Mechanisms of plastic deformation

Although at room temperatine most ceramic materials suffer fractine before the onset of plastic deformation, a brief exploration into the possible mechanisms is worthwhile. Plastic deformation is different for crystalline and noncrystalline ceramics however, each is discussed. 

For crystalline ceramics, plastic deformation occurs, as with metals, by the motion of dislocations (Chapter 7). One reason for the hardness and brittleness of these materials is the difficulty of slip (or dislocation motion). For crystalline ceramic materials for which 

However, for ceramics in which the bonding is highly covalent, slip is also difficult, and they are brittle for the following reasons (1) the covalent bonds are relatively strong, (2) there are also limited numbers of slip systems, and (3) dislocation structures are complex. 

The units for viscosity are poise (P) and paseal-seconds (Pa s) 1 P = 1 dyne-s/em and 1 Pa s = 1 N s/m. Conversion from one system of units to the other is according to 

FigMte 12.32 Representation of the viscous flow of a liquid or fluid glass in response to an apphed shear force. 

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Later, after the War, Tammann moved further towards physics by becoming interested in the mechanism of plastic deformation and the repair of deformed metals by the process of recrystallisation (following in the footsteps of Ewing and Rosenhain in Cambridge at the turn of the century), paving the way for the very extensive studies of these topics that followed soon after. Tammann thus followed... 

Information on molecular orientation can be useful in two primary ways. First, it is possible to use the orientation functions or averages to gain an understanding of the mechanisms of plastic deformation. Secondly the orientation averages can provide a basis for understanding the influence of molecular orientation on physical properties, especially mechanical properties. 

Microscopic mechanisms of plastic deformation are far too complex to be described in detail. Many attempts have been made, but they have all had a variety of shortcomings. Part of the problem is that several important deformation mechanisms involve atomic interactions which interact with one another, so not only must the interactions be described by means of quantum mechanics, but also ordinary statistical mechanics cannot be applied. Therefore, a very rough statistical approach must suffice. 

G. I. Taylor, Mechanism of Plastic Deformation of Crystals. I. Theoretical, Proc. Roy. 

The plastic deformation in several amine and anhydride cured epoxy resins has been studied. The experimental results have been reasonably interpreted by the Argon theory. The molecular parameters determined from the data based on the theory reflect the different molecular structures of the resins studied. However, these parameters are in similar enough range to also show the structural similarity in these DGEBA based systems. In general, the mechanisms of plastic deformation in epoxy resins below T are essentially identical to those in amorphouE thermoplastics. The yield stress level being related to the modulus that controls the intermolecular energy due to molecular deformation will, however, be affected by the crosslinks in the thermosets. 

Polanyi, My time with x-rays and crystals, in Fifty Years of X-Ray Diffraction, 636. G.I. Taylor, The mechanism of plastic deformation of crystals, Proceedings of the Royal Society A145 (1934) 362-415 E. Orowan, "Zur Kristallplastizitat, Zeitschriftfur Physik 89 (1934) 605-659. On recent studies of dislocation, see R. F. Service, Materials scientists view hot wires and bends by the bay, Science 272 (1996) 484-485. 

G. I. Taylor, The Mechanism of Plastic Deformation of Crystals. Part I. Theoretical, Proceedings of the Royal Society of London Series A145 vol. 855 (1934), pp. 362-387. ... 

The primary mechanism of plastic deformation in ductile materials. Slip involves chemical bond breaking and reformation. 

The results again support the point of view that the mechanism of plastic deformation of densely crosslinked networks in the glassy state is rather close to that of linear polymers and that the presence of crosslinks does not basically affect the yield behaviour of the polymer. The localized character of plasticity of networks (small Va) reflects the very local character of nucleation and the growth of plastic nuclei68,71,7S). 

Plasticity of crystals, especially in quasiliquid phase, is connected with the action of different microscopic mechanisms of plastic deformation. Comparative role of each of these mechanisms is determined by the external conditions temperature, load, deformation velocity. The atomic layers of crystal move from the surface where the compressing forces act to the area where these forces are weaker or where the stretching forces act. 

Nieminen et al. [152] observed a different mechanism of plastic deformation in essentially the same geometry, but at higher velocities (100 m/s versus 5 m/s) and with a different model for the potential between Cu atoms. Sliding took place between (100) layers inside the tip. This led to a reduction of the tip by two layers that was described as the climb of two successive edge dislocations under the action of the compressive load. Although wear covered more of the surface with material from the tip, the friction remained constant at constant normal load. The reason was that the portion of the surface where the tip advanced had a constant area. As in Sprensen et al. s work [63], dislocations nucleated at the comers of the contact and then propagated through it. 

The mechanisms of plastic deformation and mechanical states of polyclusters are described in Sect. 6.9. The comparatively high density of cluster boundaries (experimental data point to the fact that cluster sizes are about 102a, a being the average interatomic distance), peculiarities of structure and displacement of dislocations under the action of stress determine the dominant deformation mechanisms in some region of temperature T and stress a. 

Planes, Directions and Plastic Deformation. When dealing with the modern interpretation of the mechanism of plastic deformation of metals and alloys and with the associated problems of hardness and strength—resistance to plastic deformation—we find it essential to be familiar with the more important crystallographic planes and crystallographic directions. We shall, therefore, with this in mind, devote our attention now to a brief discussion of the planes and directions of the atoms in the throe main types of metal crystal lattices... 

In this work, the plastic deformation and the damage of ternary PP/PA6/POE were studied as completely as possible, and the mechanisms of plastic deformation were revealed by microscopic observation. The results obtained are original and will be helpful to well understand the mechanical behavior of polymer blends under large deformation. 

There are some obstacles to studying the kinetic peculiarities of this process on the one hand, the problems of defects detection, and the necessity of preservation of high strength without the dislocation mechanism of plastic deformation at increased temperatures, on the other hand. 

It is necessary to emphasize that the effect of uphill diffusion should be distinguished from the directional diffusion of point defects that can also take place under conditions of a homogeneous stressed state (diffusive creep). Nevertheless, the indicated mechanisms of plastic deformation have a lot in common, because they both are diffusive mechanisms of plastic deformation of crystalline solids. 

For the low-temperature region, several dislocation-related hypotheses were proposed to explain the mechanism of plastic deformation. Trefilov and Mil-man [76] suggested that during indentation the theoretical shear strength was... 

Similar extrusions were revealed by TEM in the Berkovich indentations made in (100) Si at ultralow loadings ( 10 mN) [16]. Although isolated diffraction patterns were not obtained from the extruded material due to the presence of underlying crystal, the amorphous nature of this material was inferred from the absence of any crystalline diffraction and tilting experiments. Further, at these experimental conditions (below the cracking threshold), no evidence of dislocation activity or other mechanisms of plastic deformation operating outside the clearly demar-... 

Taylor GI (1934a) The mechanism of plastic deformation of crystals. Part 1, theoretical. Proc R Soc Lond A 145 362-387... 

Demkowicz, M. J. and Argon, A. S. (2005b) Autocatalytic avalanches of unit inelastic shearing events are the mechanism of plastic deformation in amorphous silicon, Phys. Rev.,B, 72, 245206 (1-17). 

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