Severe plastic deformation techniques

A closely related technique to the SPD techniques, and which is sometimes included with them, is cold rolling. Here, the metal is deformed by passing it through rollers at a temperature below its recrystallization temperature. Here, contrary to the other SPD techniques, the cross-section of the material changes after rolling and the grain boundaries have a low angle misorientation [251]. However, for our purpose, all these techniques will be treated as SPD. 

The advantages of SPD techniques over high energy milling are a lower concentration of impurities and a lower production cost for large quantities [253]. 

Application of SPD techniques to metal hydrides is a new field of research. Skripnyuk and Rabin were the first to use ECAP to improve the hydrogen storage properties of Mg-based alloys [254, 255]. The first study on ECAP-processed magnesium alloy ZK90 showed an improvement in sorption kinetics without loss of hydrogen capacity or change in thermodynamic parameters [254]. figure 4.12 shows comparable results for the Mg-Ni eutectic alloy [255]. 

In this chapter, the fundamental characteristics of metal hydrides as hydrogen storage materials have been reviewed. Their potential for practical applications is well known but more research need to be done in order to find a system that will meet the industry s criteria. This research could be broadly divided in the following way  

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For practical applications, the synthesis process is a crucial step since large quantities will have to be produced reliably and at low cost. Therefore, more research is needed into new methods of synthesis, particularly for nanomaterials. In this respect, severe plastic deformation techniques could be an option but more thought has to be given to scaling up ball milling. 

Figure 1. Principles of severe plastic deformation techniques a - high pressure torsion, b - ECA pressing.

The top-down approach starts with a bulk material and attempts to break it down into nanoscaled materials through physical methods. Hence, most of these techniques are really forms of fabrication rather than synthesis. For nanostructured bulk phases, including powders, the common methods are milling, devitrification of metallic glass, and severe plastic deformation. For nanocrystalline thin films (films with nanosized crystallites), methods include thermal vaporization (under high vacuum), laser ablation, and sputtering (thermal plasma), all of which were... 

High pressure torsion (HPT) (Fig. la) and equal-channel angular (ECA) pressing (Fig. lb) refer to the techniques which were used in pioneer works devoted to UFG structures formation in metals and alloys [11,12] by severe plastic deformation. These methods have been further developed lately. 

The present review shows how the microhardness technique can be used to elucidate the dependence of a variety of local deformational processes upon polymer texture and morphology. Microhardness is a rather elusive quantity, that is really a combination of other mechanical properties. It is most suitably defined in terms of the pyramid indentation test. Hardness is primarily taken as a measure of the irreversible deformation mechanisms which characterize a polymeric material, though it also involves elastic and time dependent effects which depend on microstructural details. In isotropic lamellar polymers a hardness depression from ideal values, due to the finite crystal thickness, occurs. The interlamellar non-crystalline layer introduces an additional weak component which contributes further to a lowering of the hardness value. Annealing effects and chemical etching are shown to produce, on the contrary, a significant hardening of the material. The prevalent mechanisms for plastic deformation are proposed. Anisotropy behaviour for several oriented materials is critically discussed. 

Several groups tried to unravel whether the cavitation occurs before or after the onset of plastic deformation. Real-time small angle X-ray scattering [162] and light scattering [163,164] techniques were used to study the deformation of... 

To have any hope of honestly addressing the dislocation-level processes that take place in plastic deformation, we must consider fully three-dimensional geometries. Though we earlier made disparaging remarks about the replacement of understanding by simulation, the evaluation of problems with full three-dimensional complexity almost always demands a recourse to numerics. Just as the use of analytic techniques culminated in a compendium of solutions to a variety of two-dimensional problems, numerical analysis now makes possible the development of catalogs of three-dimensional problems. In this section, we consider several very important examples of three-dimensional problems involving dislocations, namely, the operation of dislocation sources and dislocation junctions. 

The destructive effects of expanding compressed air and relaxation of elastic deformation can be also reduced if the maximum pressure is held for some time, called dwell time, before it is released. Fig. 8.2 shows, that, without special technical provisions, this is only achieved in ram extruders (Fig. 8.2b, see also Section 8.4.3). In such equipment, a number of briquettes is retained in the long pressing channel and is redensified during each stroke. After the wall friction is overcome and the entire line of briquettes moves forward, the pressing force remains almost constant. A similar, but much smaller effect is obtained in pellet presses (see also Section 8.4.2). Since, as mentioned before, a dwell time and, particularly, the application of several densification cycles also helps to convert temporary elastic deformation into permanent plastic deformation, these techniques are especially suitable for the densification of elastic materials such as, for example, biomass. 

The expansion and contraction of the polymer netwoik in conjunction with the sorption/desorption of solvent molecules and ions can be described in terms of mechanical work. This mechanical contribution should be considered in the calculation of the equilibrium electrode potential (see Chap. 5). The deformation coupled to the redox reaction is elastic in nature. A plastic deformation occurs when a neutral, diy film is immersed in electrolyte solution and electrolyzed. It has been observed for a range of neutral polymer films freshly deposited on metal substrates by solvent evaporation techniques that several potential sweeps are required for the films to become fully electroactive [2,19,126,195,196]. This phenomenon has been referred as the break-in effect (Fig. 6.19). 

Early experiments with positrons were dedicated to the study of electronic structure, for example Fermi surfaces in metals and alloys [78,79], Various experimental positron annihilation techniques based upon the equipment used for nuclear spectroscopy underwent intense development in the two decades following the end of the Second World War. In addition to angular correlation of the annihilation of y quanta, Doppler broadening of the annihilation line and positron lifetime spectroscopy were established as independent methods. By the end of the 1960s, it was realised that the annihilation parameters are sensitive to lattice imperfections. It was discovered that positrons can be trapped in crystal defects i.e., the wavefunction of the positron is localised at the defect site until annihilation. This behaviour of positrons was clearly demonstrated by several authors (e.g., MacKenzie et al. [80] for thermal vacancies in metals, Brandt et al. [81] in ionic crystals, and Dekhtyar et al. [82] after the plastic deformation of semiconductors). The investigation of crystal defects has since become the main focus of positron annihilation studies. 

Prior to the advent of fracture mechanics as a scientific discipline, impact testing techniques were estabhshed to ascertain the fracture characteristics of materials at high loading rates. It was realized that the results of laboratory tensile tests (at low loading rates) could not be extrapolated to predict fracture behavior. For example, under some circumstances, normally ductile metals fracture abruptly and with very little plastic deformation imder high loading rates. Impact test conditions were chosen to represent those most severe relative to the potential for fracture —namely, (1) deformation at a relatively low temperature, (2) a high strain rate (i.e., rate of deformation), and (3) a triaxial stress state (which may be introduced by the presence of a notch). 

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