Long-range ordered alloys

Effect of rare earths in long range ordered alloys [27]. 

Long-range ordered alloys like (Fe,Co)3V, (Fe,Ni)3V and (Fe,Co,Ni)3V are suitable alloys for high temperature applications. Addition of as little as <0.1% Ce along with Ti to these alloys increases rupture ductility by two fold, lowers the creep rate and increases the rupture life of the (Fe,Ni)3V alloy. The creep resistance of long range ordered alloys is increased by the addition of Nb and Ti/Ce. 

AB, AA and BB pairs as in a simple statistical distribution) the only difference between them is that in Fig. 6.10(h) there are more like-atom pairs (AA and BB) oriented along the vertical axis than in the horizontal direction in Fig. 6.10(a) the number of like-atom pairs is nearly the same for any direction. In the case of a long-range ordered alloy, Fig. 6.10(c), there is no possibility of directional order all atom pairs are of the type AB. 

Fig. 6.10. Schematic representation of directional order in an AB alloy (a) disordered alloy, where like-atom pairs are ordered in no particular directions (b) directional order, and (c) long-range ordered alloy (no like-atom pairs). (Adapted from Graham, 1959.)...

Using the so-called "block copolymers (a block of Na A-monomers at one end is covalently bonded to a block of Nb B-monomers) one can also realize the analogy of order-disorder phenomena in metallic alloys with polymers one observes transitions from the disordered melt to mesophases with various types of long range order (lamellar, hexagonal, cubic, etc ). We shall not consider these phenomena here further, however... 

J. Banhart, W. Pfeiler and J. Voitlander, Detection of short- and long-range order in Cu-Pt alloys,... 

The study of local or long-range ordering in semiconductor alloys based upon the effects on NQCCs has been carried out in a number of cases. These studies are analogous to the study of ordering in In Ga P by means of the chemical shift interaction, as described in Sect. 3.3.1. 

Fig. 5.15 (a) Field ion image and a domain map of a N Mo alloy which has been annealed for 5 min at 800°C with a slow heating rate to the final annealing temperature. Long range ordered domains are formed within the disordered matrix which, however, already exhibits a small degree of short range order. (Courtesy of M. Yamamoto.)... 

There is no adequate theory of the Neel temperature of a random distribution of centres in a dilute alloy, of indeed one exists. For higher concentrations of the magnetic matrix, with the assumption that only nearest neighbours interact, there is considerable theoretical work, giving a percolation limit , the concentration c0 at which long-range order disappears. The behaviour of TN is as (c —c0)12. For details see Brout (1965), Elliott and Heap (1962) and Klein and Brout (1963). 

Allen reported TEM observations of a nonaligned decomposition products in long-range ordered Fe-Al alloy [22]. Such morphologies are called isotropic spinodal microstructures. Similar structures are observed in Al-Zn and Fe-Cr alloys. Such structures can be produced in systems that are elastically isotropic or in which the lattice constant does not change appreciably with composition. 

Fusion, as an order-disorder transition, is the concept that fusion of a crystalline solid is essentially a change from the almost perfectly ordered solid state to a disordered liquid slate. The vacant spaces in the crystal lattice correspond lo the other component in the binary alloys, which undergo order-disorder transition in the pure form. Evidence from x-ray diffraction measurements indicates that short-range order is retained during fusion but long-range order is lost. 

It may be of interest to note that as far back as 1934 Rienhcker (47) reported that the decomposition of formic acid over Au-Cu alloys is influenced by long-range ordering of the alloys. 

The effect of long-range order on the electrical resistivity of copper-gold alloys is shown in Figure 8.6. The increased periodicity produced by ordering during annealing at 200 °C decreases the resistivity. 

In 1919, G. Tammann predicted that long-range order might exist in alloys. He used the German term Uberstructur, which translates as superlattices, to describe the phenomenon. Papers by C. H. Johannson, J. 0. Linde, and G. Borelius... 

The analogous calculation of the solubility of the interstitial impurity as a function of the H atom concentration, temperature, long-range order have been carried out for AB and AB3 alloys of other hexagonal structures. The results of such calculations are listed below [19-22],... 

The Eqs. (23)-(25) permit for each concrete alloy to define the dependence character of solubility of interstitial impurity of definite kind on concentration a, temperature T, degree of long-range order rj, if energetic parameters a, a, p, P are known. 

Substituting into the formula (30) the numbers m, mu m2, m3, lu l2, h from Table 1 and known formulae for probabilities (7), we find the solubility of interstitial impurity in alloy of each structure correspondingly for octa-and tetrahedral interstitial sites in dependence on alloy composition, temperature and degree of long-range order. 

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