In this article we will discuss about:- 1. Meaning of Diffusion 2. Importance of Diffusion 3. Applications 4. Types 5. Mechanisms 6. Activation Energy 7. Self-Diffusion 8. Diffusion in Oxides and Ionic Crystals 9. Grain Boundary and Surface Diffusion 10. Factors that Influence Diffusion.
Contents:
- Meaning of Diffusion
- Importance of Diffusion
- Applications of Diffusion
- Types of Diffusion
- Diffusion Mechanisms
- Activation Energy of Diffusion
- Self-Diffusion
- Diffusion in Oxides and Ionic Crystals
- Grain Boundary and Surface Diffusion
- Factors that Influence Diffusion
1. Meaning of Diffusion:
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Diffusion, from an atomic perspective, is just the stepwise migration of atoms from lattice site to lattice site. In fact, the atoms in solid materials are in constant motion, rapidly changing positions.
For an atom to make such a move, the following two conditions must be met:
(i) There must be an empty adjacent site, and
(ii) The atom must have sufficient energy to break bonds with its neighbour atoms and then cause some lattice distortion during the displacement. This energy is vibrational in nature. At a specific temperature some small fraction of the total number of atoms is capable of diffusive motion, by virtue of the magnitudes of their vibrational energies. This fraction increases with temperature.
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Diffusion is the process of mixing which involves the movement of atoms from area of higher to those of lower concentration
or
Diffusion is the shifting of atoms and molecules to new sites within a material resulting in the uniformity of composition as a result of thermal agitation.
Movements in diffusion may be relatively short-range, as in allotropy, recrystallization and in precipitation.
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Diffusion is fundamental to phase changes and is important in heat treatments. It is also basic to many processes, such as casehardening of steel, production of strong bodies by sintering powders, homogenization of castings etc.
Diffusion is basically, statistical in nature, and the term applies to macroscopic flow (not individual movements) resulting from innumerable random movements of individual atoms. The path of an individual atom is random, zig-zag, and unpredictable. Nonetheless, when large number of atoms make such movements, they can produce a systematic flow.
Diffusive processes are irreversible and therefore, they increase entropy.
2. Importance of Diffusion:
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At room temperature, diffusion occurs very slowly in most solids and is of little or no importance. Diffusion occurs more and more rapidly as the temperature rises and is the basis for most metallurgical processes.
It is important in the annealing, recrystallization and grain growth of cold worked metal, in doping of semiconductors and in the formation of metallic bonds (soldering, welding, powder metallurgy).
Diffusion, or lack of it, will determine the degree of homogeneity attained in solid crystals forming from a melt. The absence of homogeneity in a solidified casting is called “dendritic segregation” or “coring.”
Diffusion is also important in the heat treatment of steel and in the precipitation hardening of alloys.
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3. Applications of Diffusion:
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The applications of diffusion are:
1. Phase changes, e.g., γ to α iron.
2. Metal bonding, e.g., welding, brazing, soldering, galvanising and metal cladding.
3. Homogenising treatment, e.g., annealing of castings.
4. Oxidation of metals.
5. Production of strong bodies by powder metallurgy.
6. Doping of semiconductors.
7. Recovery and recrystallization.
8. Surface treatment of steels, e.g., casehardening:
9. Precipitation of phases in age-hardening.
4. Types of Diffusion:
The different types of diffusion are:
1. Self-diffusion
2. Inter-diffusion
3. Volume diffusion
4. Grain boundary diffusion, and
5. Surface diffusion.
1. Self-Diffusion:
Self-diffusion is the migration of atoms in pure materials. In a pure substance, a particular atom does not remain at one equilibrium site indefinitely, rather it moves from place to place in the material.
Self-diffusion in a pure material can be detected experimentally by radioactive tracers.
2. Inter-Diffusion:
It occurs in binary metallic alloys.
Observed in binary metal alloys such as Cu-Ni system.
3. Volume Diffusion:
Volume diffusion means atomic migration through the bulk of the material.
4. Grain Boundary Diffusion:
It implies atomic movement along the grain boundaries alone.
The activation energy for grain boundary diffusion is lower than for volume diffusion.
5. Surface Diffusion:
It implies atomic movement along the surface of a phase. Example: Solid-vapour interface.
5. Diffusion Mechanisms:
In order to explain diffusion process, several mechanisms have been proposed. All of them are based on the vibrational energy of atoms in a solid.
Some of the common diffusion mechanisms are:
1. Vacancy mechanism.
2. Interstitial mechanism.
3. Direct interchange mechanism.
1. Vacancy Mechanism:
Diffusion is possible only if the atoms can shift and rearrange themselves in the lattice. If it is assumed that the lattice sites are all occupied, the atoms shall not be able to travel easily, particularly in substitutional solid solution alloys. The lattice of most metals and alloys contains a large number of unoccupied sites, called vacancies, and the diffusion is possible due to their existence.
