Alloys closed packed structure

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. ... 

Beryllium is a light metal (s.g. 1 -85) with a hexagonal close-packed structure (axial ratio 1 568). The most notable of its mechanical properties is its low ductility at room temperature. Deformation at room temperature is restricted to slip on the basal plane, which takes place only to a very limited extent. Consequently, at room temperature beryllium is by normal standards a brittle metal, exhibiting only about 2 to 4% tensile elongation. Mechanical deformation increases this by the development of preferred orientation, but only in the direction of working and at the expense of ductility in other directions. Ductility also increases very markedly at temperatures above about 300°C with alternative slip on the 1010 prismatic planes. In consequence, all mechanical working of beryllium is carried out at elevated temperatures. It has not yet been resolved whether the brittleness of beryllium is fundamental or results from small amounts of impurities. Beryllium is a very poor solvent for other metals and, to date, it has not been possible to overcome the brittleness problem by alloying. 

The holes in the close-packed structure of a metal can be filled with smaller atoms to form alloys (alloys are described in more detail in Section 5.15). If a dip between three atoms is directly covered by another atom, we obtain a tetrahedral hole, because it is formed by four atoms at the corners of a regular tetrahedron (Fig. 5.30a). There are two tetrahedral holes per atom in a close-packed lattice. When a dip in a layer coincides with a dip in the next layer, we obtain an octahedral hole, because it is formed by six atoms at the corners of a regular octahedron (Fig. 5.30b). There is one octahedral hole for each atom in the lattice. Note that, because holes are formed by two adjacent layers and because neighboring close-packed layers have identical arrangements in hep and ccp, the numbers of holes are the same for both close-packed structures. 

Laves phases form in several of the most metallic systems listed (especially alloys of Be, Mg, Zn), whereas for many 3 1 compounds the presence of geometrically close-packed structures (such as the cP4-AuCu3 and the hP8-Ni3Sn types) is characteristic. 

The simple cubic structme, sometimes called the rock salt structure because it is the structme of rock salt (NaCl), is not a close-packed structure (see Figure 1.20). In fact, it contains about 48% void space and as a result, it is not a very dense structure. The large space in the center of the SC structme is called an interstitial site, which is a vacant position between atoms that can be occupied by a small impurity atom or alloying element. In this case, the interstitial site is surrounded by eight atoms. All eight atoms in SC me equivalent and me located at the intersection of eight adjacent unit cells, so that there me 8 x (1/8) = 1 total atoms in the SC unit cell. Notice that... 

This process has been proposed to your notice in the first part of paper by the example of hexagonal close-packed structures. Many of hydrideforming metals and alloys have such crystal lattices. 

For the atoms which comprise metallic solids, and especially those having close-packed structures (fee in all the relevant cases here for which each atoms has 12 nearest neighbours) the atomic radius value which seems most appropriate is simply half the nearest-neighbour distance in the appropriate elemental metal. Some of the alloying adsorbate species, however, do not form elemental solids of this type. As a starting point in our discussion, and the constraction of Table 6, we follow a common procedure of taking the atomic radius from the value of half the interatomic spacing in the elemental solid, but corrected to a coordination number of 12 [51, 52] and take values from a... 

Apart from the intrinsic interest of the interatomic distances in metals, it is useful to have a set of radii to refer to when discussing the structures of alloys. Since the c.n. 12 is the most common in metals, it is usual to draw up a standard set of radii for this coordination number. For the metals with ideal close-packed structures the radii are simply one-half the distances between an atom and its twelve equidistant nearest neighbours. Many structures, however, deviate slightly from ideal hexagonal closest packing in such a way that sbt of the neighbours are slightly farther away than the other six, for example. 

The characteristic line sequences for cubic lattices are shown graphically in Fig. 10-2, in the form of calculated diffraction patterns. The calculations are made for Cu Kol radiation and a lattice parameter a of 3.50 A. The positions of all the diffraction lines which would be formed under these conditions are indicated as they would appear on a film or chart of the length shown. (For comparative purposes, the pattern of a hexagonal close-packed structure is also illustrated, since this structure is frequently encountered among metals and alloys. The line positions are calculated for Cu Kol radiation, a = 2.50 A, and cja = 1.633, which corresponds to the close packing of spheres.)... 

Simple metallic solids are elements or alloys with close-packed structures where the large number of interatomic overlaps gives rise to wide bands with no gaps between levels from different atomic orbitals. Metallic properties can arise, however, in other contexts. In transition metal compounds a partially occupied d shell can give rise to a partly filled band. Thus rhenium in Re03 has the formal... 

The crystal structures of metallic ruthenium and copper are different, ruthenium having a hexagonal close-packed structure and copper a face-centered cubic structure (7). Although the ruthenium-copper system can hardly be considered one which forms alloys, bimetallic ruthenium-copper aggregates can be prepared that are similar in their catalytic behavior to alloys such as nickel-copper (3,4,8). 

Tin has three crystalline modifications or allotropes, a-tin or gray tin (diamond structure), P-tin or white tin , and y-tin the latter two are metallic with close packed structures. Tin also has several isotopes. It is used in a large number of alloys including Babbit metal, bell metal, Britannia metal, bronze, gun metal, and pewter as well as several special solders. 

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