Packings, structured definition

Martensitic phase transformations are discussed for the last hundred years without loss of actuality. A concise definition of these structural phase transformations has been given by G.B. Olson stating that martensite is a diffusionless, lattice distortive, shear dominant transformation by nucleation and growth . In this work we present ab initio zero temperature calculations for two model systems, FeaNi and CuZn close in concentration to the martensitic region. Iron-nickel is a typical representative of the ferrous alloys with fee bet transition whereas the copper-zink alloy undergoes a transformation from the open to close packed structure. ... 

Here Hd is the number of atoms in a unit cell, the volume of which is V, and is the shortest interatomic distance in the arrangement. The definition contains a division by /2 so that the parameter D becomes unity for close-packing structures. Kepler s conjecture ensures that the parameter D is always less than or equal to unity. The fraction of space occupied (fi in the rigid-sphere model, which is often used in the discussion of metallic structures, is proportional to the parameter D and the relation is as follows. 

If close-packing structures are deformed in a special way, they can be transformed into other definite structures. In this section, the variations of the D and N parameters during the transformation will be examined. 

F. C. Frank, J. S. Kasper, Complex alloy structures regarded as sphere packings. I Definitions and basic principles, Acta Crystallogr. 11 (1958) 184. II Analysis and classification of representative structures, Acta Crystallogr. 12 (1959) 483. 

This brings us to a class of compounds too often overlooked in the discussion of simple ionic compounds the transition metal halides. In general, these compounds (except fluorides) crystallize in structures that are hard to reconcile with the structures of simple ionic compounds seen previously (Figs. 4.1-4.3). For example, consider the cadmium iodide structure (Fig. 7.8). It is true that the cadmium atoms occupy octahedral holes in a hexagonal closest packed structure of iodine atoms, but in a definite layered structure that can be described accurately only in terms of covalent bonding and infinite layer molecules. 

The sequence and spacings given for close packing are not artificial descriptions or approximations, as these are determined by geometry. The PTOT system is the most detailed and definitive treatment presented for close-packed structures, and many other structures can be described in this system. 

Lustrous, hard metal hexagonal, cJose-packed structure. d 12.45. mp about 2450" bp about 4150", Sp heat (O ) 0,057 cal/g/°C. Does not react with acids, even aqua regia. Net oxidized by air in the cold on heating combines readily with oxygen the powdered metal forms the dioxide on ignit -mg in air. Supeficially attacked by coned alkaline hypochlorites, The powdered metal is attacked by chlorine above 200" by bromine between 300-700. Oxidized by fused alkali hydroxides. Forms alloys with platinum, palladium, cobalt, nickel, tungsten forms definite compds with zinc and with tin. 

In close-packed structures, for example, face centered cubic (fee) austenite, the formation of a vacancy can be envisaged as a definite process in which an anpty hole is left in an otherwise not-greatly-disturbed lattice. This concept, as has been pointed out by Lomer [78], is much less tenable for the bcc structure. In such structures, the formation of relaxed vacancies has been postulated, allowing relatively easy atomic movanent inside the disturbed region. 

Frank, F.C. Kasper, J.S. (1958). Complex Alloy Structures Regarded as Sphere Packings. I. Definitions and Basic Principles. Acta Crystallogr. 11,184-190. 

Proc., 65(3), 45 (1986)] is preferred. To use this and alternate models, dimensional characteristics of structured packing must be defined. Figure 14-51 shows nomenclature and definitions of key dimensions. Not shown, but also important, is the angle the corrugations make with the horizontal (usu y 45 or 60°). Then the Rocha et al. predictive equation is ... 

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