The crystal growth and structure of some vanadium carbon alloys

Abstract

The influence of ordering on the structure of vanadium carbon alloys has been studied using electron microscopy and electron diffraction. Within the nominally cubic (rocksalt) phase field of the vanadium carbon system non-stoichiometry occurs by the formation-. of-carbon vacancies, and these have been shown to be always distributed in a non-random manner at low temperatures (below 1300°C). The cubic phase field has been shown to contain two ordered compounds both of which exist over a range of stoichiometry, and in addition a region exhibiting a form of short range order. The order-disorder temperature has been determined from metallographic changes occurring in quenched samples annealed at successively higher temperatures, and a revised equilibrium diagram proposed for this compositional region.. In order to produce the high purity single crystals of different composition required for this structural study, a floating zone melting apparatus has beer, designed and built to operate with r.f. heating under a positive pressure of ambient-inert Gas to reduce volatalisation losses. Crystal growth experiments using this apparatus are described. The ordered compound V6C5 has been found to exist over a wide range of stoichiometry from VC0.75 to VC0.86. Two forms of ordered structure having monoclinic and trigonal symmetry are obtained, and duplex structures consisting of small highly faulted regions of both structures are obtained. Possible mechanisms and reasons for the formation of this type of structure are discussed. The alloys contain a high density of planar faults and these are analysed and discussed in terms of possible fault vectors for the proposed structures. A long period superlattice is formed at high temperatures at compositions near to VC0.84 whose formation is explained in terms of a crystallographic shear structure. At higher carbon contents the cubic V8C7 compound exists from VC0.87 to VC0.90 and is characterised by a foam structure of anti-phase domain boundaries which have been analysed in terms of possible fault vectors. Both the cubic V8C7 and non-cubic V6C5 forms of ordered carbon atom arrangements are seen in alloys with intermediate compositions and the formation of such structures is discussed. Above the order-disorder temperature and at lower carbon contents (< VC0.75 ) the carbon atom arrangement indicates a form of short range order detectable by broad diffuse bands in diffraction patterns. Possible models for this type of structure are discussed. In addition these lower carbon alloys contain large numbers of intrinsic stacking faults bounded bi Shockley partial dislocations. A dislocation mechanism is proposed whereby these faults are nuclei for the transformation to the ζ-phase detected at lower carbon contents. This phase has a complex twelve layer metal-atom stacking sequences Finally an explanation for the formation and stability of the ordered compounds is given in terms of both electronic and structural considerations, and the possible role these structures might have on mechanical properties is discussed