Structure of Dislocations and Mechanical Properties of B2 Alloys
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Structure of Dislocations and Mechanical Properties of B2 Alloys V. Paidar1 and V. Vitek2 1 Institute of Physics AS CR, Na Slovance 2, 182 21 Praha 8, Czech Republic 2 Department of Materials Science and Engineering, University of Pennsylvania, 3231 Walnut Street, Philadelphia, PA 19104, U.S.A ABSTRACT Adequate mechanical properties are important for both structural and functional applications of materials. There are significant differences in mechanical behaviour of different B2 ordered alloys and these are related to the properties of superlattice dislocations. Several types of dislocations can be activated, in particular and dislocations gliding on {110} planes. Their mobility can vary markedly from material to material and this has a strong impact on the mechanical properties. With the aim to elucidate qualitatively the differences between different alloys crystallising in the same B2 structure we analyse possible dislocation dissociations. The model employed is based on the isotropic elasticity but includes an important characteristic of stacking-fault-like defects involved in the splittings, the deviation of their displacements away from the usually assumed ½ APB. INTRODUCTION Mechanical properties of B2 intermetallic compounds and alloys are controlled by the type of active dislocations. The shortest lattice vector in the B2 structure is and therefore these dislocations are likely to possess the lowest energy. Hence, the slip direction is expected to be dominant. There are, indeed, B2 alloys in which the principal slip direction is (e. g. CoTi [1-3]) but the slip dominates in others (e.g CuZn [4] or FeAl [5, 6]) in spite of the fact that the square of the Burgers vector is three times larger than that of the vector and, therefore, the energy of the dislocations is expected to be much higher than that of dislocations. However, the dislocation energy is not the only criterion determining which dislocations mediate the slip. Another factor, which may be even more important than the energy, is the dislocation mobility that depends crucially on the dislocation core structure [7], in particular on the ability of dislocations to split into partial dislocations separated by metastable stacking-fault-like defects. For example, if dislocations possess narrow non-planar cores their motion is restricted since these cores must be first transformed from their sessile form to a glissile form. The well-known example is BCC metals. On the other hand, dislocations with planar cores are usually quite mobile since their cores are already in the glissile form. The latter is usually the case if dislocations dissociate into a crystal plane like, for example, in FCC metals. It is commonly accepted that in materials with metastable stacking faults that possess sufficiently low energies the dislocations dissociate into well-separated partials which renders dislocation cores planar. It has been generally assumed that in B2 ordered alloys the APB of the type ½ is a metastable fault on {101} planes and that its energy is related to the o
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