Dislocations around indents

The plastic deformation patterns can be revealed by etch-pit and/or X-ray scattering studies of indentations in crystals. These show that the deformation around indentations (in crystals) consists of heterogeneous rosettes which are qualitatively different from the homogeneous deformation fields expected from the deformation of a continuum (Chaudhri, 2004). This is, of course, because plastic deformation itself is (a) an atomically heterogeneous process mediated by the motion of dislocations and (b) mesoscopically heterogeneous because dislocation motion occurs in bands of plastic shear (Figure 2.2). In other words, plastic deformation is discontinuous at not one, but two, levels of the states of aggregation in solids. It is by no means continuous. And, it is by no means time independent it is a flow process. 

Silicon as whisker material is entering the area of engineering application, but the dominant area of interest is the use of silicon and doped silicon as semiconductor materials. The use of hardness measurement as a probe of the properties of specially prepared silicon crystals for semiconductor device use has been attempted several times but as some of the data presented below show, doping at the levels used in the device industry does not show up as change in hardness even through etch-pit rosette studies in the area around indentations show some dependence of dislocation movement on concentration and type of dopant. 

Glide bands are observed around hardness indentations in lysozyme so dislocations (with large displacements) are associated with its deformation. 

Gragert and Meyer (Fig. 6.2.1) and Boyarskaya (Fig. 6.2.2) by observation of surface deformations induced by indentation with a tungsten carbide ball and by scratch. The observations were carried out using secondary electron beam and in cathodoluminescence. They demonstrated on MgO and LiF crystals the occurrence of cracks around the impression of the ball similar to those induced by a Vickers indenter, and also the occurrence of a concentration of screw and edge dislocations in the area of the cracks. 

Extensive TEM studies by Page et al. [65] delivered all previous low-temperature electron microscopy results to the consistent view that (i) silicon becomes amorphous in response to the high contact stresses under a hardness indenter and (ii) limited dislocation arrays are generated around the deformed volume at contact loads exceeding some threshold value. The authors also argued that the dislocation arrays might occur as a means of accommodating the displacements from the densification transformation, rather than as a primary response to the indenter intrusion. 

As a first approximation, we follow Yoffe s approach to resolve the stress configuration in the SiC ceramics directly underneath and around the indenter during indentation [13]. Here, under an elastic/plastic deformation condition, a plastic zone is produced and restricted to an area directly beneath the contact interface of the indenter and the ceramic surface. As shown in Fig. 4, this hemispherical region of plastically deformed material is equal to a pressurised blister of uniform compression with a radius, a, equal to, d, the radius of the indentation impression. Appropriately, the a term is associated with the density of dislocations which generate within the plastic zone during loading. 

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