Deformation Mechanical Twins

In particular. Fig. 4.38c shows that the twin patterns on the Y-BT surface are highly ordered. The topographical contrast results from the difference in the etching rate of twins with distinct polar directions and, hence, the interfaces separating the stripes are twin walls. 

In FCC metals, there may be a CRSS law for twinning analogous to Schmid s Law, according to Szczerba et al. [46]. Some criteria must be satisfied simultaneously for twinning to occur, namely (a) the ratio of the resolved shear stress. 

to the critical stress of a twin system must be greater than that of any slip system, namely trss/ Tc CRSS for slip (b) the trss should be greater than the threshold (namely greater than some minimum) stress necessary for twinning to occur and (c) the Trss must satisfy the character of a twin shear. It is not currently known if such a concept exists also for ceramics. 

Graphically, curves pertaining to these two relations intersect and, below their intersection, stress reduction by twinning cannot reduce the total energy of the system. Therefore, there is a grain size below which twinning does not occur. For details on energy minimization, Arlt s [1] review may be consulted. 

In a polycrystalline ceramics, such as BT, when structural phase transition occurs, a crystallite (grain) experiences slight deformation. This deformation is 

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In addition to slip, mechanical twinning may occur as a deformation mechanism. Twin boundaries are often decorated with dislocations to reheve local strain [Fig. 3(b)]. 

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... 

Twinning. Twirming is a less dominant deformation mechanism and occurs in high-purity tungsten only. Twins are mainly formed on [112 planes in [111] direction. Twins have been observed in tension below 0°C and, after rolling, compressing, swaging, and explosion deformation, up to 1500°C. 

For coarse-grained metals, dislocation movement and twinning are well known primary deformation mechanisms. Ultrafine, equiaxed grains with high-angle grain boundaries impede the motion of dislocations and... 

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... 

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. 

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. 

Mechanical twins are of special interest. They can be either type I or type II twins or even compound twins. They can appear by shear deformation of the... 

In the very thin deposits that were strained in the [110] and [120] direction TEM analysis revealed the presence of mechanical twins that occurred only near the fmeture line. As for the thin [100] specimens there was again no homogeneous plastic deformation in the gauge length. Only thicker samples showed some homogeneous plastic deformation prior to necking and rupture. 

Besides dislocation movement, there are other mechanisms of plastic deformation. These are the martensitic transformation we already discussed, diffusion creep at high temperatures (to be covered in chapter 11), and finally the so-called twinning. Mechanical twinning usually contributes only slightly to plastic deformation and is in general more difficult to activate than dislocation movement. Therefore, it will be discussed only briefly. 

Values of SFE from 20 to 60 mjm determine intensive mechanical twinning related to TWIP effect. At SFE values higher than about 60 mJm-2, the partition of dislocations into Shockley partial dislocations is difficult, and therefore the glide of perfect dislocations is the dominant deformation mechanism (Hamada, 2007). In TRIPLEX steels with a structure of austenite, ferrite and K-carbides ((Fe,Mn)3AlC) and for SFE > 100 mJm-2, the SIP (Shear Band Induced... 

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