The atoms surrounding the vacant site shift their equilibrium positions to adjust for the change in binding that accompanies the removal of a metal ion and its valency electron. Assuming, that the vacancies move through the lattice and produce random shifts of atoms from one lattice position to another as a result of atom jumping.
Over a period of time such diffusion produces concentration changes. Vacancies are continually being created and destroyed at the surface, grain boundaries and suitable interior positions such as dislocations. The rate of diffusion, therefore, increases rapidly with increasing temperature.
If the solid is composed of a single element (pure metal), the movement of atoms is called self-diffusion because the moving atom and the solid are the same chemical element.
Cu and Ni are mutually soluble in all proportions in the solid state and form substitutional solid solutions, e.g., plating of Ni on Cu.
The vacancy mechanism of diffusion in substitutional solid solutions is the dominant mechanism of diffusion in FCC metals and alloys and has been shown to be operative in many BCC and HCP metals. However, when the atoms of solute are of a small volume, they can travel easily through the interstitial sites without permanently displacing any of the atoms in the lattice.
2. Interstitial Mechanism:
When a solid is composed of two or more elements whose atomic radii differ significantly, interstitial solutions may occur. The large atoms occupy lattice sites while the small ones fit into the voids created by the large atoms. These voids are called interstices. The diffusion mechanism in this case is similar to vacancy diffusion except that the interstitial atoms stay on interstitial sites.
With interstitial diffusion, an activation energy is associated, because to arrive at the vacant site, it must squeeze past neighbouring atoms with energy supplied by the vibrational energy of moving atoms. Thus, interstitial diffusion is a thermally activated process.
This process is simpler since the presence of vacancies is not required for the soluble atom to move.
This diffusion mechanism is important in two cases-
(i) The presence of very small atoms in the interstices of the lattice greatly affects the mechanical properties of metals;
(ii) O2, N, and H, can be diffused in metals easily at low temperatures.
3. Direct Interchange Mechanism:
Two or more adjacent atoms jump past each other and exchange positions, but the number of sites remains constant. This may be two-atoms or four-atoms (zenner ring) interchange (for BCC).
Direct interchange mechanism entails following shortcomings /objections-
(i) Severe local distortion results due to the displacement of the atoms surrounding the jumping pair.
(ii) A number of diffusion couples of different compositions are produced. This is also called Kirkedall’s effect. (The inequality of diffusion was first shown by Kirkendall).
The practical importance of this effect lies in:
A. Metal cladding;
B. Sintering;
C. Deformation of metals (creep).
6. Activation Energy of Diffusion:
If the atoms are to change locations, the energy ridges (Fig. 4.5) must be overcome. The energy required to overcome them, alongwith the energy of defect formation, is called the activation energy of diffusion. As shown in Fig. 4.5 (i), the energy is required to pull the atom away from its present neighbours; with interstitial diffusion [Fig. 4.5 (ii)], energy is required to force the atom into closer contact with neighbouring atoms as it moves among them.
The activation energy varies with the following:
(i) Size of atom,
(ii) Strength of bond, and
(iii) Type of diffusion mechanism.
The activation energy required is high for large-sized atoms, strongly bonded materials such as corundum and tungsten carbide since interstitial diffusion requires more energy than the vacancy mechanism.
Kirkendall Effect:
An phenomenon in alloys is often confused with the dependence of the inter-diffusion coefficient in composition. This is the fact that the intrinsic diffusion coefficients for the two components in a given binary alloy may have different values, and thus one migrates faster than the other with no relationship to the concentration.
As a result, when two metals with different diffusion coefficients inter-diffuse, there is a net transport of material across the plane of the original interface between the two components. This is known as Kirkendall effect.
The Kirkendall effect is of considerable theoretical importance since it confirms the vacancy mechanism of diffusion.
The Kirkendall effect is also of some practical importance, especially in the field of metal to metal bonding, sintering and creep.
7. Self-Diffusion:
It is known that diffusion can occur by atoms moving into adjacent sites that are vacant. For an atom to move past other atoms, energy is required. This is the activation energy for vacancy motion.
Normally, no net diffusion is observed in a pure, single-phase material, because the atom movements are random, and the atoms are all identical. However, through the use of radioactive isotopes, it is possible to identify the diffusion of atoms within their own structure, i.e., self-diffusion.
Example:
Radioactive nickel (Ni59) can be plated onto the surface of normal nickel. With time, and depending on the temperature, there is a progressive self-diffusion of the tracer isotopes into the bulk of the nickel.
