Diffusional plasticity

Traditionally, production of metallic glasses requites rapid heat removal from the material (Fig. 2) which normally involves a combination of a cooling process that has a high heat-transfer coefficient at the interface of the Hquid and quenching medium, and a thin cross section in at least one-dimension. Besides rapid cooling, a variety of techniques are available to produce metallic glasses. Processes not dependent on rapid solidification include plastic deformation (38), mechanical alloying (7,8), and diffusional transformations (10). 

Most rigid polyurethane foams have a closed cell structure. Closed cells form when the plastic cell walls remain intact during the expansion process and are not ruptured by the increasing cell pressure. Depending on the blowing process a small fraction (5-10%) of the cells remain open. Closed cell structures provide rigidity and obstruct gaseous or fluid diffusional processes. 

Pore dimensions may have a more subtle effect on decay rate depending on component dimensions and production method of the manufactured material. Products made from pasted starch, LDPE, and EAA (2) typically appeared as laminates of starch and plastic when examined by scanning electron microscopy (Figure 1). The dimensions of inter-laminate channels (i.e., pores) were not uniform and ranged from about 50 to 325 m in cross-section (22). Since flux is dependent on diffusional path area, the smaller pores can be an impediment to movement of solutes from the interior to the surface of the films. Figure 5 illustrates two films in which the laminate units are the same thickness, but differ in length. When the starch is removed... 

Such transformations have been extensively studied in quenched steels, but they can also be found in nonferrous alloys, ceramics, minerals, and polymers. They have been studied mainly for technical reasons, since the transformed material often has useful mechanical properties (hard, stiff, high damping (internal friction), shape memory). Martensitic transformations can occur at rather low temperature ( 100 K) where diffusional jumps of atoms are definitely frozen, but also at much higher temperature. Since they occur without transport of matter, they are not of central interest to solid state kinetics. However, in view of the crystallographic as well as the elastic and even plastic implications, diffusionless transformations may inform us about the principles involved in the structural part of heterogeneous solid state reactions, and for this reason we will discuss them. 

Mass diffusion between grain boundaries in a polycrystal can be driven by an applied shear stress. The result of the mass transfer is a high-temperature permanent (plastic) deformation called diffusional creep. If the mass flux between grain boundaries occurs via the crystalline matrix (as in Section 16.1.3), the process is called Nabarro-Herring creep. If the mass flux is along the grain boundaries themselves via triple and quadjunctions (as in Sections 16.1.1 and 16.1.2), the process is called Coble creep. 

It is well established that the plastic deformation of crystalline solids occurs by the movement of lattice dislocations and/or diffusional creep. The rate of diffusion is expressed as... 

Dimethyl sulfoxide (DMSO) test6 — This test measures the diffusional resistance of the horny layer. Equal quantities of three different concentrations of DMSO are applied to three plastic wells for 5 min. DMSO provokes whealing in human skin. The wheals are scored 10 min after removal of the test fluid using a scale. Individuals with high reactivity are those susceptible to whealing with the lowest concentration of DMSO. 

At present more than 100 zeolitic structures (both natural and synthetic) have been reported and their number grows annually as new structures are continuously being discovered which opens up a wide range of possible applications [61, 62]. However, from a practical viewpoint, only a few zeolites are used as industrial catalysts such as Y, ZSM-5, Beta and mordenite (Table 3.1), mainly due to the cost and difficulties inherent to their preparation [60]. When zeolites are applied for the catalytic cracking of polymers, their microporous structure causes important diffusional and steric hindrances for the access of the bulky plastic molecules to the internal acid sites [5, 24]. 

Another way of using plastics as fuels is their direct combustion. The oxidative pyrolysis of PET has been investigated in several cases. Oxidative pyrolysis means that instead of an inert atmosphere an oxygen-containing gas was used. In this case pyrolysis can occur, if oxygen can not reach the particle surface due to efflux of volatiles or diffusional resistances at boundary layers, or if oxidative conditions do not affect the kinetics and mechanism of the pyrolytic processes [41]. 

Fig. 6.17. Map of polycluster mechanical states. Region I elastic and anelastic (shaded area) deformations Region II inhomogeneous plastic deformation Region III homogeneous diffusional-viscous flow. Curves 1-3 show the temperature dependence of the stress at different constant strain rates...

On the map of mechanical states in region I, elastic and anelastic (shaded areas) deformations take place. In the region II, the inhomogeneous plastic deformation with the formation of shear bands takes place. The horizontal broken line corresponds to the theoretical yield stress of LRC. In the region III, the homogeneous diffusional-viscous flow takes place and, in the region IV, the mixed viscous flow is realized. Curves 1,2, 3 show the temperature dependence of the stress at different constant strain rates. The continuations of these curves in regions IV and II correspond to the mixed nonuniform plastic deformation. 

The models that have been suggested to explain region I for glasses in the presence of moisture can be divided into three categories diffusional (i.e., a desintering of the material along the fracture plane), plastic flow, and chemical-reaction theories. Currently the chemical-reaction approach appears to be the most consistent with experimental results and is the one developed here. 

Two types ofDV actions occur (1) volatile components diffuse to the plastic-vapor interface (called diffusional mass transport), and (2) volatile components evaporate at the interface and are carried away (called conveaive mass transport). If (1) is less than (2), the process is diffusion-controlled. This condition represents most of the plastic devolatilization processes because plastics diffusion contents are usually low. 

The densification by plastic deformation and power-law creep is, in principle, independent of particle (grain) size. In the case of diffusion (both lattice and grain boundary), on the other hand, densification depends on not only the effective pressure but also the grain size. The densification by diffusion under an external pressure is similar to diffusional creep Nabarro-Herring creep due to lattice diffusion, and Coble creep due to grain boundary diffusion. The dependency of densification on grain size is the same as that of diffusional creep. 

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