On crack nucleation in zinc upon interaction of basal and pyramidal dislocations
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STRUCTURE OF CRYSTALS Dedicated to the memory of V.L. Indenbom
On Crack Nucleation in Zinc upon Interaction of Basal and Pyramidal Dislocations V. A. Fedorov, Yu. I. Tyalin, and V. A. Tyalina Derzhavin State University, Internatsionalnaya 33, Tambov, 392000 Russia email: [email protected] Received February 24, 2009
Abstract—The interaction of intersecting basal and pyramidal dislocation pileups in singlecrystal zinc has been analyzed. Different versions of the formation of sessile (1/3[42 2 3]) and cleavage ([0001]) dislocations (microcrack nuclei) are considered. The merging of the head dislocations in pyramidal pileups is shown to be preferred. The conditions for thermally activated dislocation merging are derived. The conditions for crack opening according to the Gilman–Rozhanskiі mechanism are discussed. It is analytically established that the breaking stress, normal to the (0001) plane in the region of microcrack nucleation, exceeds the theoretical strength. DOI: 10.1134/S1063774510010128
Let us consider the basal–pyramidal interaction leading to the microcrack nucleation. With allowance for the sign of the Burgers vector of pyramidal and basal dislocations, these interactions are schematically shown in Fig. 1. Basal and pyramidal dislocations are located in the (ХОZ) and (( −X 'ОZ ) ) planes, respec tively. The resulting dislocation is placed at the center of coordinates. The vector ξ [8] is oriented along the negative direction of the Z axis. Two types of resulting dislocations with the vector bP can be formed (Fig. 1); they can be yielded by the following dislocation reac tions:
One of the reasons for the discrepancy between the real and theoretical strength values is the presence of microcracks in materials. This circumstance was noted for the first time by Griffith [1]. Griffith’ energy approach was developed in the microscopic theory of cracks [2]. To date, many ways of microcrack nucle ation have been proposed; they were repeatedly sys tematized and discussed in the literature [3–6]. How ever, despite the variety of versions of crack nucle ation, few have been experimentally confirmed. The processes caused by deformation twinning can be selected into a separate group. In this case the twin intersection of different crystallographic systems is very hazardous to the strength. For example, the nucleation of the second kind of Rose channels, which were initiated by twin intersection, was observed in zinc single crystals [7]. It was shown that the basal– pyramidal interaction in a twin material leads to the formation of cleavage dislocations of the [0001] type which play the role of a microcleavage nucleus. The growth of such a nucleus is due to both the merging of sliding basal and pyramidal dislocations into a void and their opening as a result of the stress caused by a dislocation pileup and twin intersection.
(0001)
1 / 3[2 1 10]1.0
( 2112)
(2114)
+ 1 / 3[2 1 13]4.46 → 1 / 3[42 23]7.46 ; (а)
(0001)
(2 1 12)
( ) . (b) 1 / 3[2 1 10]1.0 + 1 / 3[2113]4.46 → [0001]3.46 Interaction (a) is
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