8. Diffusion in Oxides and Ionic Crystals:
Diffusion in oxides and other crystalline compounds that have ionic or partial ionic bonds can occur by the diffusion mechanisms earlier discussed. The diffusion process in the present case is somewhat complicated by the requirement of electrical neutrality.
Vacancies must be formed in such a way that the crystal does not acquire an electric charge. This condition can be fulfilled if the vacancies form in pairs, with each pair consisting of one cation (+ve charge) and anion (-ve charge) vacancy.
In ionic crystals, Schottky and Frankel defects assit diffusion. In the Frenkel diffusion mechanism, the cation interstitial carries the flux. In the Schottky diffusion mechanism, the cation vacancy carries the diffusion flux. The cations diffuse through cation vacancies and anions through anion vacancies. Actuation energies are not the same for both processes.
Diffusion in oxides and ionic crystals is very sensitive to the concentration of impurities as they affect the number of vacancies.
This diffusion process is very useful in fabricating parts from high temperature ceramics by the powder metallurgy technique. In the sintering process the powdered particles would bond together by diffusion.
9. Grain Boundary and Surface Diffusion:
Diffusion takes place along the surfaces of a solid in the grain boundaries of a polycrystal and through the volume of a material.
Surface atoms form fewer bonds than atoms at the interior of a solid; therefore it is expected that surface diffusion will have lower activation energy than volume diffusion.
During the solidification process, there is migration of atoms. This is called surface diffusion.
Atoms in the regions of grain-boundaries are not bonded as tightly as interior atoms and as such they diffuse more readily.
Surface and boundary diffusions are produced along paths other than those of crystal lattice diffusion.
10. Factors that Influence Diffusion:
Diffusion (and diffusion coefficient D) are influenced by the following factors:
1. Diffusing species;
2. Crystal structure;
3. Grain boundaries, dislocations and surfaces;
4. Grain size;
5. Temperature;
6. Pressure;
7. Concentration.
1. Diffusing Species:
The magnitude of the diffusion coefficient D is indicative of the rate at which atoms diffuse. Coefficients, both self- and interdiffusion, for several metallic systems are listed in Table 4.2. The diffusing species as well as the host material influence the diffusion coefficient.
Example:
There is a significant difference in magnitude between self- and interdiffusion in a iron at 500° C, the D value being greater for carbon interdiffusion (3.0 x 10-21 vs. 2.4 x 10-12 m2/s). This comparison also provides a contrast between rates of diffusion via vacancy and interstitial modes. Self-diffusion occurs by a vacancy mechanism, whereas carbon diffusion in iron is interstitial.
2. Crystal Structure:
The ease of diffusion with atoms having a fairly well-defined radius increases with decreasing density of packing.
In case of distorted crystal structure, the rate of diffusion usually increases.
3. Grain Boundaries, Dislocations and Surfaces:
Grain boundary diffusivities decrease with decreasing angle of tilt between the grains joined at the boundary.
Low-angle boundaries show anisotropy of diffusion, the mobility being higher in direction parallel to the dislocation edges than in the perpendicular direction on the boundary surface.
4. Grain Size:
Since grain boundary diffusion is faster than diffusion within the grains, it is to be expected that the overall diffusion rate would be higher in fine grained material.
5. Temperature:
Temperature has the most profound influence on the coefficients and diffusion rates. Example: The self-diffusion of Fe in α-Fe; the diffusion coefficient increases approximately six orders of magnitude (from 3.0 x 10-21 to 1.8 x 10-15 m2/s) in rising temperature from 500 to 900°C (Table 4.2).
It is observed that very small changes in temperature can have a marked effect on the diffusion rate.
6. Pressure:
Owing to strong binding between atoms in most metals, it requires high external pressures to effect an appreciable change in internal conditions, although it has been accomplished with relatively soft metals zinc and sodium.
7. Concentration:
When the concentration within a single solid solution phase varies, the diffusion coefficient also varies. For brass, in both the F.C.C. and B.C.C. phases, the diffusion coefficient increases markedly with increasing zinc content.
The following points are worth noting:
(i) The diffusion proceeds more rapidly along the grain boundaries because this is a zone of crystal imperfections.
(ii) Higher temperature provides higher diffusion coefficients, because the atoms have higher thermal energies and therefore, greater probabilities of being activated over the energy barrier between the atoms (Fig. 4.5).
(iii) Carbon has a higher diffusion coefficient in iron than does nickel in iron because the carbon atom is a small one.
(iv) Copper diffuses more readily in aluminium than in copper because the Cu-Cu bounds are stronger than Al-Al bonds.
(v) Atoms have higher diffusion coefficients in B.C.C. iron than in F.C.C. iron because the former has a lower atomic packing factor.
