visible in the lower part of the Figure 5) formed in the process. 

Experimental studies of alkali feldspars in Westerley granite with ordered microcline [63,64] and disordered sanidine [65-68] show that (010) [001] shp is active in both. Other dislocations derive from (010)[101] [Fig. 15(a)], (001) [110], and (121) [101] shp. In naturaUy deformed alkali feldspars, subgrain formation was observed, indicative of climb [357-360]. It has been suggested that shear-induced mechanical Albite and Pericline twinning in potassium feldspar may facilitate ordering [361] but this has been disputed [362]. Dislocations have no effect on diffusion in alkah feldspar [363]. 

In the Fe-rich phases and in FeAl, however, no subgrain formation has been observed even after long creep times. The dislocation density remains high (about 10 cm, and the stress exponent varies between 3 and 3.6. This indicates class I behaviour, i.e. here the creep is controUed by the viscous glide of dislocations. In both cases only <100) dislocations have been observed. Obviously the driving force and the atomic mobility that are necessary for subgrain formation are sufficient only in the Ni-rich phases. 

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... 

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. 

The well-formed subgrains are sources, emitters of mobile dislocations, which contribute to strain. Emission of mobile dislocations from sub-boimdaries leads to formation of jogs in them. It is the dislocation sub-boundary that generates jogs in mobile dislocations. The screw components of emitted dislocations keep their origin in their memory. They contain the equidistant one-signed jogs. 

Another (however not so common) mechanism of subgrain development can be observed in the rocks of the gypsum cap-rock - the process is called kinking and leads to formation of kink bands (Means Ree, 1988 fide Passchier Trouw, 1998), which are represented by narrow accumulation of kink folds see fig. 25. They are formed in brittle-ductile system and... 

Larger etch pit densities of VCo.88 than of VCo,s3 form the subgrain boundaries characterized by the presence of substructure such as antiphase boundaries due to the formation of an ordered compound (150). The hardness of NbC decreases with carbon content and the hardness anisotropy of NbCo.8 is less pronounced than that of NbCo.9 (Fig. 11), which would be due to (a) deviation from stoichiometry of the crystal and (b) ordering of carbon vacancies. A high-resolution electron microscopy (HRFM) study gives very detailed information about defect order... 

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