Synchroshear of Laves Phases
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SYNCHROSHEAR OF LAVES PHASES 3 P.M.HAZZLEDINE 1 , K.S.KUMAR 2 , D.B.MIRACLE and A.G.JACKSON
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1. UES Inc, 4401 Dayton-Xenia Road, Dayton, OH 45432 2. Martin Marietta Laboratories, 1450 South Rolling Road, Baltimore, MD 21227 3. Wright Laboratory, Materials Directorate, Wright-Patterson AFB, OH 45433. ABSTRACT The three Laves phases consist of alternating single layers and triple layers of atoms. Shear of the structure within the triple layers may be achieved by moving synchrodislocations. The dislocation with the smallest Burgers vector is the synchroshockley a/6 which has a core split over two planes. If a synchroshockley sweeps every triple layer of the cubic C15 it is twinned, if it sweeps every other triple layer, C15 is transformed into hexagonal C14. If the synchroshockley sweeps two triple layers, leaves out two, sweeps two etc. C15 is transformed into C36. Synchroshockleys travelling in pairs in any of the structures form dissociated perfect dislocations capable of giving slip. INTRODUCTION The Laves phases form the most numerous group of intermetallic compounds. Some of them have promisingly high melting temperatures and contain elements which could provide oxidation resistance. Ductility has been observed at low temperatures [1] and two-phase alloys containing Laves phase have some attractive mechanical properties [2]. Often Laves phases are hexagonal at high temperatures and transform to a cubic structure at low temperatures [3]. Electron microscopy shows crystals which are heavily faulted [31 or twinned [1] on the close packed planes. The relationship between the three crystal structures [4] and dislocation models for shear transformations [5] are both generally described by means of shears on these planes. However, it is important to realise that ordinary shear cannot occur in Laves phases and that synchroshear is necessary instead. In this paper we use the concept of synchroshear [6] and the established notation [3] to give a unified description of the crystal structures, phase transformations, twinning and slip in Laves phases. LAYERED STRUCTURE OF LAVES PHASES The three Laves phases C14, C15 and C36 ( whose archetypes are MgZn2, MgCu2 and MgNi2 respectively) have ideal chemical compositions S2L containing small atoms S and large atoms L with an ideal radius ratio of 32. The crystal structures are layered and have just two structural units, a single layer of S and a triple layer in two variants. The layers may be thought of in the following way (Fig.l): Form a close packed layer of S atoms (Fig.lA) and remove from that one quarter of the atoms. The remaining S atoms form a single layer, s, and the crystallography is defined by a hexagonal lattice with lattice points A on the holes in the s layer (Fig. 1B). The triple layers are made by keeping the quarter of S atoms on the same two-dimensional lattice and filling the interstices with L atoms. Because the L atoms are larger than S atoms they do not quite form a flat plane; half of the L atoms are slightly above the s plane (L+) and half are slightly
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