Plasticity Dislocation-based mechanism

Lately, however, some surprising exceptions have been found to the general rule of low plasticity in ceramics. One is the perovskite oxide strontium titanate, SrTiOs. Recent studies on single crystals have revealed a transition from nonductile to ductile behavior in this material not only at temperatures above 1000°C, but again, below 600°C. Even more unexpectedly, it reached strains of 7 percent at room temperature with flow stresses comparable to those of copper and aluminum alloys. At both the high and low temperatures, the plasticity appears to be owing to a dislocation-based mechanism (Gumbsch et al., 2001). 

Normally, dislocation-based plastic deformation is irreversible, that is, it is not possible to return the material to its original microstructural state. Remarkably, fully reversible dislocation-based compressive deformation was recently observed at room temperature in the layered ternary carbide Ti3SiC2 (Barsoum and El-Raghy, 1996). This compound has a hexagonal stmcture with a large cja ratio and it is believed that the dominant deformation mechanism involves dislocation movement in the basal plane. 

According to Shchukin [9], the mechanism of adsorption plasticizing is based on facilitation of dislocation movement. It was experimentally established that upon deformation of crystals (e.g., of naphthalene or sodium... 

Defects as shown in Fig. 1(a) are the subject of the present chapter. They are referred to as metadislocations, and occur in numerous structurally complex metallic materials. The concept of metadislocations addresses a central problem in the plasticity of materials with large lattice parameters In these materials, conventional dislocation-based deformation mechanisms are prone to failure. This is a direct consequence of the elastic strain energy, which, per unit length of dislocation, is given by... 

Based on the discussion in earlier sections of this chapter, one may expect atomically flat incommensurate surfaces to be superlubric. Indeed the first suggestion that ultra-low friction may be possible was based on simulations of copper surfaces.6,7 Furthermore, the simulations of Ni(100)/(100) interfaces discussed in the previous section showed very low friction when the surfaces were atomically flat and misoriented (see the data for the atomically flat system between 30° and 60° in Figure 21). In general, however, it is reasonable to assume that bare metals are not good candidates for superlubric materials because they are vulnerable to a variety of energy dissipation mechanisms such as dislocation formation, plastic deformation, and wear. 

Allen [240], Costanzo et al. [241], and Krajcinovic [242-244] that thermodynamics of appropriate internal state variables for damage in composites under mechanical loads were addressed. Similar to the metal plasticity theoreticians, these researchers employed Coleman and Gurtin s [11] thermodynamic framework to determine the kinetic equations. However, unlike the metal internal state variable community that could quantify the evolution equations for dislocations and damage, these polymer-based composites theoreticians did not propose evolutionary rate equations, but just damage state equations. 

The enhancement of creep by anodic dissolution is well known, for copper in acetic acid153 and austenitic stainless steels and nickel-based alloys in pressurized water reactor (PWR) environments. The initial vacancy injection from the surface is followed by vacancy attraction to the inside dislocations, which promotes easier glide, climb, and crossing of microstructural barriers. This mechanism illustrates the corrosion-enhanced plasticity approach.95... 

An example of a material model based on the physics of material behavior is classical metals plasticity theory. This theory, often referred to as /2-flow theory, is based on a Mises yield surface with an associated flow rule, followed by rate-independent isotropic hardening (Khan and Huang 1995). Physically, plastic flow in metals is a result of dislocation motion, a mechanism known to be driven by shear stresses and to be insensitive to hydrostatic pressure. 

Before entering into a detailed discussion of the above list and based on what has been said thus far on the subject, briefly summarized (a) creep in materials (including ceramics), namely time-dependent plastic deformation, may occur during mechanical stresses well below the yield stress and (b) in general, two major creep mechanisms characterize the time-dependent plastic-deformation process-dislocation creep and diffusion creep. Now, a detailed discussion of paragraphs (a)-(d) follows. 

The reason for this spectacular failure of the theoretical prediction is that plastic deformation does not occur by sliding of complete layers of atoms. Instead, it proceeds by a mechanism that is based on a special type of lattice defect, the dislocations. To understand plastic deformation of metals thus requires an understanding of dislocations. 

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