SOLID SOLUTION
When homogeneous mixtures of two or more kinds of atom occur in the solid state they are known as solid solutions i.e. a solid solution is simply a solution in the solid state and consists of two kinds of atoms combined in one type of space lattice.
Eg: - 1) Brass is solid solution of copper and zinc. Composition is copper 64% and zinc 36%. Cu is solvent atom and zinc atoms are solute atoms.
2) Ni-Cu alloys (Monel metal); Fe-C alloy (steels).
5.1 SOLID SOLUTION: -
A solid solution is a single phase, as it is chemically homogenous and the component atoms cannot be distinguished physically or separated mechanically by ordinary means.
Two types of solid solution formed are:
- Interstitial solid solutions
- Substations solid solutions
- Ordered.
- Disordered
- Interstitial solid solutions
This is formed when solute atoms are very small as compared to the solvent atoms and they are unable to substitute solvent atoms (because of the large difference in diameter of solvent and solute atoms.) and can only fit in to the interstices or space in crystal lattice of solvent atoms.
Atoms size is not the only factor that determines whether or not an interstitial solid solution will form. Small Interstitial solid solute atoms dissolve much more readily in transition metals like (Fe, Ni, Mn, Mo, Cr, etc.) than in other metals.
Carbon forms an interstitial solid with FCC (face centered cubic) iron during the solidification of steel. Nitrogen also dissolves interstitial in solid steel.
2. Substitutional solid solutions
In this, there is direct substitution of one type of atom for another so that solute atoms (Cu) enter the crystal to take positions normally occupied by solvent atoms (e.g nickel atoms). That is in substitutions solid solution, the atoms of the solute substitute for the solvent in the lattice structure of the solvent.
For e.g. – Atomic diameter of copper is 2.551A ° and that of nickel is 2.487 A ° and the two form Substitutional solid solutions.
Disordered substitutional solid solution: -
If the solute atoms do not occupy any specific position but are distributed at random in the lattice structure of the solvent, then this is known as disordered substitutions solid solution.
Ordered substitution solid solution: -
If the atoms of the solute material occupy similar lattice points within the crystal structure of the solvent material, this is known as ordered substitution solid solution
If the alloy in the disordered condition is cooled slowly, it undergoes a rearrangement of atoms because of diffusion to produce uniform distribution of solute and solvent atoms. This structure is known as ordered substitutions solid solution or super lattice.
E.g.- -Cu-Zn, Au-Cu etc.
Hume Rothery’s rules (or factors governing substitutions solubility):-
The different rules or factors are: -
1 Crystal structure factor: -
The crystal lattice structure of the two elements (metals) should be same (i.e. Both should be of B.C.C, F.C.C or H.C.P structure) for complete solubility for complete solid solubility the size factor must be less that 8%.
2 Relative size factor: -
If the sizes of two metallic atoms differ by less than 25% the metals are said to have
favorable size factor for solid-solution formation
E.g.: Cu-Ni, Au-Pt
3 Chemical affinity factor: -
The greater the chemical affinity of two metal, the more restricted is their solid solubility When their chemical affinity is great two metals tend to form an intermediate phase rather than a solid solution.
4 Relative valence factor: -
Consider two atoms, one with large valence electrons and other with small number of valence electrons. The metal of high valence dissolve only a small amount of lower valence metal, while the lower valence metal may have good solubility for higher valence metal.
Eg: -In Al-Ni alloy system Ni is lower in volume than Al and thus solid Nickel dissolves 5% aluminum, but higher valence Al dissolves only 0.04% Ni.
Intermediate phases: -
In binary alloy system when the chemical affinity of metal is great, their mutual solubility becomes limited and intermediate phase are rather than solid solutions.
Eg: -Cu-Al
There are two types: -
(a) Inter-metallic compounds of fixed composition: -They obey the usual valance laws.
Eg: -NaCl
(b) Inter-metallic compounds of variable composition.
They do not obey the valance laws and are known as electron compounds.
