Solidification, Phase Relation and strengthening part 2

Solid solution and solid solution strengthening The mixture of two or more liquid material i.e. solute and solvent is called solution. The solution after solidification e.g. steel (alloy of iron and carbon) is called solid solutions. There are two types of solid solutions.

1. Substitutional Solid Solution
2. Interstitial Solid Solution

Factors affecting solubility of solute atom The solubility of the solute atoms in the host solvent (also called host matrix) can be determined by several factors:

1 Atomic size factor: Solid solution forms when the difference in atomic radius between the two mixing atoms is ≤15%, else it creates substantial lattice distortion.

2.structure of crystals forming solution : Similar type of crystal structure of metals of both atom are preferred.

3. Electronegativity: The more is electropositive one element and the more electronegative the other, the more capability to form an inter-metallic compound in place of solid solution.
4. Valency: A solvent of metal will have greater tendency to dissolve another metal(solute)of valency higher than that of a lower valency.

Hume-Rothery rules

They are a set of basic rule which describes the conditions for which an element could be dissolved in a metal, composing a solid solution. There are two sets of rules, first for substitutional solid solutions, and second for interstitial solid solutions

1. Substitutional solid solutions rule

The Hume-Rothery rules for substitutional solid solutions are:

1. The atomic radius of the solvent atoms and solute must differ by less than 15%.
2. The structure of a crystal of solute and solvent must match as possible.
3. Complete solubility between 2 metals when the solvent atom and solute atoms have the same valency.
4. The solute and solvent atom should have the similar value of electronegativity.

Interstitial solid solution rules

The Hume-Rothery rules for Interstitial solid solutions are:

1.Atoms of Solute must be smaller than the interstitial sites in the solvent lattice.

2. The solute atom and solvent matrix must have a similar value of electronegativity.

Phase:

A phase has the following characteristics

1. a) A phase has the same structure of atomic arrangement throughout
2. b) A phase has almost the same properties & composition throughout
3. c) There is a definite interface between the phase and any surrounding or adjoining phases.

Phase Diagram

The properties of any material depend on its type of state, atomic number, the amount of substance , and which form of the phases present. The properties of phase can be differed by altering these quantities. To make those change happen , it is important to understand the conditions for which these quantities exist and the requirement for which the change of the phase shall occur.

The best method to read, record and store the data related to any phase changes in many alloy systems or mixture is in the form of phase diagrams, also known as constitutional or equilibrium diagrams. It is primarily important to specify three independent variables. These variables, which are usually externally controllable, are temperature, pressure, and composition. pressure is assumed to be held constant at (1.015* 10^5 pascal)atmospheric value, the phase diagram shows the structural changes that occur in any state due to change of temperature and composition. Equilibrium diagrams show the phase relationships under equilibrium conditions of external variables.

Cooling Curve of a Pure Metal

Under equilibrium conditions, all metal material(Na, Mg, Ca) exhibit a specific melting or freezing point. If a cooling curve is plotted for a pure metal. It will show a horizontal line at the melting or freezing temperature.

Cooling Curve of a Solid Solution

A solid solution is defined as a solution in the solid state consisting of two atoms combined together homogeneously in a single type of space lattice. A solution is formed of two components: a solute and a solvent. When solidification starts in the solution, the temperature in magnitude may be lower or higher than the freezing point of the pure solvent.

Inverse Lever Rule

Inverse lever rule is used to determine the percentage of each phase that is present at various temperatures.

the solid phase of either A or B =( Ws-Wo) / (Ws-W1)

the liquid phase of either A or B= 1- solid phase

Phase reactions

Eutectic system

It is a phase transformation that takes place when a single-phase liquid transforms directly to a two-phase solid.