Deformation by Twinning

Plastic energy dissipation and frictional energy dissipation, in that order of importance, where compacted polymer particulates are relentlessly deformed by twin rotor devices, which rapidly raise their temperature and create regions of melts. 

List four major differences between deformation by twinning and deformation by slip relative to mechanism, conditions of occurrence, and final result. 

Figure 3.26 Transformation twinning a cubic crystal (a) will become tetragonal on cooling if the length of one axis changes (b, c) (d) the resulting deformation is minimized by twinning.

The stress-strain curve for superelastic Fe3Be. After the initial Hookean strain, the material deforms by martensitic transformation. On unloading the reverse martensitic transformation occurs at a lower stress. Adapted from R. H. Richman, in Deformation Twinning (New York AIME, 1963), p.267, figure 23. 

On the other hand, we discussed and presented in physical terms the very powerful melting mechanisms resulting from repeated, large deformations, forced on compacted particulate assemblies by twin co- or counterrotating devices. These mechanisms, which we refer to in Section 5.1, are frictional energy dissipation (FED), plastic energy dissipation (PED), and dissipative mix-melting (DMM). 

Marshall and McLaren (1974) showed that the boundaries of deformation pericline twin lamellae in experimentally deformed anorthite (An95) involve a displacement. The specimens were cut normal to [010] so that the twin boundaries were viewed edge-on and the twins were out-of-contrast for all g = hOl, because these reflecting planes are unaffected by the twin. However, in DF images with b-reflections (h+k = ln- -, l = 2n- - ) and with c-reflections (h+k = 2n, l = 2n+l), the twin boundaries are seen... 

The deformation microstructures of monoclinic pyroxenes are considered in Section 9.9.2. Optical microscope observations (Griggs, Turner, and Heard 1960 Raleigh 1965) indicate that the dominant slip system in monoclinic pyroxenes is also (100)[100]. However, van Duysen and Doukhan (1984) found by TEM that in naturally deformed a-spodumene the activated slip systems are (110) [001] and [lT0) <110>. In specimens of a-spodumene deformed by scratching, they also observed interesting microstructures of dislocations and faults that may be related to the twins observed in deformed diopside by Kirby and Christie (1977). 

In BCC structures, the twin plane is (112) and the twinning shear is in the direction [llT]. The only common example of such twins is in a-iron (ferrite) deformed by impact, where they occur as extremely narrow twin bands called Neumann bands. It should be noted that, in cubic lattices, both 112 and 111 reflection twinning produce the same orientation relationship however, they dilfer in the interatomic distances produced, and an FCC lattice can twin by reflection on 111 with less distortion than on 112, while for the same reason 112 is the preferred plane for BCC lattices. 

In addition to the importance that attaches to rigid body motions, shearing deformations occupy a central position in the mechanics of solids. In particular, permanent deformation by either dislocation motion or twinning can be thought of as a shearing motion that can be captured kinematically in terms of a shear in a direction s on a plane with normal n. 

Fig. 8. Shear deformation of B crystals by twinning. Dotted line indicates the formal location of the crystal.

Inelastic deformation can occur in crystalline materials by plastic flow . This behavior can lead to large permanent strains, in some cases, at rapid strain rates. In spite of the large strains, the materials retain crystallinity during the deformation process. Surface observations on single crystals often show the presence of lines and steps, such that it appears one portion of the crystal has slipped over another, as shown schematically in Fig. 6.1(a). The slip occurs on specific crystallographic planes in well-defined directions. Clearly, it is important to understand the mechanisms involved in such deformations and identify structural means to control this process. Permanent deformation can also be accomplished by twinning (Fig. 6.1(b)) but the emphasis in this book will be on plastic deformation by glide (slip). 

Miigge and others found that the only minerals that could easily be deformed under ambient conditions were the alkali halides and a few sulfides and carbonates. An exception to this was periclase (MgO), which deformed by 110 (110) dodecahedral glide in the same way as halite (NaCl). A more recently discovered exception is SrTiOs with the cubic perovskite structure, which can be deformed plastically at ambient and high temperatures but is brittle at intermediate temperatures (see Section 9.4.7). Other oxides and silicate minerals either cleaved or twinned when attempts were made to deform them at normal temperatures and pressures [1]. 

The FED and FED terms are not easy to describe mathematically, since thqr are not strictly speaking homogeneous sources, the reason being that particulate assemblies are not continua, but made up from discreet bodies. Nevertheless, dominance of co-TSEs in processing equipment is due to their capability of very rapid melting caused by repeated deformations imposed on compacted particulates by twin kneading elements. Eor extensive discussion, please refer to References [5, 45]. 

Fig. 4.37 Schematic hypothetical illustration (no change in shape is shown) a before deformation b after deformation by slip only c after deformation by slip and twinning. Note the twin bands in some crystallites [7]...

In hexagonal metals, which glide preferentially in the basal plane (cf. section 6.2.4), the rotation of the lattice by twinning can lead to a more favourable orientation of the crystal and thus increase the deformability by subsequent dislocation movement. 

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