Accommodation at deformation twins in bcc crystals
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1. I N T R O D U C T I O N D E F O R M A T I O N twins constitute homogeneously sheared regions where strains are very localized. It is therefore relatively easy to visualize that stress concentration must develop in the matrix if twin growth is obstructed. In a polycrystalline metal and alloy undergoing deformation by slip and twinning, this situation could arise when: i) a twin ceases to grow inside the lattice, ii) twins with nonparallel shear vectors intersect, and iii) a twin impinges on a high angle grain boundary. It is the objective of this paper to review the available experimental evidence concerning the accommodation processes occurring under these distinct situations and to reconcile these observations with those where twins have been identified as a source for crack nucleation. 2. A C C O M M O D A T I O N A T TERMINATING TWINS Slip bands emanating from tips of 1/6 (111) [112} twins terminating inside a crystal have been observed by several investigators) 4 Some of the typical examples are reproduced in Figs. 1 and 2. It is clear from Fig. 1 that the termination of twin_x, lying on the (121) plane, has resulted in slip on the (101), (0il) and (110) planes. Another example giving a clear demonstration of the occurrence of slip ahead of a terminating twin is shown in Fig. 2. A twin ends up within the crystal at A, but it causes displacement of a sub-boundary at B which is exactly in line with the twin. Recently, the crystallographic features of twins and of associated slip in Mo-35 at. pct Re alloy have been correlated by means of transmission electron microscopy. 6,7 Figure 3 shows an example of a twin that stops propagating within the crystal. It is evident that the stress concentration which is likely to exist at the twin tip is accommodated by slip occurring ahead of the twin. Furthermore, the observed variation in the background intensity on either side of the dislocation image S. MAHAJAN is Member of the Technical Staff at Bell Laboratories, Murray Hill, NJ 07974. This paper is based on a presentation made at a symposium on "The Role of Twinning in Fracture of Metals and Alloys" held at the annual meeting of the AIME, St. Louis, Missouri, October 1519, 1978, under the sponsorship of the Mechanical Metallurgy Committee of The Metallurgical Society of AIME. METALLURGICALTRANSACTIONSA
suggests that the slipped region consists of dislocations of both signs. Electron diffraction results coupled with trace analysis show that T~ and slip band lie on the (211) plane. Since the slip dislocatio_ns are in contrast for g -- 110 and invisible for g = 110, the possible Burgers vectors are + 1/2 [111] and + 1/2 [111]. Combining the two preceding assessments, the activated glide system must be _+ 1/2 [111](211). It may be emphasized that emissary slip bands need not always be present at terminating twins. The evolution of slip structures appears to depend on the thickness of twins, which in turn determines the magnitude of stress concentration, and microstructure. These aspects have been well documented by the re
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