Twins, annealing

Figure 1.30 Comparison between (a) bright-field and (b) dark-field images of AISI 1080 high carbon steel. In addition to grain boundaries and oxide particles, annealing twins are revealed in the dark-field image. (Reproduced with permission of ASM International . All Rights Reserved. www.asminternational.org. G.F. Vander Voort, Metallography Principles and Practice, McGraw-Hill, New York. 1984 ASM International .)...

Annealing twins, such as occur in FCC metals and alloys (Cu, Ni, a-b ass, Al, etc.), which have been cold-worked and then annealed to cause recrystallization. 

Annealing twins in FCC metals are rotation twins, in which the two parts are related by a 180° rotation about a twin axis of the form <111>. Because of the high symmetry of the cubic lattice, this orientation relationship is also given by a 60° rotation about the twin axis or by reflection across the 111 plane normal to the twin axis. In other words, FCC annealing twins may also be classified as reflection twins. The twin plane is also the composition plane. 

Occasionally, annealing twins appear under the microscope as in Fig. 2-23(a), with one part of a grain (B) twinned with respect to the other part (.4). The two parts are in contact on the composition plane (111) which makes a straight-line trace on the plane of polish. More common, however, is the kind shown in Fig. 2-23(b). The grain shown consists of three parts two parts (Ai and A 2) of identical orientation separated by a third part (B) which is twinned with respect to Ai and A2. B is known as a twin band. 

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

In Fig. 4.45, SEM microstructures of air- and Ar-sintered samples in tetragonal BT, show growth (annealing) twins. Two single twins can be identified by the distinctive contrast across the twin boundary, as indicated in Fig. 4.45a. The twin... 

In this section, the formation of deformation and annealing twins were discussed with the resulting stress-strain relation characterized by serrated curves. As indicated here, there is no basic difference in twin formation between ceramics and metals. 

Figure 4.14 shows an annealing twin in pure nickel. It is to be noted that the intersection of the twin with the grain boundaries results in only a small deflection of the grain boundaries, which is consistent with the low energy of the twin boundary. The intersection of twins with grain boundaries has been analyzed extensively by Murr [12]. 

Fig. 3. Austenitic structure with annealing twins of 26Mn-3Si-3 Al-Nb steel after the thermo-mechanical rolling and immersion in IN F12S04 (a) and X-ray diffraction pattern (b).

C.S. Pande, B.B. Rath, M.A. Imam, Effect of annealing twins on Hall-Petch relation in polycrystalline materials. Mater. Sci. Eng. A-Struct Mater. Prop. MicrostrucL Process. 367(1-2), 171-175 (2004)... 

Annealing twins, found in alloys having the FCC crystal structure (Section 4.6), may be observed in this photomicrograph for austenite. They do not occur in BCC alloys, which explains their absence in the ferrite micrograph of... 

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