Eg: - Cu-Zn.
5.2 Solid phases and its type: -
Here the solids are described in terms of their phases, such as single-phase solids two phase solids etc. and this requires compositional approach, which involves temperature, pressure and concentration of chemical elements present in the solids This is useful in material science to know the state or condition at which the solids in question exists and how to improve the properties by changing its from thermodynamic point of view.
Basic terms: -
In the study of phase and phase diagram some terms are frequently used. So these should be clearly defined before different aspects of the topics are explained.
System: -
A system is a substance or group of substances so isolated from its surroundings that it is totally unaffected by the surroundings and is subjected to changes in over all composition, temperature, pressure or total volume only to the extent allowed by the person who investigates it.
State: -
The state of a system is a physical condition as defined by any combination of quantities, which fixes the condition.
Components: -
The components of a system are the chemical or compounds that make up the system.
Phase: -
A phase is physically and chemically homogeneous portion of a matter.
Equilibrium: -
Equilibrium in a system is the state of minimum free energy under any specified combination of overall composition, temperature, pressure and total volume.
Degrees of freedom: -
These are defined as the number of independent variables whose values must be specified in order to exactly define the state of a system.
Solid phases: -
Solid, liquid & gas are quite different in their characteristics and the freezing or at the boiling point where liquid and solid or gas can exist; these are two homogeneous type of matter. That is each of three forms constitutes a separate and distinct phase.
Single and multi-phase solids: -
A single crystal of a material can consist of one phase only, while polycrystalline materials may be single or multi-phase, depending upon the nature of the individual crystals or grains present. Solid solubility of the metals melted together to form a particular alloy. The most common multi phase materials are the naturally occurring solids called rocks. It is composed of two or more phases. Ceramic materials, wood, a number of polymers, fiberglass are common multiphase alloys. A number of structural features are important in multiphase materials, namely number of each phase and size of its domains. The structural Property relationship in multi phase materials depends upon the physical and chemical natures of the phase present, the percentage amount of each present distribution of phase in relation to one another and a size of the domains occupied by each phase.
Alloys: -
An alloy is a substance that possesses metallic property and composed of two or more element of which at least one is metal, is called an alloy. The base metal of an alloy is the metal present in greatest proportion, while other constituents are called alloying elements.
Phase diagram: -
A phase diagram or equilibrium diagram or constituent diagram is a graphical method of representation of the different compositions. Phase diagrams have temperature on the vertical axis and percentage composition by weight along horizontal axis. Phase diagram indicates the temperature at which the solid alloy will start and finish melting and the possible phase changes, which will occur as a result of altering composition, or temperature.
Phase rule:-
Gibb’s phase rule: -
This establishes the relationship between the number of degrees of freedom (F), the number of component (C) and a number of phases (P).
Mathematical representation is F=C-P+2, That 2 stands for two variable say temperature and pressure.
Unary Phase diagram: -
In unary or one component system, only temperature and pressure may be varied and coordinates of unary phase diagrams are temperature and pressure. In unary, temperature
Is shown along x-axis. For one component system, the phase rule becomes
F = 1-P+2 =3-P
This implies that the maximum number of phases in equilibrium is three when F=0. As shown figure, which has three regions, solid, liquid and vapor. In any one of these regions or phases temperature or pressure may be varied with no change in phase as there are two degrees of freedom (F=3-P, i.e. F=3-1=2) for two phases to continue to coexist only temperature or pressure can be varied independently. The point of intersection of these lines of phases is known as “Triple point”. At this point, the solid, liquid and vapor coexist in equilibrium.
Binary phase diagrams: -
In binary or two-component system, the maximum number of variables is three. Those are changes of temperature, pressure and concentration. Only one concentration is required to define the composition of two components, since the second component is found by subtracting from unity. The graphical representation of such a system requires a three dimensional diagram which is complicated. In order to simplify it, binary phase diagrams are drawn at constant pressure showing variations in temperature and composition only. The phase rule reduces to
F = C - P + 1
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