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CHAPTER 9
PHASE DIAGRAMS |
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lecture:2
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So far…..
We have concentrated on the important definition of mechanical properties.
We have reviewed some important structure you are studied it in pervious class.
In this chapter we start to examine alloying, and see why alloys are stronger than pure metals
Introduction
The understanding of phase diagrams for alloy systems is extremely important because there is a strong correlation between microstructure and mechanical properties.
The development of microstructure of an alloy is related to the characteristics of its phase diagram.
In addition, phase diagrams provide valuable information about melting, casting, crystallization, and other phenomena.
Some important definition in this chapter:
It is necessary to establish a foundation of definition and basic concepts relating to alloys, phases, and equilibrium before interpretation and utilization of phase diagram .
Component: A chemical constituent (pure metals or compounds) of an alloy, which may be used to specify its composition.{Ex. In a copper-zinc brass, the components are (Cu & Zn)}.
Solvent: The component of a solution present in the greatest amount. It is the component that dissolves a solute.
Solute: One component or element of a solution present in a minor concentration. It is dissolved in the solvent .
System: Two meanings are possible: (1) a specific body of material that is being considered (e.g. a ladle of molten steel), and (2) a series of possible alloys consis-ting of the same components (e.g. the iron-carbon system).
In previous class you are studied :
Solid solution: consists of atoms of at least two different types; the solute atoms occupy either substitutional or interstitial positions in the solvent lattice, and the crystal structure of the solvent is maintained.
THE SOLUBILITY LIMIT :
Solubility limit: The maximum concentration of solute that may be Added without forming a new phase.
The addition of solute in excess of this solubility limit results in the formation of another solid solution or compound that has a distinctly different composition.
For example the sugar-water (C12H22O11-H2O) system :
Initially, as sugar is added to water, a sugar-water solution or syrup forms.
As more sugar is introduced, the solution becomes more concentrated until the solubility limit is reached or the solution becomes saturated with sugar.
At this time the solution is not capable of dissolving any more sugar , and farther additions simply settle to the bottom of the container.
This solubility limit of sugar in water depends on the temperature of the water.
The solubility limit can be represented in graphical forms on a plot of temp. in vertical coordinate and composition (in weight percent sugar)in horizontal Coordinate as shown:
From this fig.:
The sum of concentration at any composition will equal 100wt%.
The solubility limit is represented as the nearly vertical line in the fig..
For composition and temp. to the left of the solubility line, only the syrup liquid solution exists.
To the right of the line syrup and solid sugar coexist.
The solubility limit at some temp. is the composition that corresponds to the intersections of the given temperature Coordinate and the solubility limit line.
Example:
Question: What is the solubility limit at 20oC?
Answer: from above fig. the solubility is 65wt% sugar.
If Co < 65wt% sugar: syrup
If Co > 65wt% sugar: syrup + sugar
Hint : Solubility limit increases with temperature .e.g., if T = 100oC, solubility limit = 80wt% sugar
Phases:
To understand of phase diagrams the concept of phases must be defined:
A phases: A homogeneous portion of a system that has uniform physical and chemical characteristics.
Every pure material is considered to be a phase, so also is every solid, liquid, and gaseous solution.
For example, the sugar – water syrup solution just discussed is one phase and solid sugar is another. (How?)
When two phases are present in a system, it is not necessary that there be a difference in both physical and chemical properties ,a disparity in one or other set of properties is sufficient.
Example :
When water and ice are present in a container, two separate phases exist, they are physically dissimilar (one is a solid, the other is a liquid) but identical in chemical makeup.
When a substance can exist in two or more polymorphic forms (e.g. Having both FCC and BCC structures), each of this structure is a separate phase because their respective physical characteristics differ.
Sometimes:
Single phase system is termed (homogeneous).
System composed of two or more phases are termed (mixture) or heterogeneous system like (metallic alloys, ceramic, polymeric, and composite materials.
Microstructure
Microstructure : is the structural features of an alloy (e.g. grain and phase structure) that are subjected to observation under a microscope.
Microscope observation are:
Optical microscope,
Electron microscope.
In metal alloys microstructure is characterized by the number of phases present their proportions and the manner in which they are distributed or arranged.
The microstructure of an alloy depends on such variables as the alloying element present, their concentration, and the heat treatment of the alloy (i.e. the temp., the heating time at temp. and the rate of cooling to room temp.).