Dislocation Interaction with Point Defects in Transition-Metal Disilicides
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Dislocation Interaction with Point Defects in Transition-Metal Disilicides Man H. Yoo Metals and Ceramics Division, Oak Ridge National Laboratory Oak Ridge, TN 37831-6115, U.S.A. ABSTRACT Energetics of the formation of jog-pairs and kink-pairs on a straight dislocation are analyzed using anisotropic elasticity theory for the equivalent slip systems of seven transition-metal disilicides. While glide loops of the active slip systems are stable in all cases, having positive line tension, the interaction energies of two opposite segments in kink-pairs and jog-pairs are found to be very anisotropic with respect to dislocation orientation. The anisotropic interaction plays an important role in the glide resistance due to dislocation-point defect interactions. A dislocation model is proposed for the glide resistance on edge and near edge dislocations based on jog-pairs resulting from the contact interaction between dislocations and intrinsic point defects. The available data of yield strength anomaly and dislocation structures of disilicide crystals are discussed in view of the proposed model for jog-pair pinning and dynamic breakaway. INTRODUCTION Transition-metal disilicides (MSi2, M: transition metals in group IV-VII) have non-cubic crystal structures, viz., hexagonal C40 for the four elements in groups V-VI (M = V, Nb, Ta, and Cr), tetragonal C11b for the three elements in groups VI-VII (M = Mo, W, and Re), and orthorhombic C54 (M = Ti) and C49 (M = Zr and Hf) for the three elements in group IV. The results of ab initio self-consistent band-structure calculations by Carlsson and Meschter [1] show that the structural energies of these disilicides are determined primarily by electronic-band effects rather than by atomic-size effects. Single-crystal elastic constants have been determined for most of these disilicides [2,3], and the directional nature of atomic bonds in MSi2 compounds with the C11b, C40 and C54 structures were analyzed in terms of the bond-stretching and bond-bending components of interatomic forces [4]. Geometrically, the atomic arrangements of a (110) plane in the C11b structure are identical to those of the (0001) plane in the C40 structure if the axial ratio of the tetragonal structure is set to c/a = √6 = 2.449 instead of the actual value (e.g., 2.450 for WSi2 or 2.452 for MoSi2). Moreover, the atomic arrangements of the (001) plane, the pseudohexagonal plane, in the C54 structure correspond also to those of the C40 basal plane. So, (110) [111], (0001)[1120], and (001)[110] are called the equivalent slip systems in the C11b, C40, and C54 structures, respectively [5]. Plastic deformation behavior of MSi2 single crystals has been reviewed by Ito et al. [6], including the role of the equivalent slip systems. In many cases (M = V, Nb, Ta, Mo, and Ti), an anomalous increase in the critical resolved shear stress (CRSS) with increasing temperature was observed over the temperature ranges in which serrated stress-strain curves were recorded. Therefore, the yield strength anomaly may be related to the Portevin
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