Mechanical twins

Properties. Table 1 hsts many of the physical, thermal, mechanical, and electrical properties of indium. The highly plastic nature of indium, which is its most notable feature, results from deformation from mechanical twinning. Indium retains this plasticity at cryogenic temperatures. Indium does not work-harden, can endure considerable deformation through compression, cold-welds easily, and has a distinctive cry on bending as does tin. 

Twins can also form in perfect crystals by a number of processes. The deformation of a crystal due to the application of a shear stress can result in mechanical twins, which form in order to reduce the strain so produced. The formation of mechanical twins in deformed metals has been extensively investigated, and a number of mechanisms have been proposed to account for the differences that occur between one structure and another. These mechanisms often involve the passage of a sequence... 

Figure 3.25 Mechanical twinning in vanadium pentoxide, V205 (a) the formation of needlelike twins at the tip of reduced oxide and (b) the idealized structure of the twin. Arrows in (a) represent the direction of shear forces.

Mechanical twins that form to reduce internal stress have also been observed in nonmetallic crystals. Mild reduction of vanadium pentoxide at low temperatures produces regions of reduced oxide that induce considerable shear stress in the matrix... 

The wire which would normally go to the spark plug is replaced by a wire which is taken to a box containing one pre-war mechanical twin-coil relay or vibrator per cylinder. Each of these wires drives its own dedicated relay , the current energises the relay coil but the other side of the relay coil is left unconnected. The wiring arrangement is shown in the diagrams below. 

Abstract—Pseudo-twinning and mechanical twinning have been observed in a transmission electron microscopy study of TiMNi47Fe3 and Ti49Ni5i alloys which have the B2(CsCI) structure. Observation of twinning in ordered alloys is rare and this is the first observation of twinning reported in a B2 structure. The twin planes are the 112 and 114 planes. For 112 pseudo-twins, the composition plane is not the twin plane and the pseudo-twin does not have the B2 structure. For 114 mechanical twins, the composition plane is the twin plane and the twin does have the B2 structure. It is shown that a shear on the 114 plane plus a shuffle of the atoms results in the ordered B2 structure in the twinned region. 

M.V. Klassen-Neklyudova, Mechanical Twinning of Crystals, Consultant Bureau, New York... 

Kirby, S. H., Christie, J. M. (1977). Mechanical twinning in diopside Ca(Mg, Fe)Si206 structural mechanism and associated crystal defects. Phys. Chem. Minerals, 1, 137-63. 

Deformation mechanisms of Crl8Re and Cr35Re alloys differ from those of pure chromium by availability mechanical twinning (figure 4) up to relatively high temperature (600°C for Cr-35Re). Twinning is result of... 

Figure 4. High density of mechanical twins on Cr-35Re deformed 71% at room temperature...

Thermal shock resistance of Cr-Re alloys was sufficient to withstand conditions similar to those imposed by the application. The ability to mechanically twin permitted the accommodation of thermal deformation in the boundary area between the hot and cold zones of the combustion chamber, which is acknowledged to be the its most solicited area. Grain boundary micro cracking on the thermally shocked surface might be overcome by strengthening of the grain boundaries through thermo mechanical treatment. 

The elastic and plastic deformation behavior, including mechanical twinning, of... 

Klassen-Nekludova M.V. Mechanical twinning of crystals. Moscow, 1960 (in Russian). 

From an atomic point of view, the origin of plastic deformation is most readily understood by studying single crystals. The deformation is revealed as a series of steps or lines, where parts of the crystal have moved relative to each other. The process can be one of slip, in which atom planes have slid sideways, or one of twinning, known as mechanical twinning. In slip, a small slice of crystal is moved sideways (Figure 10.12a). The slices are usually the order of a few hundred atoms wide and, during slip. 

Figure 10.12 (a) Slip in a rod, characterised by diagonal planes across which atoms in the crystal have sheared because of an applied load (b) shp band, in which slip planes are aggregated into narrow regions (c) mechanical twin planes, across which the atoms in the crystal are reflected because of an apphed load (d) twin band, in which twin planes are aggregated. Note slip and twimiing are both caused by stress and are difficult to distinguish in macroscopic samples... 

We shall study the following topics twinning elements, mechanical twin formation, twin junction and twin intersection, twin by crystal growth and grain boundaries, the real structure of B, and A, B phase transformation and the domains of B by phase transformation. 

Recently Yangui (1981) has shown the existence of 111 twin and of (192) twin. (Ill) and [192] are reciprocal twins as well as (ill) and [192]. These twins had not been seen previously because the diffraction pattern can be easily confused with the diffraction pattern of the other well-known twins except in a systematic study. They have been suspected by observation of both the diffraction pattern and the image (the way of crossing twin boundaries for the Z.A.P. was not in agreement with the diffraction pattern for the well-known twins). Like the other ones they can be mechanical twins and are indeed quite common. Table 3 gives their twinning elements Ki, K2, tji, t)2- Such twins cannot be related, as are the well-known 313 and (132) twins, to symmetry elements of the A structure. Nevertheless both can be related to a possible 10.0 twin of the A structure. This twin would be a rational twin, i.e., all the twinning elements Kj,... 

The observation, inside an electron microscope, of the mechanical twinning of B shows that this twinning occurs when crystals are strained so as to relax the stress. For instance, a mechanical twin can appear at the tip of a fracture inside a crystal (fig. 15). Except for this situation it always appears at the boundary of a crystal when, for example, a mechanical twin has just occurred at the neighbouring crystal. In this case a wedge microtwin starts from the grain boundary and passes through the crystal, giving rise to a twin layer (fig. 16a,b), and this for a type I as well as for a type II twin. 

Fig. 16. (a) Mechanical twins start from grain boundaries in polycrystalline samples. They can go through grain boundaries so that the shear deformation is nearly the same on each side of the grain boundary. 

The mechanical twins of the B structure appear by stressing the crystals shear, tension or extension. The first case is related to the simplest phenomenon where only one kind of twin layer is generated. In the other cases, generally two kinds of twin layers are generated at once, which makes the phenomenon rather complex. 

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