Plastic Deformation at Elevated Temperatures

Deformation at elevated temperatures is the commonly observed case in ceramics. Once again using the example of polycrystalline ceramic MgO, the following stress-strain curves are illustrated (Fig. 4.8). One of the possible differences in these stress-strain curves reflects the difference in grain-size the different grain size before deformation is shown in Fig. 4.9. The composition and porosity were also different in the otherwise nominally pure and dense specimens, as seen in Table 4.1. 

This equation is related to the strain rate at small strains and the symbols have the usual meaning, namely stress (tr), strain (a) and time (t). The stress-strain curves of the polycrystalline MgO are indicated as a function of ternperamre. Above 1200 °C, deformation occurred by grain-boundary shearing accompanied, in some cases, by slip, as seen in Fig. 4.10. 

Considerable work was done on MgO, both single crystals and polycrystalline materials alike, toward an understanding of deformation behavior in ceramics. Furthermore, when polycrystalline MgO is ductile, its strain hardening is comparable to that of (111) oriented single crystals [16]. 

In summary, ductility at high-temperature deformation is exhibited by most of ceramics, but is best studied in MgO, which, unlike most ceramics, is not brittle at ambient temperatures. 

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The lonsdaleite Si-IV phase can be obtained either from the metastable Si-III phase after heat treatment at 200-600 °C [53,56,57] or from Si-I after plastic deformation at elevated temperatures (350-700 °C) and under confining pressure [55, 58]. The Si-I -> Si-IV transformation is closely related to deformation twinning and was described as a martensitic transformation taking place at twin-twin intersections or after secondary twinning [58-60]. 

As a result of this process, a joint is produced in solid state. Because of various geometrical features of the tool, the material movement around the pin can be quite complex. During FSW process, the material undergoes intense plastic deformation at elevated temperature, resulting in generation of line and equiaxed recrystallized grains. The fine microstructure in friction-stir welds produces good mechanical properties of the joints, both static and dynamic. 

Do not heat plastic membranes at elevated temperatures because the filter melts or becomes deformed. 

Higher temperature may result in a weaker Rehbinder effect as well. This occurs due to the facilitation of a plastic flow at elevated temperatures. Thermal fluctuations result in the relaxation of deformational microheterogeneities. As a result, at elevated temperatures local concentrations of stresses are too low to initiate the formation of primary microcracks. An increase in temperature thus often leads to a transition from brittle fracture in the presence of adsorption-active medium to plastic deformation. The decrease in the rate of deformation of a solid has an analogous effect slow deformation also results in an increased probability of the thermally activated relaxation of locally concentrated deformations and stresses. 

Grain boundaries are obstacles to slip, since the slip direction of a favorably-oriented crystal may change when it crosses a grain boundary. As a result, the strength of polycrystaUine materials is higher than that of single-crystal materials. A polycrystalline ceramic can deform plastically (mainly at elevated temperatures. 

Some examples of these processes and effects are offered below under sections that deal with (1) ductility at low tem-peratmes, (2) deformation at elevated temperatures (forming), (3) transformation-assisted plasticity, (4) deformation textures, and (5) deformation microstructures. 

Wax usually refers to a substance that is a plastic solid at ambient temperature and that, on being subjected to moderately elevated temperatures, becomes a low viscosity hquid. Because it is plastic, wax usually deforms under pressure without the appHcation of heat. The chemical composition of waxes is complex all of the products have relatively wide molecular weight profiles, with the functionaUty ranging from products that contain mainly normal alkanes to those that are mixtures of hydrocarbons and reactive functional species. 

Precipitation (Age) Hardening Alloys. Only a few copper alloys are capable of responding to precipitation or age hardening (7). Those that do have the constitutional characteristics of beiag siagle-phase (soHd solution) at elevated temperatures and are able to develop iato two or more phases at lower temperatures that are capable of resisting plastic deformation. The copper alloy systems of commercial importance are based on iadividual additions of Be, Cr, or Ni + X where X = Al, Sn, Si, and Zr. 

Designing plastic Basically the general design criteria applicable to plastics are the same as those for metals at elevated temperature that is, design is based on (1) a deformation limit, and (2) a stress limit (for stress-rupture failure). There are, of course, cases where weight is a limiting factor and other cases where short-term properties are important. 

Annealing in metals can first lead to stress relaxation in which stored internal strain energy due to plastic deformation is relieved by thermally activated dislocation motion (see Figure 5.18). Because there is enhanced atomic mobility at elevated temperatures, dislocation density can decrease during the recovery process. At still higher temperatures, a process known as recrystallization is possible, in which a new set of... 

The method of molecular dynamics (MD) provides a remarkable opportunity for the observation of various mechanisms of processes taking place on a micro-(nano-) level, and for the evaluation of the probability of such processes by repeating experiments dozens of times. Figure IX-37 shows the MD simulation of the deformation and fracture of a two-dimensional crystal. Plastic deformation and formation of a dislocation (AB) at elevated temperature (upper part) and the formation of a brittle crack at low temperature (lower part) are shown in Fig. IX-37, a, while simultaneous processes of crack nucleation influenced by the presence of foreign atoms, and their propagation to the tip of the crack, taking place at elevated temperature, are illustrated in both lower and upper portions of Fig. IX-37, b [40,41]. 

Fig. IX-37. The molecular dynamic simulation of deformation and fracture of two-dimensional crystal a - plastic deformation and formation of a dislocation at elevated temperature (upper portion) and development of brittle crack at low temperature (lower portion) b - simultaneous processes of crack nucleation and foreign atom propagation at elevated temperature (both upper and lower portions)...

If all or some of the particles to be agglomerated by pressure are elastic, under certain conditions, the temporary elastic deformation can be converted into permanent plastic change of shape. For this to occur, the most important parameters are time and temperature. If elastically deformed solids are kept under pressure for some time, certain structural features, such as lattices, dislocations, etc., move into stable new positions by creep. At elevated temperatures, but still well below the softening point, most solids, even those that are brittle or tough at ambient temperatures, become more malleable and deform readily under pressure. 

A second source for accidental orientation is deformation of the material in the solid state, e.g. with cold-forming or solid-phase forming, in which a plastic sheet or billet is formed into an object at a temperature below the softening point of the plastic. The relaxation process can hardly take place under these conditions and although the article may be stable at room temperature, the deformation is (almost) completely recoverable at elevated temperatures. ... 

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