Alloy: Metallic materials having a combination of more than one metal along with some other element or metal are usually termed as an alloy.
Types of alloys: it is based on the base metal which will be the main material or element and it is combined with other elements in small portions.
Example: stainless steel- the combination of iron and carbon, chromium in small proportions.
Solid solutions: When two different metals are melted and mixed with the liquid melt of one metal completely dissolves in the other results in a solution and when it solidifies it is said to form a solid solution.
Types of solid solution
Substitutional solid solution: It is formed when atoms of the base metal are replaced or substituted by the atom of the alloying metal.
Example: Monel, it’s an alloy of Copper and Nickel
Disorder substitutional solid solution: when the atoms of the alloying element do not occupy any specific position but are distributed at random in the lattice structure of the basement.
Ordered substitutional solid solution: in the atom of the alloying elements substitute the atoms of the base metal in an orderly manner occupying fixed positions in the lattice structure of the base metal.
Interstitial solid solution: When the atoms of the base metal are bigger than the atoms of the alloying element, substitutional of atoms is impossible instead is a small- sized atom of the alloying element fit in the open space between bigger base metal atoms.
Rules governing the formation of solid solubility
Hume Rothery rule:
Crystal structure: Two or more metals should have the same crystal structure for complete solubility.
Atomic size: For complete solubility two elements should have approximately the same atomic radius.
Chemical affinity factor: chemical affinity should be low to form a proper alloy.
Electronegativity: atoms of the two elements must have approximately the same electronegativity.
Valence factor: the valency of the base metal and the alloying elements should be the same.
Basic terms:
Components - Independent chemical species which compressor system.
Phase - Physically Distinct, homogeneous chemically and mechanically separable regions of a particular System.
Phase diagram - A map that provides relationship between the phase in equilibrium System as a function of temperature, pressure and composition.
Solubility limit - The maximum concentration of elements for which a single-phase solution exists.
Equilibrium:
Equilibrium Is usually described in terms relating to free energy.
The free energy is nothing but the internal energy of a system or in other words, it is the disorderliness and randomness in the arrangement of atoms and molecules of the material.
A system is said to be in equilibrium when free energy is minimum for a given temperature, pressure and composition. Which means that the property of the material does not change with the change in the independent variable.
Uses of Phase Diagrams
It helps to identify various phase present in different temperature and composition.
It helps to identify solubility(solid) of one element in the other.
It indicates the temperature range for which solidification/liquefaction of system occurs.
It indicates the specific temperature at which various phases start and melt.
Unary phase diagram
The term unary indicates that the phase diagram is off single component. The above-mentioned figure is a Phase diagram of water. The point O the figure indicates the phase equilibrium. Water exists in different faces with change in temperature and pressure as indicated in the unary phase diagram of water.
Binary Phase diagram
The phase diagram of the two components is called a binary phase diagram.
The above-mentioned binary phase diagram is of copper Nickel system. In addition to temperature and pressure, a third variable i.e., composition also involves in this diagram. For simplicity in plotting, temperature is plotted on the y-axis and alloy composition on x-axis thereby assuming pressure as 1 atmospheric. The x-axis contains the composition of both Cu and Ni. The composition of Ni labelled from left to write in the x-axis and for Cu it is in opposite direction as shown in the figure. Which means that , when the wt% of Ni is 100%, wt% of Cu is 0.
Solidus - The temperature at which the alloy is completely solid. Above this line, the liquification initiates.
Liquidus- The temperature at which the alloy is completely liquid. Below this line, the solidification initiates.
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