Long-distance order


engineering materials, some materials atom by covalent bonds, van der Waals bond binding, their atoms are arranged according to certain rules in the molecular range, but molecules with molecular then randomly coupled together between irregular. E.g. SiO 2 , is a covalent bond to four oxygen atoms bound to a silicon atom, silicon and oxygen tetrahedral structure. However, SiO 2 tetrahedral units may be randomly irregular together, i.e., an amorphous glass body. Usually the molecular range law called short range ordered arrangement, since the regularity of the arrangement relates to only atoms adjacent areas. Most polymers having short-range ordered arrangement of atoms.

engineering materials, some materials as metal bonds, ionic bonds binding, their atoms are arranged according to certain rules, but the regularity of atomic arrangement, not only in the vicinity of the atomic range, but throughout internal material. The entire interior material generally has a regular arrangement of atoms, referred to as long-range order. Atomic long-range order, i.e. constituted crystals. Such as metals, ceramics, and some of many polymer materials, which are expressed as atomic arrangement long range order, i.e., the atoms constituting crystals of these materials.

A-B-based binary alloy

long range order of

in. If the sequence element is formed a solid solution, the lattice points in the lattice may be arbitrarily occupied by A or B atoms; if an ordered solid solution, α-point bursts point should occupy the position of atoms A, B beta] position should occupy as atoms, that is, each in their own certain atoms occupy the correct position. Atomic percent now provided A component for C A , atomic percent B component of C B ;
occupied α position A atom probability,
probability β position is occupied by B atoms; p is the probability of a, the group B element in one of the group element atoms (i.e., a or B) in the correct position, C x for the respective atomic percent of component, the long-range order of the [omega], is defined as:

visible, when p = C X when, ω = 0, was completely disordered solid solution; when p = 1, ω = 1, was fully ordered solid solution. Thus, in the AB type alloys,
, the
; of AB 3 type alloys, when starting from the A component, which
, the
; 3B type alloy, still starting of A from the A component, which
, the

Long-distance order

Further, AB type alloys, [omega] may be represented by the following formula expressed:

formula, N is the total number of atom; R is accounted for "on" atoms lattice points (i.e., a representing α position din, B representing β position); W is accounted for as "fault" to the original number of lattice points (i.e., a β representing position, B representing α position). When no crystal defects due to,
, and
, the formula can be derived therefrom.


long-range order due to

in the left as shown in Figure 1, when the solute atoms elongated distribution range order, may be formed between the sliding surfaces on both sides atoms AB type matching relationship atoms. When there is movement of dislocations in the slip plane, we will continue to disrupt this orderly relationship to form APB. Therefore, only a single dislocation motion in the additional external force to compensate for the energy required in the antiphase boundary. When assuming that the antiphase boundary is γ, so that shear stress is applied to a single dislocation motion is required:

of formula, b is the Burgers vector dislocations. In high antiphase boundary alloys, possible to achieve the conditions given in Formula only stress concentration exists.

in the long-range ordered alloy, the dislocation is easily form pairs of superlattice dislocations motion. This superlattice dislocations by the full number of dislocations with two connected in antiphase boundary consisting of dislocations (FIG. 1 right panel). Each full dislocation and broken down to form an extended dislocation. When To superlattice dislocation motion, without additional force. Antiphase boundary formed by the leading dislocations,

may be paired therewith through subsequent dislocation motion tracking destroyed, so the result is an integrated energy system unchanged. However, if the crystal APB already exist, it can be an effective barrier superlattice dislocation motion. Left in FIG. 1 (b), when the superlattice dislocations crossing APB, APB will produce two atoms long steps, resulting in increased energy system. This will not only formed on the main surface slip APB, resulting in difficulties median secondary slip dislocation motion, and, when there is the main slip plane superlattice dislocations continue to slip, but also left the following form configuration shown in FIG. (a), the main slide blocked. By the time the leading dislocation APB slip surface, can eliminate the cause of superlattice dislocations disintegration. So, trailing the leading dislocations and dislocation they were blocked in two antiphase boundaries disconnected.

Because of the above superlattice dislocation motion mechanism, the flow stress will lower the initial ordered alloy, and a high rate of work hardening, high strength is obtained easily by flash work hardening. When ordered alloy single crystal modification, only the linear hardening stage, as shown in FIG. 1 on the right. This indicates superlattice dislocation slip plane prone to slip on the main surface of the cross slip is difficult to occur. Results superlattice dislocation slip plane, are piled up easily formed at grain boundaries and cause stress concentration. Therefore, by cold deformation ordered alloy is significantly hardened, while filling the plastic losses.

long-range order structure

ordered distribution of solid solution atoms, or a similar compound known as an ordered solid solution or superlattice. Surface atoms and each atom location is different superlattice enriched atoms thereof. Such equivalent atomic plane spacing than the atomic plane spacing equivalents disordered solid solution is several times longer. Superlattice atomic ratio generally has a simple, such as AB, AB 2 , AB 3 and the like, and therefore is also called superlattice structure long-range order. Surface lattice atoms such equivalent variations and generate new additional diffraction spot.

solid solution and disordered superlattice structure under certain conditions can be converted into each other, but the degree of ordering in a superlattice which is within a certain range of fluctuation, so people accustomed to superlattices corresponding to the disordered structure is referred to as a source of the basic structure of the structure, while the long-range order or superlattice structure derived structures considered, additional structures. Some under various conditions are ordered structure, which itself is similar to phase compound, and is not disordered structure corresponding to transformation, but the diet was also called superlattice.

and the source of the relationship between the flow of such a structure, the basic and the additional views are taken to electron diffraction analysis, the diffraction spot is generally referred to as a random solution substantially spots, while the superlattice diffraction spots called excess superlattice additional spots. From electron diffraction we can clearly distinguish the two types of diffraction spots, substantially as bright spots, additional more weak spots.

superlattice ordered and atomic vacancies layer category can be divided into three classes: substitutional atoms leads to long-range order ordered substitutional called super lattice; gap caused by long-range atomic ordering ordered called gap type superlattice; atomic vacancy layer results in long-range order ordered called a nonconservative displacement superlattice. No longer explain each of the three types here.

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