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Ordered solid solution
Chemistry, materials science terms
When the distribution of solute atoms in the solid solution reaches an ordered state, it is called long-range order or complete order. Solute atoms in a completely ordered distribution of solid solutions, called ordered solid solutions, this ordered structure is called superstructure or
superlattice
.
During ordering, due to the increase of interatomic binding force, lattice distortion and the presence of reverse phase domains, the properties of the solid solution will change dramatically, in addition to the increase of hardness and yield strength, the decrease of resistivity, and even some non-iron
Magnetic alloy
It has obvious ferromagnetism after ordering.
- Chinese name
- Ordered solid solution
- Foreign name
- Ordered solid solution
- connotative
- solute Atoms are all ordered in a solid solution
- Application field
- Chemistry, materials science
Through definition and division, the concept of ordered solid solution can be clarified.
A structure in which the nearest neighbor of an atom in a solid solution is a heterogeneous atom is called an ordered structure; This process of tending to an ordered structure is called ordering, and the solid solution of the ordered structure formed is called ordered solid solution. The driving force of ordering is the mixing energy parameter ε
m
<0, and the resistance of ordering is configuration entropy; The contribution of the latter to the free energy (-TS) increases as the temperature rises. When the critical temperature is reached, the disordered solid solution becomes more stable and the ordered solid solution disappears.
The degree of ordering is called the degree of order, and the degree of order has the difference between short and long range, so there are short range order (σ) and long range order (ω) respectively.
Many solid solutions will form ordered solid solutions at low temperatures, when dissimilar atoms tend to be adjacent, this phase is also called superlattice or superstructure. Only when ideally proportioned ingredients (e.g. A
3
B, AB, AB
3
And in an ideal single crystal with a simple metal crystal structure, it is possible to obtain a completely ordered state. In fact, due to the existence of various defects and grain boundaries in the crystal, in the vast majority of cases, it is impossible to exist in a completely ordered state. It has been known that there are some ordered domains in ordered solid solutions, in which the solute and solvent atoms can be arranged in complete order, but the arrangement of atoms between the ordered domains is not consistent, and as a result, there will be more similar atoms adjacent to each other at the boundaries of adjacent domains. Ordered domains are sometimes called antiphase domains, and the number of them is quite large within a grain. The existence of antiphase domains has been confirmed by thin film transmission electron microscopy. When the composition of the solid solution is not at the ideal ratio, the degree of ordering is reduced, and the temperature of ordering is lower than that of the ideal composition.
[1]
There are several structural types of ordered solid solutions.
This type of solid solution was first found in Cu-Au alloys and has Cu
3
Au structure and CuAu structure. Cu
3
Au type is a disordered solid solution above 390℃. When annealed and cooled slowly below 390℃, Cu and Au atoms are arranged regularly in the lattice, Au atoms are located in the angular center and Cu atoms are located in the face-center, as shown in the figure. Alloys with such superlattice also have Ni
3
Fe, Ni
3
Mn, Zr
3
Al, CO
3
V, Zn
3
Ti, etc.
Ordered solid solution structure of Cu3Au
The Cu and Au molar fractions of CuAu (Ⅰ) type ordered structures are 50% respectively. Above 385℃ is a disordered solid solution, below 385℃ is an ordered solid solution, Cu and Au atoms arranged in layers, as shown in the figure. Because of the small size of the Cu atom, the vertical axis of the lattice becomes shorter, becoming
The tetragonal lattice. Alloys with such structures also include AgTi, AITi, CoPt, HgZr, FePt and so on.
Ordered solid solution structure of CuAu (I) type
The ordered structure of CuAu (Ⅱ) type is shown in the figure. This structure exists between 385 ℃ and 410℃. The superlattice is a long-period structure with five small cells at intervals
The category of atoms on the surface changes, and the crystal plane of the original Au atoms becomes the Cu atomic plane. The original Cu atomic plane becomes the Au atomic plane. An interface is created at half of the long cell, called an antiphase domains boundary. The distance between the two antiphase domains is
. M is the half-period of a long-period lattice, and δ is a small expansion in the b direction. Long period lattice in Cu
3
Au also exists, and not only in one long period, but sometimes in three long periods.
Ordered solid solution structure of CuAu (II) type
The most typical is Fe
3
AI, whose crystal structure is shown in Figure 2-18a. The four atomic positions are represented by a, b, c, and d. Above the ordered transition temperature, the four positions are occupied by Fe and Al atoms randomly. When the temperature is lower than the ordered transition point, Fe atoms occupy positions a, c, and d, and Al atoms occupy positions b, as shown in Figure 2-18b. When Al exceeds 25% (at), the excess Al atoms will occupy the c position. When the Al content is 50% (at), the C position is all occupied by Al atoms, as shown in Figure 2-18c, which becomes the FeAl superlattice. Other alloys with this superlattice are Fe
3
Si, Mg
3
Si, Mg
3
Li et al.
Body-centered cubic ordered solid solution structure
A typical alloy with a close-packed hexagonal ordered structure is the MG-CD series Mg
3
Cd, MgCd, MgCd
3
, Mg
3
The cell of Cd alloy is shown in the figure. Alloys with such an ordered structure also include Co
3
W, Co
3
Mo, Cd
3
Mg, etc.
Mg3Cd ordered solid solution structure
Ordering is the process of rearrangement of atomic positions, so the cooling rate has an effect on the degree of ordering, and the alloy changes from the ordering temperature (T
0
At rapid cooling, ordering may be inhibited, retaining the disorder at high temperatures. If the alloy is below T
0
. The temperature is maintained for a certain time, and the ordering can continue.
When the alloy deviates from the ordered ideal composition, it will produce an incomplete ordered structure. Strong plastic deformation also affects the degree of ordering.
[2]
Ordered solid solutions and microheterogeneity of solid solutions
in
Replacement solid solution
In interstitial solid solutions, the distribution of solute atoms is generally disordered, that is, the solute is statistically distributed. But under certain conditions, they may be partially or completely ordered. At this time, solute atoms and solvent atoms occupy fixed positions respectively, and the ratio of solute and solvent atoms in each cell is certain. Such a solid solution is called an ordered solid solution. This ordered structure is called a superlattice.
In fact, completely disordered solid solutions do not exist in nature. It can be considered that in a thermodynamically equilibrium disordered solid solution, the distribution of solute atoms is uniform on the macro scale, but they are not uniform on the micro scale. The diagram shows the distribution of solute atoms in a solid solution. In figure (a), the solute atoms are completely disordered. In the figure (b), (c), and (d), the cases are partial, partially ordered (also called short-range order), and fully ordered (also called long-range order). This depends mainly on the binding energy E between atoms of the same kind (i.e. A-A, B-B)
AA
And E
BB
Binding energy E between and heterogeneous atom (i.e. A-B)
AB
The relative size of... If E
AA
≈E
BB
≈E
AB
The solute atoms tend to be disordered. If the binding energy between similar atoms is greater than the binding energy between different atoms, the solute atoms are prone to a state of segregation. When the binding energy between different atoms is larger than that between similar atoms, the solute atoms will be partially ordered. For some alloys, when the solute atomic concentration reaches a certain atomic fraction, the arrangement is completely ordered.
[3]
Solute distribution in solid solution
