Twinning, deformation

Microstructural examinations revealed deformation twins (Neumann bands) in metal grains at wasted surfaces. The surfaces in these areas have a jagged, undercut profile. 

Second, deformation twins were observed in metal grains at the damaged surfaces. Deformation twinning cannot result from corrosion but is the consequence of shock loading of the metal, precisely the effects of microjets of water impacting on the metal surface. 

Figure 6.4. Bright-field micrograph and SADP of deformation twins in Al-4.8 wt.% Mg shock loaded to 13 GPa at 100 K.

Figure 6.5. Dislocation networks and deformation twins in 4340 steel shock loaded to 15 GPa.

J.N. Johnson and R.W. Rohde, Dynamic Deformation Twinning in Shock-Loaded Iron, J. Appl. Phys. 42, 4171-4182 (1971). 

Anonymous, Shock-Loading Effect On, and Deformation Twinning of Beryllium, John Crerar Library, Research Information Service Bib. No. 236, Chicago, IL, 6 pp., June 1967. 

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. 

Fig. 6. Taper section of a ground zinc surface. A recrystallized surface layer and a zone containing deformation twins are present. Taper ratio 16.2. X KMX...

Dislocation movement requires only a small stress compared with that required for the simultaneous movement of one atomic plane over another because only a few atoms are directly involved in the slip process at any instant (see Figure 9.2). However, at higher temperatures, edge dislocations can move out of their slip planes by a process called climb, in which atoms (or vacancies) diffuse to, or away from, the dislocation core (Figure 9.3). The climb of dislocations is, therefore, an important process in high-temperature deformation. In some materials, deformation twinning may be important, especially at low temperatures. 

Barber, D. J., Wenk, H.-R. (1979). Deformation twinning in calcite, dolomite, and other rhombohedral carbonates. Phys. Chem. Minerals, 5,141-65. 

Fig. 2-23 Twinned grains (a) and (b) FCC annealing twins (c) HCP deformation twin.

Deformation twins are found in both BCC and HCP lattices and are all that their name implies, since, in both cases, the cause of twinning is deformation. In each case, the orientation relationship between parent crystal and twin is that of reflection across a plane. 

U. Homemann and W.F. Mttller, Shock-induced deformation twins in clinopyroxene. Neues Jahrb MineralMonatsh 6, pp. 247-255 (1971). 

Climb, deformation twinning, and prismatic loops are also important in the multiplication of dislocations (6, 7). 

The base metal had equiaxed grains with an average diameter of 25 pm. Welds were produced with no apparent defects. Optical microscopy revealed that the microstructure of the stir zone was characterized by a high density of deformation twins within the grains. The twin density varied with position relative to the position of the tool shoulder, with denser twins found near the upper part of the weld. The... 

Reed-Hill, R.E., J.P. Hirth and H.E. Rogers, 1963, Deformation twinning (Gordon and Breach, New York). 

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]. 

Hay, R.S. and Marshall, D.B. (2003) Deformation twinning in monazite, Acta Mater. 51, 5235. [And Hahn T. ed. (1985) Space Group Symmetry, International Tables for Crystallography, Brief Teaching Edition, D. Reidel Publishing Co., Dordrecht.]... 

Zha] Electron diffraction, optical microscopy Deformation twinning of martensite 20 mass% Ni, 0.8 mass% C... 

The availability of sizable single crystals has led to a significant literature on the deformation of sapphire of various orientations, and at various temperatures. As already noted, the first such study was by Wachtman and Maxwell in 1954 [6], who activated (0001) 1/3 (1120) basal slip at 900 °C via creep deformation. Since that time, it has become clear that basal slip is the preferred slip system at high temperatures, but that prism plane slip, 1120 (1100), can also be activated and becomes the preferred slip system at temperatures below 600°C. Additional slip systems, say on the pyramidal plane 1012 1/3 (1011), have very high CRSSs and are thus difficult to activate. Both, basal and rhombohedral deformation twinning systems, are also important in AI2O3 (these are discussed later in the chapter). 

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